ABSTRACT

MICROSCOPIC ANATOMY OF THE
INTEGUMENTARY SYSTEM OF THE HORSE

by Amir Hossain Talukdar

The microscopic anatomy of the integumentary system of
the horse is not adequately described in the available vet-
erinary literature. This investigation was undertaken with
the view of filling this void.

For this histologic study, skin specimens from 35 areas
of two stallions, two geldings and two mares were obtained
from freshly killed normal animals two to seventeen years
old. A few additional sections were obtained for the histo-
chemical studies of glycogen and periodic acid-Schiff (PAS)
positive substances and for frozen sections to study the
nerve endings of the lip. For routine paraffin sections,
the specimens were fixed in formalin-acetic acid-alcohol.
For histochemical study, 16% formalin was the fixative of
choice.

The following stains were used for histologic study:
(1) Harris hematoxylin and eosin, (2) Gomori's aldehyde
fuchsin for elastic fibers, (3) periodic acid, silver,
orcein and aniline blue for all the fiber components of the
-dermis, (h) Bielschowski-Gros for the revelatiOn of nerve
endings and (5) Domici's mast cell stain. For histochemical
studies Bauer-Feulgen technic was used for glycogen and

periodic acid Schiff reagent for PAS positive substances.

Amir Hossain Talukdar

The surface of the skin contained ridges and grooves.
The hairs erupted in the grooves and formed rows on the sur-
face. The average skin thickness of general body skin was
3.8 mm. The epidermis consisted of 3 layers: stratum
germinativum, stratum granulosum and stratum corneum; the
stratum lucidum was absent. Keratohyalin granules were
observed in the cells of the suprabasal layer of the epider-
mis and increased gradually until the cytOplasm of the cells
of the stratum granulosum was completely filled. Basal
epidermal processes anchored the epidermis with the underlying
tissue. The basal layer also contained dendritic melanocytes
in addition to germinative cells. The dermis consisted of
two well demarcated layers: the stratum papillare and the
stratum reticulare. The collagenous fibers in the papillary
layer were fine and loosely arranged, but in the reticular
layer were compact and tended to form a third layer in the
lower part. An extensive network of elastic and reticular
fibers was present in the superficial part of the papillary
layer and penetrated the PAS positive basement membranes. A
modified dermal region extended from the posterior part'of
the dorsum to the end of the croup with lateral extensions
over the gluteal regions. While the dermal thickness of the
general body skin, excluding the croup, was 3.7 mm that of
the croup was 5.5 mm. As in cattle the hairs occurred singly.
The medulla of the hair contained two layers of rectangular
cells which were attached to adjacent cells by desmosomes.

The larger hair follicles contained follicular folds similar

Amir Hossain Talukdar
to those observed in the other domestic animals. The cells
of the external root sheath of the hair follicles were
strongly PAS reactive and contained glycogen. Usually two
sebaceous glands, larger in the regions with fine hairs,
were associated with each hair follicle. The upper lip and
hoof margin contained large, branched sebaceous glands.

Sweat glands were the apocrine type containing a brush border
on the luminal side and canaliculi at the basal part of the
intercellular space. Both an apocrine and canalicular
secretory mode has been preposed. Numerous arterio-venous
anastomoses were present in the lips, nostrils and coronary
border. In this study arterial cushions were observed only
in the dermis of the lips and thigh. ’Morphologically, four
different types of nerve endings were present in the lips:
lamellated endings, capsulated end organs, free nerve.endings
and non-capsulated balls. In addition, nerve nets were

demonstrated on the hair follicles.

MICROSCOPIC ANATOMY OF THE

INTEGUMENTARY SYSTEM OF THE HORSE

BY

Amir Hossain Talukdar

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Anatomy

19 67

AK/f/V57g ‘

BISMILLAH-IR-RAHMANER-RAHIM
(Holy Quran)

Begin in the name of Allah the

most gracious and merciful.

VITA
Amir Hossain Talukdar

Candidate for the degree of

Doctor of Philosophy

Einal Examination: November 17, 1966. 10:00 a.m.

Dissertation: Microscopic Anatomy of the Integumentary
System of the Horse.

Outline of Studies:
Major Subject: Histology

Minor Subject: Physiology

Biographical Items:

Born: February 1, 1928. Parents: Mvi. Ghulam Ismail
Talukdar and Mst. Aftabunessa.
Chhotasundar, District-Comilla, East Pakistan.

Undergraduate Studies: D. V. M. S., East Pakistan
Veterinary College, Dacca, 1952.
Scholarship (A. I. D.) 1957-
1960. Be S. (A. 80.), Texas
A.& M University, College
Station, Texas, 1959.

Graduate Studies: M. S. (Veterinary Anatomy), Texas
A.& M University, College Station,
Texas, 1960.

Professional §x2erience:

Veterinary Assistant Surgeon, Department of Animal
Husbandry, Government of East Pakistan, 1952-1957.

Lecturer, East Pakistan College of Veterinary Science
and Animal Husbandry, Mymensingh, East Pakistan, 1960.

Senior Lecturer, East Pakistan Agricultural University,
Mymensingh, East Pakistan, 1961.

Head of the Department of Anatomy, East Pakistan
Agricultural University, Mymensingh, East Pakistan,
1962-19640

Egggg£_gf: Phi Zeta (a national honor society of Veterinary
Medicine) and Sigma Xi (a national honor society
of scientists).

ll

ACKNOWLEDGMENTS

The author wishes to sincerely express his deep
appreciation and gratefulness to Dr. M. Lois Calhoun, Profes-
sor and Chairman of the Department of Anatomy, for her
untiring guidance, supervision, encouragement and valuable
advice during the course of this investigation and the
preparation of the manuscript. A sincere note of grateful-
ness is extended to Dr. Roger Brown for his help in procur-
ing the specimens and to Dr. Carles Titkemeyer for his
timely advice and help. The author is greatly indebted to
Dr. Al. w. Stinson for his suggestions and guidance in the
technique of photography and preparation of the manuscript
and to Dr. Esther Roege for very useful help in microtechnique.

The entire faculty and staff of the Department of
Anatomy has been most co-operative and helpful during the
course of this investigation. Special note of thanks to
Dr. Arlene Seaman for supplying many valuable references
relating to the subject. A note of thanks to the members
of the graduate committee for their co-Operation and useful
criticism in the course of this investigation. Special
thanks are due to those who helped in typing. The author
is grateful to his wife, Shaida, for her continuous encourage-
ment and for taking the long and lone responsibility of rear-
ing the children.

With great pleasure the author acknowledges the financial
support of the government of the United States (AID) and the

East Pakistan Agricultural University, Mymensingh.
iii

TABLE OF CONTENTS

INTRODUCTION 0 O O O O O O 0 0 O 0
REVIEW OF LITERATURE . . . . . . .
General feature . . . . . . .

Epidem180000000000

Stratum germinativum . .
Stratum cylindricum
Stratum spinosum. .

Stratum granulosum . . .

Stratum lucidum. . . . .

Stratum corneum. . . . .

The pigment cells of the

Skin.

Origin of melanocytes . . . .

Location of melanocytes in the skin

The fate of the melanocytes .

Pigment and pigmentation o o o o o

Keratin and keratinization . .

Histochemistry of epidermis. .

Glycogen. e o o o 0

Enzyme activities in the epidermis.

Cutaneous innervation . . . .

Nerve supply to the hair follicles

The organized nerve endings. .

Dermis or corium. . . . . . .

The cellular components of dermis.

The fibrous components of the dermis

iv

0

an C: va

\O\OCD\}0\0\4:‘

10

12
14
15
18
23
23
25
26
29
33
35
37
u2

TABLE OF CONTENTS (continued)

The collagen fibers . . .

Elastic fibers. . . .’. .

Reticular fibers. . . . .

The ground substance . . . . .

The basement membrane . . . . . . .
Dermo-epidermal junction. . . . . .
Blood supply. . . . . . . . . . . .
Arterio-venous anastomoses . .

Thehair..............

Arrangement of hairs . . . . .

Development of hair and hair follicles.

Replacement and regeneration of hairs.

The histologic structure of the hairs.

The structure of the follicle . . .
The inner root sheath. . . . .
The outer root sheath. . . . .
The connective tissue sheath .
Follicular folds . . . . . . .
Tactile hair. . . . . . . . . . . .
Arrector pili muscle. . . . . . . .
Histochemistry of hair follicle . .
Glycogen in the hair follicle.

The sebaceous glands. . . . . . . .

Histochemistry of sebaceous glands

,Development of sebaceous glands. .

TABLE OF CONTENTS (continued)

Page

The sweat glands . . . . . . . . . . . . . . . . 73
The apocrine glands . . . . . . . . . . . . 73
Thernerocrine or eccrine glands . . . . . . 7h
Histologic feature. . . . . . . . . . . . . 7h
Apocrine glands. . . . . . . . . . . . 74
Composition of apocrine secretion. . . 75

Mode of secretion. : . . . . . . . . . 76

Merocrine or eccrine glands. . . . . . 77

Nerve supply . . . . . . . . . . . . . . . . . . 78

MATERIALS AND METHODS . . . . . . . . . . . . . . . . 80
SKIN AREAS AND SKETCH . . . . . . . . . . . . . . . . 82
RESULT AND DISCUSSION . . . . . . . . . . . . . . . . 8h
General feature . . . . . . . . . . . . . . . . 84

Skin thickness . . . . . . . . . . . . . . 84
Components of skin . . . . . . . . . . . . . . . 85

BlOOdSllpply00000000000000. 85

Arterio-venous anastomoses . . . . . . 86

Hair . . . . . . . . . . . . . . . . . . . . . . 8?
Classification, density and arrangement . . 87
Structures of hair. . . . . . . . . . . . . 88

Follicular folds . . . . . . . . . . . 89
Tactile hairs . . . . . . . . . . . . . . . 89
Arrector pili muscle. . . . . . . . . . . . 90
Histochemistry of hair follicles. . . . . . 91

EPIdBHfliS-o0.0000000000000000 92

vi

& 83

TABLE OF CONTENTS (continued)
Stratum corneum . . . . . .
Stratum granulosum . . . .
Stratum germinativum. . . .
Stratum cylindricum. .
Stratum spinosum . . .

Pigmentatlon. o e o o o o o

Keratinizationéccrnifieaticn)

Dermis (Corium) . . . . . . . .
. Stratum papillare . . . . .
Stratum reticulare. . . . .
Cellular components . . . .
Fibroblasts . . . . .

Mast cells . . . . . .
Macrophages. . . . . .

Dermal chromatOphores.

:ibrous components . . . .
Collagenous fibers. .

Elastic fibers . ',’ .

Reticular fibers . .-

The basement membrane. . . . . .
Dermo-epidermal junction . . . .
The sebaceous glands . . . . . .
Distribution and morphology
Histologic structure. . . .

vii

Page

93
9a
95
95
96
96
97
98
99
100
101
101
101
102
102
102
102
103
10a
105
105
106
106
107

TABLE OF CONTENTS (continued)

The sweat glands .

Distribution and morphology

Histologic structure. . . .

SPECIAL AREAS . . . .

Croup and adjacent areas . .

Externalear........

Circumanal region. . .

The dorsal neck and the tail

Eye lids e o o o

Chestnut and ergot

Coronary border.
Prepuce. . . . .
Nostrils . . . .
The lips . . . .
Upper lip .
Lower lip .

Nerve supply of the

Innervation of

lips. e o o 0

hair follicle

Organized nerve endings.

Lamellated endings . . .

Capsulated end organs.

Free nerve endings . .

Non-capsulated ball.

TABLES. O O O O O O O
PLATES. O O O O O O O

Page
108
108
109
112
112
113
11a
115
116
117
118
118
119
120
120
121
121
123
124
125
125
126
126
127 - 13“
135 - 182

TABLE OF CONTENTS (continued)

SUMMARY AND CONCLUSIONS . . .
BIBLIOGRAPHY. . o . . . . . .

ix

I.

II.
III.
IV.

V.

LIST OF TABLES

Page

Average measurement of total skin thickness in

mm o o o e e o e o o o o e o o e o e o o o e o o 127
Average measurement of epidermis in microns. . . 129
Average measurement of dermis in mm. . . . . . . 131

Dermal thickness of croup in mm and its comparison
With adjacent Skin areas 0 o o o o o e o o o o o 133

Sex differences in skin thickness in mm. . . . . 134

I.
[1.
111.
IV.

V.
VI.

VII.
VIII.
IX.
X.

XI.

XII.

XIII.

XIV.
XV.

XVI.

LIST OF PLATES
Page
General skin areas, vertical section . . . 135
Lateral surface of thigh, horizontal section 136
Skin of the abdomen, vertical section . . . 13?
Skin of the abdomen, vertical section . . . 138

Dermo-epidermal junction, nostril, vertical
SCOtLOH e o o o o o e o e o o o o e e e o o 139

Skin of the nostril showing melanocytes
and their processes, vertical section . . . 140

Skin of the nostril, vertical section . . . 141
The chestnut, vertical section. . . . . . . 142

Skin of the region of the croup, vertical
section 0 o e o o e o e o o o e o o o e e o 143

Reticular layer of the croup, showing the
different arrangements of collagen fibers . 1&4

Upper reticular layer of the croup showing
the collagen bundles arrangement and the
detail of individual fiber. . . . . . . . . lfij

The chin (level 15), showing the attachment

of individual skeletal muscle fibers with

the the superficial dermis by a strong bundle

0f elaStic fibers 0 o e e e o e o e o o o o 146

Horizontal section of general skin, showing
the arrangement of the elastic fibers in
papillary layer and attachment with hair
fOllLCleo o o e e o o o o o o o . e e e o o 147

I

Upper lip, showing reticular fibers . . . . 1&8
Skin of the croup, showing the branched
arrector pili muscle and its attachment with
epideTMIS by elaStic fibers 0 o o o o o o e 149

Skin of the back, showing different sizes
of hair follicles, horizontal section . . . 150

xi

LIST OF PLATES (continued)
Page

XVII. Skin of the abdomen, showing thin hair
distribution, horizontal section . . . . . . 151

XVIII. Hair and hair follicle, vertical section . . 152

XIX. Hair follicle, showing follicular folds,
vertical section 0 o o o e o e e o o o o o o 153

XX. Hair follicle, showing the strong reaction
for glycogen in the cytOplasm of the cells of
external root sheath, vertical section . . . 154
XXI. Showing the comparison of the PAS reaction
and the Bauer-Feulgen Reaction for glycogen in
the cells of the external root sheath of hair
fOIILCle e o e o e o e e o o o o o o o e o e 155

XXII. Hair follicle (submandibular region). Open-
ing of 3 sweat glands in one follicle. . . . 156

XXIII. Apocrine sweat gland (circumanal region),
cross section. 0 o o e e o o e o o o o e o o 157

XXIV. Sweat gland, general body region . . . . . . 158

XXV. Skin of the upper lip, vertical section. . . 159
XXVI. Skin of the lower lip, vertical section. . . 160
XXVII. Skin of the nostril, vertical section. . . . 161

XXVIII. Skin of the nostril, showing the arrangement
of the skeletal muscle in the upper dermis . 162

XXIX. Skin of the external ear, vertical section . 163

XXX. Skin of the circumanal region, vertical
SGCtione o c o o o e e o o o o e o o o o o o 164

XXXI. Skin of the prepuce, showing branched elastic
fibers attached to the basement membrane,
vertical seCtlon e o e e e e o o o o o o o o 165
XXXII. Chestnut (hind limb), vertical section . . . 166
XXXIII. Coronary border, vertical section. . . . . . 167

XXXIV. Cross section of an arterio-venous
anastomoses, upper lip . . . . . . . . . . . 168

XXXV. Longitudinal section of an arterio-venous
anastomoses in the skin of croup . . . . . . 169

xii

LIST OF PLATES (continued)
Page
XXXVI. Glomus formation, lower lip. . . . . . . . 170

'XXXVII. Cross section of an artery containing
arterial cushion, upper lip. . . . . . . . 171

XXXVIII. Arterial cushion cut in cross section,
showing arteriole leaving the main artery
from the raised central part of cushion,
thigh, lateral surface . . o o o e o o o o 172

XXXIX. Skin of the upper lip. Plexus of nerve
fibers, vertical section . . . . . . . . . 173

XL. Paired capsulated nerve ending, upper lip. 174
XLI. Thin capsulated nerve ending, upper lip. . 175

XLII. Disk-like capsulated nerve ending, upper
lip. o o o e o o o o o o o o e e o e o e o 176

XLIII. Innervation of the sinus hair, upper lip 177

XLIV. Nerve ending along hair follicle, upper
lip e o o o o e o o e o o o o e e o o o o 178

XLV. Highly organized capsulated nerve ending,
upper lip. o o o o o o o o o o o o o o o o 179

, XLVI. Lamellated nerve endings, upper lip ,
cross and longitudinal section . . . . . . 180

XLVII. Innervation of the part of hair follicle,
upper lip e o o o e e e e e o o o o o o o 181

XLVIII. Skin of the nostril, vertical section. . . 182

xiii

INTRODUCTION

There has been considerable progress in the study of
the histology and histochemistry of the skin of both man
and laboratory animals during the last few decades. with
the exception of sheep, very little attention was paid to
the histology and histochemistry of the domestic animals.

A better understanding of the function of the skin of man
resulted in a remarkable improvement in the diagnosis of
dermatological problems. Since dermatology is a very impor-
tant branch of veterinary medicine, the Department of Anatomy
of Michigan State University has taken the lead to investi-
gate the integumentary system of domestic animals.

rhe histologic study of the skin of the horse is the
latest in the series of studies on the skin of domestic
animals. This department has already completed research on
the skin of the mongrel dog, (Webb and Calhoun, 195M); cattle,
(Goldsberry and Calhoun, 1959 and Sinha, 196M); eye adnexa of
sheep and goat, (Sinha 1965); cat, (Strickland and Calhoun
1960 and 1963); swine, (Smith and Calhoun 196k, Fowler and
Calhoun, l96h, and Marcarian and Calhoun, 1966); rat, (Holmes
1960); Goat (Sar and Calhoun 1966); and sheep, (Kozlowski
1966).

The skin of the horse, in particular, has received so
little attention that no comprehensive histologic studies
are available. Smith (1888) investigated the skin of the
horse more thoroughly than any one else during that time.

Since then, investigation on horse skin has been limited to

l

2
isolated areas only; for example on the sweat glands (Evans
g£;§l. l957,'Eakagi and Tagawa, 1959, 1961 and Aoki 22 a1.
1959); on the group region (Schgnberg 1926 and Odoni, 1951)
and some contribution on the skin in general (Szelingrowski,
1962).

The horse has had the distinction of becoming the con-
stant companion of human beings through the course of history
and civilization. It is one of the first animals to have
been domesticated. Although most of the work of the horse
has been replaced by modern mechanization, the importance of
the horse in Sports and recreation continues at a high level.
The horse industry, even in this most mechanized country, is
valued at 33.5 billion (Sandifer, 1966). It is hOped, there-
fore, that this investigation will be very helpful for a
prOper diagnosis and treatment of the diseases of the skin of

the horse, an old companion of human beings.

REVIEW OF LITERATURE

An extensive search for literature related to the skin
of the horse was most discouraging as insufficient amount of
work has been undertaken on the skin of the horse in particu-

131‘.

general Feature

The skin consists of (a) epidermis--epithelial layer,
(b) dermis--connective tissue layer, (0) subcutis--loose
connective tissue layer with panniculus adiposus.

Smith (1888) categorized the horse into two groups, race
horses and cart horses and observed thin delicate skin with
short fine hair in race horses. He observed that the skin
of the horse differs in appearance depending upon its posi-
tion. Around the lip it is closely attached to the muscular
structures below and the whole lip is felt as a muscular
mass. Behind the angle of the mouth, the skin is exceedingly
thin and supple and is nearly the thinnest of the body;
over the head and cheek it is thin and never has fat beneath
that part covering the external masseter muscle. On the
side of the neck the skin is still thin, but at areas where
the mane grows it is much thicker and more firmly attached
below. Over the back and loins the thickest skin is found.
It is also thick down the quarter but inside the thigh and up
to the groin is the finest skin covering the body. The skin
over the limb is of medium thickness. On removing pigmented

hairs from the skin, the integument is black but in the
3

u
area of white hair it is pink.

. According to Ellenberger (1906) and Krglling and Grau
(1960), the skin of the horse is 1-5 mm in thickness and
differs according to individual, body region, age, sex and
race. Ellenberger (1906) found the thickest skin on the tail
and around the penis. The skin of the horse is thinner than
that of cattle but thicker than that of other domestic
animals. Trautmann and Fiebiger (1957) stated the skin is
very thick on the tail. Sisson and Grossman (1953) stated
that the greatest thickness of the skin occurs at the attach-
ment of the mane and on the dorsal surface of the tail.

The dermis contains smooth muscle associated with hair
follicles. Smooth muscle fibers are found in the dermis of
the scrotum, and penis and in the nipple and areola of the
breast of man (Montagna, 1962). All authors agree that
integumentary appendages such as hairs, horns, claws, nails,
hoofs and cutaneous glands grow directly from the epidermis

and are an integral part of the skin.

Epidermis

The epidermis covers the entire outer surface of the

A

body and consists of stratified squamous epithelial cells.
According to the position of these cells in the epidermis,
they may be cuboidal, columnar, fusiform, or polyhedral.
Scattered between the lower cells of the basal layers are
the dendritic melanocytes, the cytOplasm of which produces

melanin pigment (Montagna, 1962)-

5

The preperties of the epidermis shows tepographic differ-
ences. In the palms and sole of man, the thickest outer
dead layer is compact, but in the epidermis of the general
body surface the dead outer layer is flaky (Montagna, 1962).
The epidermis is nonvascular and presents Openings of cuta-
neous glands and hair follicles. The epidermis is character-
ized by a high degree of elasticity. Its free surface may
appear smooth or show elevations caused by the underlying
papillae (Trautmann and Fiebiger, 1957). On the nose and
foot pad of the dog, the epidermis forms independent eleva-
tion (Trautmann and Fiebiger, 1957). In cat and mongrel dog,
microscopic demal papillae are covered by thickened epider-
mis (Strickland and Calhoun, 1963, and Webb and Calhoun,
195“). According to Ellenberger (1906), the epidermis of the
horse is 15-80 u thick. The epidermis is thick on the fore-
head and on the borders of the natural Openings. Smith (1888)
observed in the horse that the epidermal elevations and
depressions are extreme on the lip, straight on the mane and
tail, and wavy on the general body surfaces. Szeligowski
(1962) found the epidermis of horse skin comparatively thin.
Strickland and Calhoun (1963), and Lovell and Getty (1957)
found in cat and dog respectively the characteristic small
hairless, knob-like projections of the skin, designated as
integumentary papillae.

Electron microscOpic observation revealed that the mito-
chondria, endOplasmic reticulum, and the submicroscOpic cyto-

plasmic particulates of the epidermal cells gradually decrease

6

in quantity from the base of the epidermis to the stratum

lucidum (Selby, 1957).
On the basis of histologic organization of the cells,

I

the epidermis is generally divided from within outwards into
stratum germinativum, stratum granulosum, stratum lucidum
and stratum corneum.

Stratum germinativum: This layer is also known as
stratum malpighii. On the basis of the cellular differentia-

tion, this layer is divided into stratum cylindricum end
stratum spinosum.

(a) Stratum czlindricumx This is a layer of columnar
cells of varying height (Trautmann and Fiebiger, 1957),

resting on the basement membrane (Copenhaver, 196“). The
contour of the basal surface is undulating due to irregular
tepography of the epidermal ridges (Odland, 1966). Each
basal cell is provided with several cytoplasmic processes
which extend a short distance into the dermis (Odland, 1966
and flu and ;a:de11, 1962). COpenhaver (196“) believed that
these processes anchor the cells to underlying connective
tissue. According to Trautmann and Fiebiger (1957). they
penetrate the basement membrane. The nucleus of these cells
is oblong and the cytoplasm has a fibrous appearance due to
the presence of a large number of tonofilaments. Tonofila-
ments organize to form tonofibrillae (Hu and Cardell, 1962,
«and Selby, 1957). The cell membrane between the basal cells
1.8 folded considerably, giving the impression that the cells

Eire compressed together (Hu and Cardell, 1962, and COpenhaver,

7

196“). The cells of this layer possess abundant RNA (Odland,
1966). The cells of this layer show mitotic frequency.
(Trautmann.& Fiebiger, 1957).~

(b) Stratum spinosum: This layer, just above the
stratum cylindricum, consists of varying numbers of poly-
hedral cells. The cells of the upper part tend to be flat-
tened. The cells within the stratum germinativum are grad-
ually changing their shape and cytOplasmic constituents
through cellular differentiation. As the cells migrate
upward, there is dimunition of cytOplasmic baSOphilia (Odland,
1966). The plasma membrane of the cells of the stratum
germinativum in general are in apposition throughout most
of their extent, but in the process of tissue preparations,
they shrink and remain attached only at the point of the
desomosome (Copenhaver, 196“). The connections to the adja-
cent cells are known as desmosomes. Electron microscOpy shows
that the point of apposition does not represent bridges of
protoplasmic continuity (Cepenhaver, 196“). Montagna (1962)
stated that thickening of the plasma membrane at the point
of contact in the desmosome constitutes two attachment
plaques, one from each epidermal cell, about 750 9A apart.
According to Selby (1957), tonofilaments are, for the most
part, organized into tonofibrillae in the malpighian layer
and appear to be equally plentiful in basal cells as in the
cells of more superficial layers. The tonofilaments are
attached at both ends to the cell membrane. At these points

or attachment the desmosomes adhere so that the tonofilaments

8
of the whole epidermis are linked together. (Charles and
Smiddy, 1957). The tonofilaments do not pass through the
cell membrane (Hu and Cardell, 1962).

The mitosis may occur in the cells of the spinous layer
(Trautmann and Fiebiger, 1957). Mitotic frequency is greatest
during sleep (Odland, 1966; Trautmann and Fiebiger, 1957).
According to Bloom and Fawcett (1962), mitotic figures in the
Malpighian layer correspond with the intensity of desquama-
tion in a region. This was also reported by Thuringer (1928).
Variation of mitotic rate in the epidermis of rat was discussed
by Bertalanffy, 33 a1. (1965) during the growth, maturity,
senility, and regeneration.

Stratum granulosum: This layer consists of 2-5 rows
of flattened rhombic shaped cells with their long axis par-
allel to the surface of the skin. The cytOplasm contains
numerous keratohyalin granules (Copenhaver, 196“). Electron
microscopic examination has shown that the keratohyalin
granule consists of a dense substance deposited about the
pre-existing tonofilaments. The keratohyalin granules are
actively synthesized in these cells (Odland, 1966). The
tonofibrillae in some cases appear lacking in filamentous
structure which is proposed as the macromolecular rearrange-
ment (Montagna, 1962; Selby, 1957). When keratohyalin
{granules attain their'maximum size, the cells of the stratum
granulosum. lose their nuclei and their organelles. The cells
-loee much of their water content and become flattened, emerg-
idmg as fully keratinized cells in the stratum corneum (Odland,
1966).

9

Stratum lucidum: This is a shiny acidophilic layer of
homogenous appearance (Trautmann and Fiebiger, 1957). in
which cellular outlines are indistinct and nuclei are rarely
perceptible (Odland, 1966). The cells of this layer are more
firmly attached to each other and break when an attempt is
made to separate them (Chamber and Benji, 1925). Cepenhaver
(196“), and Trautmann and Fiebiger (1957) stated that the cells
of this layer are replaced by a substance known as eleidin.
According to Odland (1966), the stratum lucidum is probably
a reflection of high order of organization of keratin in the
lowermost lamellae of the stratum corneum. The ultrastructure
of the fibrils in this layer is altered but the hyalin
fibrils terminate at the desmosomes, as do those in the cells
of lower layers (Montague, 1962). This layer does not occurv
in thin skin and is found only in the thick skin of the human
species and some other mammals.

Stratum corneum: This layer consists of anucleate,
broad, flat, scaly cells whose edges interdigitate with the
edges of the adjacent cells (Trautmann and Fiebiger, 1957;
Odland, 1966; and many others) which are constantly des-
quamated. The cells have definite horny membrane and are
closely packed (Cepenhaver, 196“) so that no desmosomes are
present. The morphological changes occurring within the
desmosomes are related to the eventual loss of cohesion
(Selby, 1957). According to Keddie and Saki (1965), the
normal surface of the horny layer is a more or less contin-
uous sheet of intact cells. Parting of the cells in the

usual process of desquamation is by the separation of a single

10

cell or a block of cells due to the lack of lateral adherance
between the cell membrane, rather than disintegration of the
containing wall of the individual cells. The cells of this
layer are birefringent, a characteristic associated with the
presence of the fibrous protein (Odland, 1966). Electron
microscopic analysis revealed that the cyt0plasm of these
anucleate cells consists entirely of a highly organized mass
of filaments about 70 to 90 %h enclosed in an amorphous

dense matrix (Odland, 1966).

The Pigment Cells of the Skin

Melanocytes, the dendritic cells of the adult skin, are
the main source of skin pigments.

Origin of melanocytes: The melanocytes, according to
Leydig (1876), Ehrmann (1885), Aeby (1885), and Babel (1896),
are of mesodermal origin. They also thought, on circumstan-
tial evidence, that the macrOphages of the dermis phagocytose
the red blood cells and other cell debris in the dermis and
then migrate to the epidermis. Here they discharged their
contents as the excretory products of the dermis which, in
turn, along with the epidermal cells, are pushed to the
cornified layer and finally expelled, from the body. Block
(1921) held the view that the melanocytes are of ectodermal
origin and their fate is similar to that of the epidermal
cells.

The most convincing and modern theory of the neural,
crest origin was possible to postulate after the d0pa

technique of Bloch (1927) was established. By this technique,

11

the site of melanogenesis was established and also the fact
that the melanocytes were the sole producer of melanin pig-
ments was proved. Shane (1939), by tranSplanting neural
crest cells from white amphibians to black and from black to
white, observed precisely the differentiation of the neural
crest to the pigment cells. Similar experiments with tissue
of the rat, mouse and chicken conducted by Willier (1953),
Bowles (19“0, l9“7, 1953), HOpkins (19“9), Hu 32 31. (1957),
Shane (l9““), Weissenfels (1956), and many others, both
in gitgg_and in 21123 and placed the neural crest origin
theory of melanocytes on a sound foundation. Bowles (19“0,
19“7, 1953) undertook perhaps the most convincing and elab-
orate eXperiments. She was not only able to show the neural
crest origin of melanocytes which gradually migrated to the
epidermis for functioning as adult cells but also proved that
the immature melanoblasts migrate dorsoventrally. Their
migration starts in the mouse embryo of 8-10 somites at the
mesencephalic region and gradually Spreads towards the caudal
region. She found that the melanoblasts reach all the body
regions of mouse embryo by the 12th day of gestation.
Zimmermann and Becker (1959) studied the phenomenon of migra-
tion of the melanoblasts and their location in human fetuses
of different age groups and reached the conclusion that the
melanoblasts are of neural crest origin. They observed that
the migration in human fetuses starts at about 10 weeks of
fetal life and reaches the normal distribution at 1“ weeks.

Okun (1965) postulated a theory through biochemical,

histochemical and ultrastructural study that the melanocytes

 

 

iii

66

‘II‘

\A.U

12
and mast cells are of the same group and the melanocytes
migrated from the dermis to the epidermis while the mast
cells remained in the dermis. According to him, the entire
dermis originated from the neural crest and thus the mast
cells of the dermis and the melanocytes are of neural crest
origin. In summarizing the neural crest origin theory, it
postulates that when the two neural folds finally come
together and fuse to form the neural tube, some of the cells
at the two fusing edges break loose and move down between
the tube and the overlying ectoderm and thus become separated
from the ectoderm. These cells eventually form the pigmented
cells, ganglion cells, and schwam cells. The pigmented cell
precursors gradually migrate to the permanent sites and start
functioning (Waddington, 1956).

Location of melanocytes in the skin; In man melanocytes
usually appear in the dermis at the age of 10 weeks of fetal
life (Zimmermann and Becker, 1955, 1959, and Breathnach, 1963
and Breathnach g§_al, 1963). At birth, all the melanocytes
are functioning at the Junction of the layer of pigmented
basal epithelial cells and not infrequently abutt slightly
into the dermis Just below this general level. Each melano-
cyte perikaryon, in most instances, is slightly elongated
horizontally and gives rise to a variable number of primary
branches, usually “-5. Billingham and Silvers (1960)
observed that the branches diverge repeatedly, subdividing
and becoming attenuated as they wave horizontally and upward

between the Malpighian cells which separate them. In man

13
many extend as far as 100 u from the parent cell body. Each
branch ends in a sort of swelling or button which is strongly
d0pa positive and intimately applied to the terminal pole of
a Malpighian cell. The distribution of melanocytes and their
mode of branching is normally such that scarcely a basal
layer of cells is without contact with one of their end cups.
Billingham and Silvers (1960) also stated that they.are most
densely concentrated at the summits and sides of the epidermal
ridges. Bloch (1921, 1927), Odland (1966), and Krglling and
Grau (1960) also stated, in general, the same principle of
the location and branching of melanocytes.

During the deve10pment of the animal, while the basal
cells of the Malpighian layer invaginate in the formation of
the hair follicle, some of the melanoblasts, still undifferen-
tiated, are included in the original/follicular invagination
(Chase, 23.51. 1951). These cells are the forerunner of the
melanocytes of the bulb. Berry (1953) observed many branched
pigment cells in the dermis of the scalp of newborn which
are often associated with blood vessels. Zimmermann and
Becker (1959) also observed similar dermal melanocytes in the
dermis of newborn fetuses. The dermal melanocytes are
common in the apes and monkeys (E1 Bahrawa, 1922; and Adachi,
1903). There are a large number of amelanotic melanocytes
present in the outer roots of hair follicles at the middle
and lower part. According to Renato (1963), these cells are
normally non-functional and they become functional if for

some reason the epidermal melanocytes are lost. Melanogenic

1“
cells identical to dendritic cells were observed by Chase,
g£,§1. (1951) in mouse skin on the surface of sebacious
glands. He also stated that the dermal melanocytes are
present in mice. Danneel and Cleffmann (195“) showed that
the various species of rodents have the same pigmentary
condition in the dermis as that of monkeys.

The fate of the melanocytes: The melanocytes first
appear in the dermis at the third fetal month as immature
melanocytes, the melanoblasts, and contain premelanin
(Zimmermann and Becker, 1959). The melanoblasts settle in
the epidermis and hair follicles at 5 fetal months in human
fetuses and gradually mature to produce melanin pigments.
From this time on throughout life they are invariably seen
in the basal layer. The cells, or their replacement, func-
tion as melanin producers. Much evidence was available that
they are replaced through regular mitotic and amitotic cell
division (Billingham and Silver 1960, Masson l9“8 and many
others). According to these investigators, relatively
undifferentiated melanocytes in the epidermis are capable of
cell division. One of the daughter cells becomes a melano-
cyte and the other is pushed to the suprabasal layer where
it also remains as a highly branched ceIl with clear cyto-
plasm, never produces melanin granules and contains a
characteristic granule (Billingham and Medawar, 1953: Willier,
1953; and Masson, l9“8). These non-melanin producing cells
are known as Langerhans' cells. The cells that are present
in the basal layer also appear clear in hematoxylin and easin

sections, but they are dopa positive. 0n the contrary the

15

cells of Langerhans in the suprabasal layer may show some
mature pigments (melanin) in their cytoplasm, but are never
dopa»positive (Billingham and Medawar, 1953) and contain a
characteristic cytoplasmic granule revealed by gold chloride.

The suprabasal dendritic clear cells occur up to the
stratum granulosum in the epidermis and are considered by
many to be the cell division product of the melanocytes,
which have been pushed up in the process of degeneration
(Billingham gt_§1. 1960). Zelickson (1965) observed that
the Langerhans' cells have characteristic granules which are
different from the melanin granules. It was thought that
both the endOplasmic reticulum and the Golgi complex play a
major role in melanogenesis. Alteration in either or both
of these organelles might account for the relatively dif-
ferent structure of the granules and that the Langerhans'
cells are the transitional cells. The Langerhans' cells
become effete dendritic cells. The effete form of the cells
is thought to be related to lysosomes which have been
associated with hydrolytic enzymes of the cells. When the
lysosomes membranes rupture, release of the enzyme lysin

occurs.

Pigments and Pigmentation
Edward and Duntly (1939) found five pigments and in

addition an Optical effect which are responsible for the color
of normal skin. The pigments are; melanin in the deeper
layer of epidermis; an allied diffused substance, melanoid,

present throughout the whole epidermis; carotine found in

16
the stratum corneum as well as in the fat of the dermis and
subcutaneous tissue, and reduced and oxyhemoglobins found in
the vessels of the dermis and subcutaneous tissue.

The colored races owe their characteristic colors only
to the variations in amount of melanin and melanoid. General
plan of the pigment distribution is identical in the two
groups. Therefore, the color of the skin is attributed to
the functional activity of the melanocytes. Melanocytes are
stimulated in the presence of sunlight. When an individual
is exposed to sun continuously for a few days, melanin produc-
tion is stimulated; this continues even after a few days of
its withdrawal. The melanin is considered to protect the
underlying soft tissue against the injurious effect of the
ultraviolet ray of the sun.

Hyperpigmentation of the skin is also associated with
the action of pituitary hormones, particularly Melanin
stimulating hormone (MSH), and adrenal insufficiency as in
Addison's disease (the lack of adrenocortical hormones which
normally inhibit the release of MSH from the pituitary).
Besides, noradrenalin, adrenalin, hydrocortisone, progesterone,
estrogen, androgens and thyroin have all been cited as having
some effect on pigmentation (Billingham and Silvers, 1960).
In pregnancy, for example, hyperpigmentation is observed
accompanied by increased excretion of MSH. Hyperpigmentation
appears in women at the time of menstruation (McGuire and
Lerner, 1963). Copenhaver (196“) stated that certain patches
of human skin are specially rich in pigment, for example,

circumanal region, the areolae, nipples, the axilla, labia

1?

majora, the penis and the scrotum. The pigment is stored as
fine granules within the cells of the germinative layer. The
pigments have the tendency to spread from the dark skin to
the white skin area as observed by Billingham and Silvers
(1960). According to Chauveau (1873) the deep layer of the
skin of the horse is composed of soft nucleated pigmented
cells. Ellenberger (1906) observed that the skin of the
udder of the horse is highly pigmented. Trautmann and
Fiebiger (1957) stated the epidermis of domestic animals,
except albinos, bear pigment either diffusely or in patches.
The pigment granules are present in highest concentration in
the stratum cylindricum. They gradually decrease in the more
superficial layers of the epidermis. Melanoma, a pathologic
condition which occurs in gray and white horses, was mentioned
oy Chauveau (1873) and described in detail by Hadwens (1931).

On histologic study, it is’founfl that the pigments are
usually in the processes of the melanocytes or in the cyto-
plasm of the Malpighian cells. They form a characteristic
supranucleer cap in the Malpighian cell layers (Odland, 1966;
and Krglling and Grau, 1960). The melanocytes themselves
remain relatively clear though occasionally some pigment may
be found in the cytoplasm. It is generally assumed that
tyrosine, indirectly, and 3,“-dihydroxyphenylalanine, directly,
are the precursors of the melanin pigments (Calvary, gg’al.
l9“6). According to Copenhaver (196“), the production of
melanin pigments is due to the activity of the enzyme

tyrosinase, which appears as a polypeptide. On being activated

18
by ribosomes they are transferred to the Golgi complex,
where they are condensed and packed into units surrounded
by membranes. These protyrosinase molecules are arranged
in an orderly manner as the membrane bound lamellar units.
The units of this stage are known as premelanosomes. When
protyrosinase becomes activated as tyrosinase (perhaps with
tyrosine), melanin biosynthesis begins and the unit which
now contains melanin in addition to tyrosinase is known as
a melanosome. The melanin pigments continue to increase in
the melanosome complex until the latter is transformed into
amorphous melanin granules which do not contain tyrosinase.
This differentiation of melanosomes is accompanied by a
change in their intracellular position. The premelanosomes
appear in the Golgi complex, the melanosomes in the basal
portion of the processes and melanin granules chiefly in the

peripheral portion of the processes.

Keratin and Keratinization

 

Keratin is a modified protein and forms the cornified
layer of the epidermis--horn, claws, hair, hoofs, chestnut
and nails. In the process of keratinization, the epidermal
cells undergo a drastic transformation to form the horny
tissue consisting of dead cells. The keratin molecule is
believed to consist of closely packed polypeptide chains
which are held together by the disulfide bond of cystine;
the resistance to solvents and enzymes being associated with

the close packing of the chains (Montagna, 1962). Thus, the

l9

horny tissue of the cornified cells is well established by
-S-S- bonds and some free -SH groups. The major part of
the hair, horn, hoof, feather, nail and stratum corneum of
the skin is made up of albuminoid proteins. The keratin of
hair, nails and other cutaneous appendages contains from 3
to 5% sulfur, while that of skin contains 1 to 3%, nearly
all of which is cystine (Hawk, gt g1. l9“7, cited by
Montagna). Calvery, gt.§1. (19“6) discussed the constituents
of keratin. They were of the opinion that the keratins are
fibrous structures possessing the phenomenon of elasticity
and regular intermolecular folding into grid-like structures
composed of polypeptide chains with -S-S- and possibly other
cross linkage in the polypeptide grid. According to Menefee
(1955) in the epidermis keratin is laid down in sheets, while
in the hair it is formed into fibrils. The basis for the dif-
ference is thought to be the difference in pressures brought
about by active growth of the two differently oriented
tissues. A

The keratin has been divided into soft and hard keratin.
The soft keratin covers the skin as a whole. The formation
of soft keratin is characterized by the epidermal cells that
are becoming keratinized, accumulating keratohyalin granules
in their cytoplasm. Hence, an area where soft keratin
occurs, shows a stratum granulosum and the cornified cells
of which it is composed, continuously desquamate from its
surface. Hard keratin constitutes the nails, the cuticle,

the cortex of the hair, feather, claws, the hooves, the horns,

20
etc. In this there is a gradual transformation from living
epidermal cells into keratin. Hard keratin is solid and does
not desquamate (Ham, 1961).

Keratinization is the process through which the cyto-
plasmic constituents are transformed into keratin, a horny
albuminoid protein. The keratinization was divided into
three forms by Matcltsy (1962); (l) Keratinization through
the formation of amorphous cytoplasmic granules. (2) Through
the production of cytoplasmic fibrils-~hoof, hair, etc.

(3) Both fibrils and granules. The keratohyalin granules
which are present in the cells of the stratum Spinosum are
considered to be the precursor of keratin. Meirowsky and
Behr (19“8) stated that keratohyalin is not the by-product

of keratinization but a prokeratin. The nucleus, cytOplasm
and intercellular substances all take part in the process

of keratinization. Then the nucleus passes through various
stages of metamorphosis. According to Bothman (195“), the
process of keratinization is not only the transformation of
'cytOplasmic proteins into keratin fibers but also a complete
disintegration of the keratinizing cells. Bothman (195“)

and Selby (1957) pointed out that the tonofilaments are the
precursors of keratinization which themselves are fibrous in
structure and are the crystallization center of the keratin
formation. Keratinization starts very early in the periphery
of the cell. According to Matoltsy (1962), the epithelial
cells of the mammalian epidermis keratinize individually, and

the process runs its own course in each cell. The main

21
constituents of the horny component of cornified cells are
derived from cytoplasmic fibrils and keratohyalin granules.
The keratohyalin granules disintegrate at an advanced stage
of cell maturation and their material mixes with the fibrous
cell constituents. Keratohyalin granules contain no -SH
groups or -S-S- bonds. Matoltxy (1962) observed that the
keratohyalin granules in the cells are closely assOciated
with a fibrous cytoplasmic network. In the cells of the
upper granular layer some of the nuclei appear to have
degenerated, and the mitochondria occur in small quantity.
In the cells of the lower granular layer, the nuclei are
intact, mitochondria are abundant and the cytoplasmic partic-
ulates occur in large quantity. The keratohyalin granules
show no preference with regard to location. The granules
are from 1.5 to “.5 microns in the upper cells and .2 to
1.5 microns in lower cells. The granules are not separated
by the limiting membrane. Bern (195“) visualized keratiniza-
tion as the holocrine transformation of epithelial cells.
He observed abnormal transformation of epithelial cells
after estrogen administration and vitamin A deficiency
resulting from trauma. He thought that the local lack of
inactivation of vitamin A is responsible for keratinization.
In the process of transformation of the cells of the basal
layer in which cytoplasmic fibrils accumulate, the cell
division ceases, which means that the cell's synthetic
activities are altered from producing materials needed for

division to producing keratin precursors (Mercer, 1961).

22

Keratinization is not a degenerative phenomenon as a
consequence of poor nutrition, of dissociation, or other
deteriorating factors. Pullar (196“) found in tissue cultures
in_1i£gg that the skin cultured under strict laboratory condi-
tions undergoes keratinization with the production of histolog-
ically normal keratinized cells. The keratohyalin differs
chemically from keratin only due to higher content of cystine
in keratin. The increase in cystine content is associated
with the decrease in sulphydryl containing amino acids. The
eleidin granules that occur in the stratum lucidum are
thought to be the degenerative remnants of the nuclei and
other cell elements (Ludford, 192“). Calvery, 32 91. (19“6)
described eleidin as a fibrous protein and an intermediate
stage of the keratohyalin of the granular layer and of the
keratin in the horny layer. Stratum granulosum and stratum
lucidum are the transition layers between the acidic stratum
corneum and slightly alkaline stratum spinosum. According to
Trautmann and Fiebiger (1957), the keratohyalin granules are
replaced by a stainable diffuse substance called eleidin in
the stratum lucidum. They considered this process is not
essential to keratinization. Bothman (195“) stated that
the keratinizing cell loses most of its water. The water
content varies greatly, mainly because all horny structures,
being on the surface and hygroscopic, may take up moisture
from the atmosphere. He preposed that the dehydration might
require uncoiling of the polypeptide chains and the close

packing of the fibrillary crystallites. While the lipids in

23
the cells remain bound to protein as lipoprotein, they
appear separate in the horny structure. Nuclear decomposi-
tion was thought to be secondary to cytoplasmic keratiniza-
tion.

As to the protein metabolism of keratinization, Calvery,
g§,gl. (19“6) stated that the process differs from the pro-
tein metabolism of internal cells. There is selection of
polypeptides, high in cystine and tyrosine containing large
numbers of -S-S- bonds and there is straightening of the
coiled polypeptide chains of the cell proteins of lower
layers. The constant desquamation of the horny layer is a
gradual process in which few cells are shed at a time. The
gradual desquamation prevents the inward movement of material
which happens to contact the surface of the skin. The pro-
cess of cornification in the epidermis is constructed to
serve as the barrier to the inward transfer of materials.

The vital processes of the mammalian skin, whether these be
secretion, excretion, or self-regeneration, all illustrate
one principle, a process intended to exclude and expel

materials rather than absorb or allow penetration (Calvery,

g; 2;. l9“6).

Eistochemistry of the Epidermis
Glycogen; In contrast to the stratified squamous

epithelium of the mucous membrane, normal mammalian epidermis
contains a relatively small amount of glycogen demonstrable

with histochemical methods (Montagna, 1962). The glycogen is

2“

found in the cells of the upper stratum spinosum (Montagna,
gt g1, 1951). Glycogen-rich cells around the pilosebaceous
orifice and the sweat duct may be found. Montagna (1962)
believed that the demonstrable glycogen may be seen in the
epidermis where the normal rate of keratinization has been
slowed down or impaired.

Glycogen was first observed in the embryonic epidermis
by Bernard in 1859 (Bradfield, 1951). He observed only a
trace or an absence of glycogen in undamaged skin. On the
other hand, glycogen was abundant in the regenerating epider-
mis but not in the basal cell layers. The epidermis becomes
free of glycogen after the wound heals. In early human
embryos, the epidermis is rich in glchgen except in rudi-
ments of the developing hair follicles where it is absent
(Montagna 1962, Berlin and Pridvizhkin 1965). The amount
of glycogen in the epidermis diminishes as the fetuses
gradually become older, and at 6-7 months of fetal life
reaches a minimal concentration. The glycogen in the develop-
ing epidermis is rich in epidermal cells which are poor in
RNA. Berlin and Pridvizhkin (1965) Bernard (1859), (cited by
Montagna, 1962) found the fetal epidermis of pig, cat and
calf rich in glycogen. .Acccrding to Montagna, 33 a1. (1952
amil962), glycogen is seldom found in the areas of active
mitotic activity such as in the lower layers of the stratum
germinativum and in the hair bulb. Glchgen appears in the
form of fibrils in the epidermal cells and external sheath of

hair follicles which sweep from cell to cell along the

25
intercellular bridges (Montagna,zgg_§l. 1952 ). In general,
there is inverse relationship between the presence of glycogen
and the process of keratinization (Montagna, 1962). Accord-
ing to Bothman (195“), the most important sites of glycogen
in the skin are the keratinized zones of epidermis and hair.
The epidermis stores glycogen as long as the liver has not
taken up this function. With the develOpment of multiple
layers in the skin, the amount of glycogen gradually dimin-
ishes. The epidermis also contains PAS positive, monoglycogen
that is not hydrolyzed by saliva or diastase._ The amount of
PAS reactive materials may vary inversely to the amount of
keratin present in a given epithelium (Montagna, 1962).

In the epidermis of laboratory mammals there are peri-
nuclear lipid Spherules, demonstrable clearly with sudan
black. The distribution of the bodies corresponds to the
osmiOphilic elements in comparable tissues and the structural
pattern is similar to that of the Golgi elements in other
tissues (Montagna, 1950; and 1962; and Baker, l9“6).

Enzyme activities in the epidermig: With apprOpriate
histochemical methods, large numbers of active enzymes are
found in the epidermis of the mammalian Skin. The coverage
in detail of all enzymes is beyond the scepe of this paper.
However, some very important enzymes that have been demon-
strated will be summarized here.

Yoichiro (1965) demonstrated the localization of phos-
phorylase a, b and kinase in biOpsy specimen of human skin.
In the epidermis a weak to moderate activity of all three

enzymes was observed. Cholinesterase activity of the nerve

26
fibers in the epidermis and the hair’follicles was observed
in human skin by Montagna and Ellis (1957) and in the skin
of the dog, cat, rat, rabbit and guinea pig by Winkelmann
and Schmidt (1959). Montagna (1962) stated that the finding
of cholinesterase in the nerves investing the hair follicles,
which subserves the touch, is evidence of the presence of
cholinesterase in some sensory nerve fibers. In adult skin,
only the small, presumably unmyelinated terminal nerves,
have variable amounts of cholinesterase. The large, recogniz-
able myelinated nerves rarely show a reaction (Montagna,
1962). Succinic dehydrogenase, carbonic anhydrase monoamine
oxidase, phOSphorylase, dermOpeptidase, phOSphomidase (in
keratinizing cells) B-gluconidase and esterases, are the

important enzymes present in the epidermis (Montagna, 1962).

Cutaneous_;nnervation

Mammalian skin is the most elaborate sensory organ in
the body. It receives the peripheral ramifications of the
sensory fibers frOm the Spinal or the cranial nerves, accord-
ing to the body region. The nerves vary widely in numbers
in different parts of the skin (Sisson and Grossman, 1953).
Upon reaching their destination, the nerve fibers divide
distal to the node of Banvier into several twigs. They may
or may not be myelinated (Trautmann and Fiebiger, 1957).
The terminal fibers either end free in the epidermis and in
certain parts of the corium or form special microscOpic
corpuscles of several kinds (Sieson and Grossman, 1953).

Besides sensory innervation, the skin receives extensive

27
efferent supply from sympathetic branches of the autonomic
nervous system. The efferent innervation, unlike sensory
nerves, is limited to the sweat glands, arrector pilorum
muscles and the blood vessels. This enables the organism to
perform various physiological activities relating to the
adjustment with the environment.

Near the skin, the large nerve trunks form a nerve
plexus in the panniculus adiposus. Many fibers follow tor-
tuous patterns to the papillary body where they form a superfi-
cial cutaneous plexus. A third, non-myelinated sub-papillary
plexus 18 formed just beneath the dermo-epidermal junction
(Montagna, 1962). All nerves innervating the skin pass
through a cutaneous plexus and grow finer and finer (Weddell,
at. 2.1... 1955).

Smith (1888) observed large branches of nerves in
bundles of three or four in the deeper part of the horse's
lip. These nerves are wavy while passing through the skin
to the papillary region. In each dermal papilla there are
as many as three nerve 100ps. -

According to Wall (196“), there is a definite pattern
of the sensory fibers to the skin. The nerve fibers of the
neurons in the dorsal root ganglia and of the nucleus
gracilis of the medulla oblongata converge to their respec-
tive cells. The cells act as points of convergence of
fibers from different parts of the skin. Each cell sub-
serves a larger area of skin than one afferent fiber. The

fibers come together in the dermis where they form intricate

28

interconnected networks (Montagna, 1962; and Weddel gglgl.
1955), probably its principal sensory end organ (Winkelmann,
1960a, b). Each sensory nerve emerges from a wide variety
of morphologically different end organs (Montagna, 1962).

The terminal fibers of sensory nerves do not unite.
According to Cauna (1959) in simple receptors they end in
free endings, filaments or terminal condensations, with or
without ramification. In complex receptors, free endings are
uncommon. Instead the nerve fibers increase their receptive
surface by ramification into a neurofibrillar apparatus in
the form of loose bundles, networks or end bulbs. The free
nerve endings that are numerous in the epidermis reach up to
the stratum granulosum. In addition there are innumerable
numbers of fine nerve endings in the papillary region of the
dermis. According to Cauna (1959), the nerve endings are
extracellular. Endings in epithelia and in terminal cor-
puscles are intimately attached to the surface membranes of
the related cells which may belong to receptor mechanism and
play a part in discrimination of sensory modalities. Human
hairy and hairless skin is devoid of intra-epidermal nerves,
but the epidermis of the nose of certain quadrupeds contains
nerves which grow continuously with the epidermis and undergo
atrophy. 0n reaching the horny layer they end in fading
filaments, in solid condensation or break into segments.

Axons have a fibrillar structure, neurofibrils in the
receptors undergo degeneration and are continuously replaced
by the eXpansion and growth of the terminal fibers. Loss

and replacement of the neural material in nerve terminals is

29

a physiologic process which facilitates change of a nerve
ending in its adaptation to changing functional conditions.

According to Weddell (19“1 ), the nerve plexus below the
epidermis consists of medullated fibers of various diameters
and fine non-medullated fibers. The thick medullatei fibers
is not branch. They and in Meissner corpuscles. Smaller
medullated fibers go to Krause's end bulbs. The fine fibers
undergo repeated dichotomization within the nerve plexus and
may give rise to a nerve net, which in turn gives rise to fine
beaded terminals, scatterei beneath and between the cells of
the deeper layer of epidermis. The plan of the dermal nerve
network varies with the density of the hair of the skin. In
the hairy skin they are distributed mostly around hair fol-
licles, sweat glands, sebaceous glands, and arrector pili
muscles (Winkelmann, 1963a). Capsulated specialized end
organs are found only in the dense connective tissue under-
neath the glabrous mucocutaneous epidermis, (Winkelmann 1957,
1959). Winkelmann (1959) stated that without the follicle
the nerve net could form a ball and rise up higher in the

dermis, simulating a mucocutaneous end organ.

Nggve Supplyto the Hair Follicles,

As a group, the hair follicles comprise the most impor-
tant tactile organ in the mammalian skin and are heavily
innervated by the dermal nerves. The nerves of the hair
follicles and the dermal networks constitute almost all of
the nerve tissue of the mammalian skin. The innervation of

hair follicles is admirably suited to the tactile sensation

3O
(Winkelmann, 1959). The hair shaft acts as a lever to
increase the range of sensitivity following any minute
mechanical change (Winkelmann, 1959; and Montagna, 1962).
As to the sensory function of hair there are two types:
(1) the general hairs of the body coat and (2) highly sensi-
tive tactile hairs of the lower and domestic mammals. Both
types of hair follicles are extensively supplied by nerve
fibers. As regards the supply to the general hairs, the
nerve supply varies directly with the size of the hair pro-
duced by the follicles (Winkelmann, 1959). The arrangement
of the nerve fibers that supply the follicles is essentially
similar to that of the dermal nerve networks (Montagna, 1962
and Winkelmann, 1959). According to these authors, human
follicles are' surrounded by a collar of nerves. Stem nerve
fibers reach the follicle just below the entrance of the
sebaceous gland duct (Weddell and Palli, 195“, Weddell 32 31.
1955, and Montagna, 1962). They give rise to ensheathed
branches which extend along the outer layer of the dermal
coat, parallel to the hair (Weddell and Palli, 195“, Weddell
.gt_§l. 1955). One series of stem fibers change direction
and pass towards the middle layer of the dermal coat (Weddell
and Palli, 195“), where they encircle the hair and give rise
to the arborization of fine nonmyelinated fibers which
interdigitate with one ancter and terminate freely. The
remaining skin fibers lose their sheath as they pierce the
vitreous layer of the follicle and give rise to fine axo-

plasmic filaments which lie parallel to the hair shaft and

31
terminate freely among the cells of the outer root sheath
(Weddell and Palli, 195“, Singer and Salpeter, 1966, COpenhaver.
196“, Dixon, 1961, and Winkelmann, 1959). The fibers of the
inner layer also end freely among the cells of the inner
root sheath and in large hairs may terminate in small but-
tons (Montagna, 1962). The hair papilla does not contain any
nerves (Winkelmann, 1959).

Tactile hairs: In the mammals with the exception of man,
these stiff hairs occur around the face. These tactile hairs
are also referred to as sensory hairs, tantacle and erroneously
vibrissea (Montagna, 1962). Sinus hairs are not vibrissae
(Trautmann and Fiebiger, 1957).

Smith (1888) observed tactile organs in the lips of the
horse. The development of the tactile hair follicle and the
nerve endim;s are not proportional to the size of the animals
(Vincent, 1213). According to Vincent (1913), the rat has a
far more dv sloped tactile hair than the horse, ox or other
rodents. Coauveau (1873) observed that these hair follicles
are heavily innervated in the horse. Messenger (1890)
described the nerve supply to the sinus hairs of mammals.
Vincent (1913) gave a detail description of the nerve supply
to these hair follicles and reported that the follicles are
supplied by two sets of nerves. According to Smith (1888),
the dermal nerves give off large branches to these hair fol-
licles and to the external connective tissue sheath. He
concluded that no part of the skin is as highly endowed with
nerves as this sheath. According to Dixon (1961), who studied

the sinus hairs of the upper lip of several mammals, there

32
are two sets of nerves-~one set innervates the follicle from
below through the fibrous capsule to spread out over the
epithelial root sheath and the other derived from the cuta-
neous nerve plexus acts as the supplementory supply to the
neck of the follicle. The clear description that has been
given by Winkelmann (1959) about the innervation stated that
two to five large nerve trunks penetrate the connective tis-
sue sac of the hair follicle at its base. These trunks
separate into myelinated subdivisions that run through the
vascular sinus to the external root sheath of the hair fol-
licle. At the external root sheath a coarse myelinated net-
work is formed and extend towards the neck of the follicle
where it forms a circular nonmyelinated network. Some of
the nerves terminate in this area below the rudimentary seba-
ceous glands.

Nerve networks are found outside the connective tissue
capsule at the neck and these nerves penetrate: the capsule
to join the nonmyelinated network.

As regards the terminal nerve endings in the hair follicle,
the problems of network or bulb nerve endings has not been
settled (Winkelmann, 1959). Tactile disks in association
with hair follicles were also discussed.

Straile (1960) observed large hair follicles that show
structural Specialization sparsely scattered on the skin sur-
face of rodents. Straile (1961“ described in detail the fol-
licles and their sensory function and termed them a tylotrich

hair follicles.

33

The Organized Nerve Endings

In the skin of all mammals there are organized nerve
endings which are usually found in the mucocutaneous junc-
tions and the glabrous skin. They vary in shape and struc-
ture considerably (Montagna, 1962). In the human skin,
Weddell and Sinclair (1952) observed numerous organized
nerve endings in the finger, nipple, lip, labium major,
clitoris, glans penis, and foreskin. In their observations
they did not find any evidence of absolute morphological
boundaries between one type of endings and the other. One
region could be distinguished from another on a neuro-
histological basis. In the cat, Winkelmann (1957) observed
similarities in end organs in the non-hairy skin. End
organs were reported by Kawata (19“3) in the horse hoof.
Smith (1883) observed pacinian-like corpuscles in the lip of
the horse. Pacinian-type and organs in the corium of the
muzzle of the cow were observed by Sinha (196“). Kuntz and
Hamilton (1938) described lamellated and organs in the skin
of the cat's forelimb and human skin. Winkelmann (1957) and
Strickland and Calhoun (1963) noted encapsulated end organs
in the mucocutaneous junctions and the paw of the cat. Tac-
tile disks were described by Winkelmann (1959) which were
found to be very close to the hair follicles. Winkelmann
(1960a) observed papillary nerves which had some distinct
morphological characteristics in mucocutaneous junctions.
He considered these a special type of nerve ending. These
nerve endings are formed of nonmyelinated nerve fibers, do

not possess a terminal expansion and are not lobulated. The

3“
presence of capsulated end bulbs in the mucocutaneous junc-
tions was described by COpenhaver (196“) and Singer and
Salpeter (1966). Human digital touch corpuscles, Meissner's
corpuscles, are described in all standard histology texts.
According to Winkelmann (1957) the end organs in the car are
morphologically similar to those in hairless skin and he
postulated a common physiologic function for them. These
bodies are supplied by heavy myelinated fibers, and due to
their superficial position he suggested acute touch as their
function.

Frey (1896) conceived the skin as a mosaic of sensory
Spots, each of which subserves a single sensory modality.

He recognized four primary modalities--touch, cold, warmth,
and pain-~and believed that each was associated with a
morphologically specific nerve ending. He associated hairs
and Meissner's corpuscle for touch, Krause end bulb with
cold, Haffini corpuscle with warmth, and free endings with
pain. Baird gt g1. (19“2) associated pressure, warmth and
cold with specific receptors, and pain with free nerve end-
ings. The skin in general has a specific pattern of distribu-
tion of specialized endings. Usually, the number of hair
follicles and the number of Specialized endings in the skin
are in inverse ratio to each other (Montagna, 1962).

Skin perceives so many madalities of sensations that it
would be impossible for each of them to be subserved exclu-
sively by an anatomically distinct and organ (Montagna, 1962).
An attempt to demonstrate Specific sensory perception by

anatomically distinct nerve endings can be related to touch

35
(Montagna, 1962). Waterston (1933 ). considering various

physiological findings, concluded that the nerve for touch

in man does not convey pain producing impulses. Orimea (1961)
and Miller, 2£.§l. (1960) (cited by Montagna 1962) and
Winkelmann (1960a) considered that the Specialized and organs
are modified according to the region in which they grow and
not according to the function that they serve. The only
Specialized organ which serves a Specific function is the

pacinian corpuscle for pressure (Montagna, 1962).

Dermis or Corium

The dermis lying between the epidermis and the subcuta-
neous tissue consists of dense connective tissue fibers which
are arranged irregularly. It is well supplied with vessels
and nerves, and contains the cutaneous glands, the hair fol-
licles and smooth muscle (Sisson and Grossman, 1953). Two
layers can be distinguished although they blend without dis-
tinct demarcation (Cepenhaver, 196“). All of the three
fibrous connective tissue components are present in both layers
of the dermis. The most superficial layer of the corium is
the papillary layer, narrower of the two zones, composed of
fine collagenous fibers interwoven with fine elastic and
reticular fibers (Hook and Walton, 1965). The superficial
surface, beset with blunt conical prominences, the papillae
project into the corrSSponding depression of the epidermis
(Sisson and Grossman, 1953). The papillae contain 100ps
of capillaries (Odland, 1966, and‘Trautmann and Fiebiger,
1957). The papillae are small and poorly differentiated in

36
the hairy skin. Wherever the hair coat is dense, the dermis
between the hairs is either smooth or shows slight elevations
and a few small folds (Trautmann and Fiebiger, 1957). The
papillae vary considerably in size and number in different
parts of the body (Odland, 1966). There are simple papillae,
which may be long and slender or short and thick. Others are
large and have a bifurcated end, especially at the mucocuta-
neous junction (Trautmann and Fiebiger, 1957). According to
Ellenberger (1906), the papillary body is absent in hairy
skin. Smith (1888) observed well develOped papillae on the
lips of the horse which gradually increase in length until
the mucous membrane is reached. They are well supplied with
nerves. In the mane, the corium is well develOped and the
papillae are mostly square tOpped. In the papillary body,
widely separated delicate collagenous, elastic and reticular
fibers, enmeshed with superficial capillaries, are surrounded
by abundant viscus substances (Montagna, 1962). The hairs
are accompanied by the cutaneous glands and the smooth muscle
in the papillary layer (Schgnberg, 1926).

The stratum reticulare is not sharply marked off from
the stratum papillare. It is the deeper and heavier layer
of the corium (Trautmann and Fiebiger, 1957). It is composed
of dense, coarse, branching collagenous fiber bundles which
form layers mostly parallel to the surface. Alternate layers
are at an angle to each other (Montagna, 1962). In addition
to collagen fibers, the reticular layer is composed of

abundant network of coarse elastic fibers (Copenhaver, 196“).

37

According to Schgnberg (1926), the reticular layer in the horse
is poor in elastic fibers, the number of fibroblasts are not
as numerous as in the papillary layer, and heavy blood vessels
pass through the area. In the skin of the croup and the whole
lumbar and sacral region, extending to the hip joint, there
is still another wide layer of collagenous fibers underneath
the reticular layer. He called this part "Spiegel" (mirror)
as it glazes when tanned. Odoni (1951) described the histology
of this layer and obServed that the thickness of the corium
of the area is 53“0 u (papillare layer--27“0 u and reticular
layer--2600u), whereas in the general skin area the corium
is 3“20 u on an average (papillare layer--l6“0 u and retic-
ular layer--1780 u). The collagen fiber bundles are thin
and finely interwoven.

In many reSpects, the dermis of the horse is similar to
that of the cow through its arrangement of connective tissue
fibers and the network of fine elastic fibers (Ellenberger,

1906).

The Cellular Components of Dermis

(In general, the cells of the dermis consist of the
connective tissue cells. The cellular elements can be com-
pared favorably with those of the cells of the subcutaneous
tissue (Bloom and Fawcett, 1962). In normal skin fibroblasts
are the most numerous cells with mast cells next in abundance.
The cells are most numerous in the papillary layers rather
than in the reticular layer (Montagna, 1962, Bloom and

Fawcett, 1962, Porter, 1966). In addition to fibroblasts and

38

mast cells, the dermis contains macrophages, plasma cells
and wandering cells from the blood (Montagna, 1962, COpenhaver,
196“). These authors reported dermal chromatOphores are
sparingly distributed in the dermis, particularly in heavily
pigmented skin. Large numbers of undifferentiated mesenchymal
cells are found in the dermis between the various dermal
tissues (Robb-Smith, l9“5). Under suitable stimuli, the
reticular cells in the dermis may proliferate and differen-
tiate to form various mature cells (Robb-Smith, 19“5).

In this review, the description of the commonly occurring
cells of the dermis only will be included.

Fibroblasts: According to Porter (1966) and COpenhaver
(196“) the fibroblasts are reSponsible for fiber formation
and also for ground substances. Their shape is determined
by their environment. In the reticular layer of the dermis,
they are usually very thin, long and compressed and in the
papillary layer they are very large and resemble mesenchymal
cells (Montagna, 1962). Under the light microscope, they
appear ameboid and are Spindle shaped, with‘processes Spread-
ing out from them. The cell membrane is extremely delicate
and difficult to see under the light microscope. The nucleas
is oval or Spherical. The cytoplasm stains lightly with basic
dyes buffered in acid range and generally appears homogenous.
According to the aboveauthors, the fibroblasts in the pap-
illary layer are more baSOphilic. Fibroblasts enlarge follow-
ing tissue injury and become particularly active in tissue

formation (Copenhaver, 196“). Montagna (1962) maintained

39
that perhaps the fibroblasts are the stem cells from which
arise all the other connective tissue cells. It is for this
reason that some investigators call fibroblasts the relatively
active cells while the fibrocytes are called inactive cells
(COpenhaver, 196“).

Mast cells: The mast cells occur in varying numbers in
all the connective tissue. Any connective tissue cell con-
taining cytoplasmic granules which stains metachromatically
with toluidine blue is a mast cell (Montagna, 1962). The
mast cell tends to congregate along small blood vessels
(Montagna, 1962, Porter, 1966, Cepenhaver, 196“, Nicholas,
1963, and many others). The average mast cell count in human
skin per cubic mm was 7225 (Mikhail and Milinska, 196“).

In human skin they occur in the various layers of the corium
in and around the hair follicles, even in some cases in the
epithelial layer (Nicholas, 1963). All Skin areas of the
donkey, examined by Dozza and Hampichini (1963), contained
mast cells, localized prevalently in the upper dermis, around
vessels and in the papilla. Waldeyer (1875) observed much
larger dermal connective tissue cells with basic staining
granules, occurring usually on the perivascular zone.

The mast cells are large connective tissue cells with a
central nucleus and ample cytoplasm with more or less densely
packed granules (Hansen, 1957). The form of the mast cells
may be round, oval, pyriform, Spindle or star-shaped (Nicholas,
1963). The granules of the mast cells stain not only in a
basic tone but also metachromatically (Ehrlich, 1879). The

granules are highly refractile, more or less water soluble

“O
and stain yellow when treated supravitally with neutral red
(Montagna, 1962, and Porter, 1966). The granules may appear
in the ground substance, either by a disruption caused by some
violent physical action or a chemical reaction (Hansen, 1957).

Mast cells increase in number in several itching skin
diseases. In the skin carcinoma of man and animals, they
seem to form a barrier between the tumor and the normal tissue
(Montagna, 1962). It is difficult to make a generalization
in the mast cells of different animals as there are consider-
able Species differences (Hiley, 1959). The mast cells have
been shown to produce heparin (Porter, 1966, and Holmgren,
and Wilander, 1937) or at least an anticoagulant substance
similar to heparin (Montagna, 1962). The mast cells contain
and release histamine (Porter, 1966, Copenhaver, 196“, and
Montagna, 1962) which is liberated from the cells into the
surrounding tissue in response to allergic conditions.
Serotonin, a vasoconstrictor substance, is also thought to
be released by mast cells (COpenhaver, 196“, and Porter, 1966).
Mast cells can increase by mitosis (Porter, 1966). All
observations lead to the fact that they also form from the
fixed connective tissue cells or nongranular precursors
(Porter, 1966, and Bates, 193“).

MacrOphages or_histiocytes: These are also known as
"resting wandering cells" as they resume their wanderings
upon suitable stimulation (Trautmann and Fiebiger, 1957).

They resemble fibroblasts (COpenhaver, 196“, Montagna, 1962,
and Porter, 1966). They have sharper outlines than do fibro-

blasts; the cytoplasm is granular and the nucleus is condensed

“1
and more basophilic. When they are activated, the entire
cell becomes larger, with a prominant nucleus and the cyto-
plasm becomes granular vacuolated and contains ingested
materials (COpenhaver, 196“, and Montagna, 1962). Though
in normal skin they are not easily identifiable, when partic-
ulate matter has been injected into the skin, histiocytes
can be recognized easily by their capacity to ingest par-
ticles (Montagna, 1962). They act as scavengers, engulf
extravasated blood, bacteria and inert foreign materials.
If the particle is large, several macrOphages join together
to form a foreign body giant cell, a multinucleated cell, to
engulf the particle. The organic materials which they engulf
are usually digested by proteolytic enzymes, and the foreign
matter which resists digestion remains in the cytOplasm'
(Porter, 1966).

There are various conflicting Opinions about the origin
of the macrOphages. According to pOpular belief, they belong
to the reticulo-endothelial system, a system consisting of a
group of cells having phagocytic activity. Montagna (1962)
stated that the macrOphages arise from a variety Of different
cells, such as fibroblasts, lymphocytes, skeletal muscle and
Schwann cells. Chevremont (19“8, cited by Montagna) observed
that a surprisingly high number Of macrophages appear
Spontaneously in cultures of skeletal muscle or of subcuta-
neous connective tissue. When the tissues in 3;£;g_are
crowded in the flask, abundant transformation Of muscle fibers

to macrOphageS takes place. This was later confirmedby

“2
Fazzari (1951). According to Porter (1966), monocytes of
blood and macrophages of tissue are the different functional
phases of the same cell type. They may arise by mitotic
division, by activation of fixed macrOphages, and by further
' differentiation of monocytes emigrating from the blood stream.
From the above discussion it may be concluded that the macro-'
phages may be differentiated from a variety of cells in the
.tissue according to the environmental condition in which they

are functioning.

 

The Fibrous Components Of the Dermis

The dermis is composed of all the three fiber components
of connective tissue: 1. Collagen fibers, 2. Elastic fibers
and 3. Heticular fibers. Their orientation, depending on
the region, is different.

Collagen fibers: The collagen fibers vary considerably
in their individual fiber thickness. The fibers are thickest
in the reticular layer and are intermingled with a few coarse
elastic fibers. The collagen fibers in the papillary layer
are very fine and entwine with the medium to fine elastic
fibers and fine reticular fibers.~ The collagenous fiber
components are fibrils with diameters Of 30-200 mu (Porter,
1966). The fibers are formed by submicroscopic fibrils.
There are also subfibrilar units, the filaments and proto-
filaments (Bear and Morgan, 1957). The electron microscopic
study of the fibrils was reported by Reed (1957). The fibrils
appear to be composed of two sets of filaments which croSs

each other in Spiral fashion.

“3

The configuration suggests that the protofibrils, the
coiled-coil system of three polypeptide chains, as indicated
by the x-ray diffraction studies, are twisted together to
form filaments (Reed, 1957). According to Borustein and
Karl (1965), the isolated polypeptides show wide differences
in amino acid composition, having different molecular weight
and the primary structure of each chain appears to be unique
throughout its length. The filaments are then organized
together in a steep, spiral fashion to form two independent
identical strands. The two strands then Spiral around each
other to make up the complete fibril’(Heed, 1957). Highly
refractile constricting fiber rings, the Henle's rings, were
observed in large numbers in ox hide and calf hide. Since
they did not stain with any common elastic fibers stain, it
was concluded that the Henle's rings resemble non-swelling
collagen (Felsher, 1966).

Orekhovitch and Shpikiter (1957) prOposed a phasic
derivation Of collagen from procollagen which is usually
present in the skin. The procollagen is higher in the skin
Of young animals and diminishes greatly as the animals become
older. Grassmann (1955) suggested three stages in the forma-
tion of mature collagen. The tropo-collagen gives a periodic
cross-striation of fibers Of 2000 A0, which is metabolically
very active, and present in ground substance Of connective
tissue. The next stage, which is pro-collagen, is meta-
bolically less active and present in newly formed collagen

fibers which gives a periodic cross-striation of 650 A0.

#4
The final stage is the condensed fibers, metabolically very
inactive with cross-striations of 650 A9. This constitutes
the principle mass of collagen fibers.

Elastic fibers: In addition to collagen fibers, the
dermis is provided with a dense network of elastic fibers
which are interwoven with the collagenous elements. In the
region of the basement membrane, the elastic fibers are
extremely delicate, whereas in the papillary layer they are
much coarser. They are thickest in the reticular layer where
the elastic networks are particularly abundant (Porter, 1966).
Sinha (1964) described the elastic fibers in the lower strata
of the corium of Holstein cattle. These fibers are all thick,
abundant, and generally parallel the surface of the skin.

Dick (1947) reported that in the deeper layer they may be
arranged in bundles in man. They are condensed about the

hair follicles and the sweat and sebaceous glands and from

the papillary layer extend into the papilla (Bloom and Fawcett,
1962). The fine elastic net becomes greatly enlarged and

the fibers are more numerous in connection with the origin

of the arrector pili muscle. The fibers extend close to the
epidermal cells and are Spread over a considerable area in
relation to the origin of each muscle (Dick, 19h7). In general,
the elastic fibers are very thin, highly refractile strands
which branch and anastomose freely to form a taut network.

When broken, they kink and curl as they recoil like tense

wire. The elastic fibers are optically homogenous even at

higher resolutions of electron microscopy (Porter, 1966).

“5

Jerrett (1958) considered the skin in zixg a gel in which
polypeptide macromolecules are oriented. The latter become
the elastic fibers noted in the fixed tissue. He argued
that the gel itself is elastic and the fibers may act as
tightening agents. According to him, polymerization results
from fixing agents and histological preparations show only
artifacts.

Elastin makes up only 2% of the dry weight of human
skin and its chemistry is not much advanced (Montagna, 1962).
Hall, gt 5;. (1955) suggested that the elastin could be
considered in two phases--one amorphous and one fibrous, the
former surrounding the latter and affording it protection
against solubilization. This outer covering was described as
containing a mucoprotein and its presence was regarded as
the stabilizing factor for the fiber as a whole. Hall 32 al.
(1955) postulated a theory that the enzyme elastase is also
dual in nature, one enzyme being specific for the inner
fibrous protein and the other for the outer amorphous muco-
protein. This introduces the possibility that mucopolysac-
charides act also as a stabilizer in the sense of a protec-
tive sheath. According to Hall (1957), some of the collagen
fibers do apparently convert completely to elastin. He con-
sidered that there may not be any single entity 'elastin'
but rather a series of elastins differing in their amino-acid
composition. Elastic fibers are isotropic but begin to show

uniaxial birefringence when stretched 100 to 150% of their

46
original length, which suggests that elastin molecules lie
in a randomly crumbled position and when pulled tend to go
back to the original position (Montagna, 1962).

Heticular fibers: The dermis, particularly the pap-
illary layer, contains a delicate network of freely branch-
ing non-elastic reticular fibers interwoven with the collagen
and fine elastic fibers. They are also called argyrophilic
fibers. They are very closely associated with the basement
membrane at the dermoepidermal Junction, around the hair
follicles (Dick, 19“?) on the wall of the blood vessels
and surrounding the cutaneous glands.

Beticulin consists of fine (l-h) isotropic fibers which
show true branching usually at right angles (Robb-Smith,
1957). In mature connective tissue, reticulin often appears
to merge with collagen fibers and the angular branching
which is seen in relation to the basement membrane between
the connective tissue is very characteristic and predominant
(Robb-Smith, 1957).

The electron microscOpe shows these fibers are made up
entirely of very fine fibrils ZOO-300 A9 in diameter (Porter,
1966). In the embryologic deve10pment and in the healing of
wounds, the fine argerphilic fibers which appear early are
subsequently replaced by coarse fibers of collagen (Torter,
1966). There is an imperceptible emergence from the fine
argyrophil reticular fibrils to the non-argyrophil collagen
fibers (Robb-Smith, 1945). The only differences between

collagen and reticular fibers probably is physical in nature

47
and only the fine fibers take up the silver stain (Mallory
and Porter, 1927). The reticular fibers are probably pro-
collagenous elements and do not constitute a separate type
of fiber (Montagna, 1962). Therefore, the principle differ-
ence between reticular and collagen fibers is one of size
‘ (Porter, 1966). Also x-ray diffraction patterns of the fibers
from the dermis of newborn rats are like those of the collagen
of the adult (Bensley, 1934, cited by Montagna, 1962). On
histochemical studies, the reticular fibers are strongly PAS
reactive while the collagen fibers are mildly PAS reactive
(Montagna, 1962, Copenhaver, l96h, Porter, 1966, and Robb-
Smith, 1957). Both fibers are stained alike by acid aniline
(Montagna, 1962). All these evidences suggest the reticulum

as the precursor of collagen (Montagna, 1962).

The Ground Substance

The ground substance of the dermis is a semifluid, non-
fibrillar, amorphous substance that fills the spaces between
the fibers and cells. There is more ground substance in the
papillary layer than the reticular layer (Montagna, 1962).
Ordinarily in normal tissues, the gelatinous character of
the ground substance prevents the spread of foreign substance
(Porter, 1966).

The chemical composition of ground substance is very
complex. Acid mucOpolysaccharides are the most important
components of the ground substance, which is mildly PAS
reactive and metachromatic (Montagna, 1962). Acid muc0poly-

Saccharide has been divided by Meyer, §£_al. (1957) into

mg
two different types according to chemical composition:
(1) non-sulphated mucopolysaccharide which consists of
hyaluronic acid and chondroitin and usually occurs in connec-
tive tissue other than hyaline cartilage, and (2) sulphated
mucopolysaccharide, which does not contain hyaluronic acid
and occurs in cartilage. Besides muccpolysaccharides, ground
substance contains neutral heteropolysaccharides, proteins,
metabolites and antibodies (Montagna, 1962). Acid mucopoly-
saccharides in connective tissue may be concerned in regula-

tion of the nucleation and growth of fibrils (Montagna, 1962).

Thg_§asement Membrane

The basement membrane, a thin homogenous membrane is
situated between the epidermis and the dermis. The basement
membrane provides the mechanical basis upon which cells can
develop complexities (Pease, 1958). It serves to anchor
the epidermis to the dermis (Odland, 1958). The conventional
basement membrane is known to be rich in mucopolysaccharides
and strongly PAS positive (Hay, 1966, Mercer, 1961, and
Pease, 1958). Frieboes (1920L(cited by Montagna. 1962)
visualized the basement membrane as a complex argyrophil
reticulin. In general, the reticulin is a component of the
mammalian basement membrane (Robb-Smith, 1957). The stain-
ing reaction of the basement membrane is not uniform (Lillie,
1952), but varies from place to place (Robb-Smith 1957, and

Mercer, 1961). The reticular fibers in the basement membrane

“9

form a dense network separating the epithelium from the
connective tissue (Bloom and Fawcett, 1962). Thus, the
basement membrane, visible under the light microscOpe, does
consist of a continuous mesh of argerphil fibrils (Montagna,
1962). According to Bloom and Fawcett (1962), when the
ground substance surrounds certain structures or is at the
base of certain epithelial structures, it is so modified that

together with the enclosed reticular fibers it forms the

basement membrane. Hay (1966) considered the basement mem-

branes to be the product of the secretory activity of some
of the epithelial cells which they underlie or other cells
of the dermis.
Odland (1958), using the electron microscope confirmed
the existence of a moderately dense homogenous membrane lying

below the basal surface of the basal epidermal cells and
:not penetrated by dermal collagen filaments in the vertebrate

skin. A membrane about 350<%.thick follows the basal contours

of the epidermal cells and is separated from them by a space
<Df 350 9A. No filaments, either epidermal or dermal cross

tflnis membrane (Montagna, 1962). In describing the general

txasement membrane, Hay (1966) divided it into (1) basal lamina,
tame most consistent dense filamentous sheet which attaches to
‘tdue plasmalemma of the basal epithelial cells and the under-
lying reticular tissue; (2) the reticular lamina of the base-

IIlent membrane which is composed of condensed ground substance

Eilnd fine reticular fibers. According to Hay (1966), reference

t3<3 the basement lamina as a basement membrane by the electron

50
microscOpists, creates confusion because it ignores the

reticular lamina which is usually the component seen under

the light microscope.

It, therefore, may be considered as a two-layered mem-
brane. The basal lamina is intimately associated with the
cells of the basal layer and a reticular lamina composed of

reticular fibers enmeshed with ground substances, associated

with the dermis.

Qermo-epidermal Junction

The contact surface of the epidermis and dermis is

 

wavy. Epidermal cones and ridges of different sizes,
commonly known as rete pegs, but better called ridges
(Montagna, 1962) project into the dermis. These ridges

'may be branching. There are corresponding furrows and
ridges on the surface of the corium to fit exactly with

the epidermal ridges and furrows. Thus, the dermis enclosed
‘Within the ridges of the epidermis forms the vascularized
(dermal papillae (COpenhaver, 196k, and Montagna, 1962).

fIhe network of epidermal ridges complement the dermal
Papillae, which are different in different parts of the
Eflkin (Ross, 1965). Montagna (1962) considered the char-
Eicteristic regional differences in the architecture of the
epidermal ridges and papillae are related to the arrangement
or hairs and sudoriferous glands. The epidermal ridges tend

t3c: converge towards the ducts of sweat glands and hair fol-

licles in a rosette pattern.

51

The cells of the basal layer send a number of delicate
protOplasmic processes into the dermis (Montagna, 1962,
COpenhaver, 1964, Odland, 1950, Trautmann and Fiebiger, 1957,
and many others). These fine processes fit into the meshes
of the reticular fibers of the basement membrane. Odland
(1958) considered the processes variable in width and depth
of penetration. The undulating cytOplasmic membrane of the
basal cell is provided on all its basal surfaces with dense‘
thickenings. Bundles of tonofilaments appear to attach to
these thickenings.

Dick (1947) noted almost complete absence of fibrils
or fibers under the epidermis of 5-year-old children. At
20 years a well developed fibrous plexus appears which
increases and the individual fibers thicken as the indivi-
dual becomes older. Most of these fibers are reticular
fibers and are very closely associated with the cytoplasmic
processes of the epidermal cells. Under light microscOpy, I
these fibers appear to terminate in a bulbous end in con-
tact with the basal epithelium. Reticular fibers possess
:10 free endings, instead they form continuous networks.
(Ddland (1950) concluded that the reticular free endings
interdigitating with cell processes are actually the cut
sands of the reticular meshes. In the papillae of an obliquely
(cut lip Dick (1947) observed that the fibers are wavy and
:Just below the epidermis the short blunt ends reach the
Iaasal cells. He could not say that these fibers actually

Einchor the epidermal cells with the dermis.

52.

The relationship between the elastic fibers of the
dermis and the cells of the basal layer is also very inti-
mate and these fibers could anchor down the epidermis
(Montagna, 1962). The dermo-epidermal Junction is easily
separated by trypsin digestion (Medawar, 1941). According
to Medawar (1941), the elastic fibers of the basement mem-
brane play a major role in anchoring the epidermis with the
underlying tissue. Pease (1958) concluded that definitive
elastin is deposited within a precursor matrix of polysac-
charides. Under some circumstances the basement membrane
is attached with the underlying connective tissue by direct
continuity with the elastic fibers. Comparing the photo-
graphs of skin stained for reticular tissue with those
stained for elastic tissue, Dick (1947) stated that they
have a very different arrangement and distribution, and are
distinct from each other. There is no continuation of the
fine elastic fibers to the basal cells, instead the reticu-
lar fibers led to the base of the cells. There is always
a distinct space separating the epidermis from the fine

elastic plexus.

lilood Supply

The skin has a very rich and efficient blood supply
Ifor the nourishment of the tissue, and temperature regulation.
'UPhe skin also participates in the general blood pressure
Iregulation. It receives about 10% of the total blood supply
( Burton, 1966) which increases significantly during hot

‘erather. The basic vascular distribution pattern is similar

53
in all the animals, though there is considerable variation
in respect to number and size of the vessels occurring in
different areas of the skin. The vascular pattern is
influenced by the thickness of the skin, type and number of
cutaneous appendages present and the relation to the skin

of the underlying tissue (Montagna, 1962). In man, two

arterial pleXuses are present in the skin: (1) the cutaneous

plexus and (2) the subpapillary arterial plexus. In the
skin of cattle, Goodall and Young (1955) recognized three

plexuses. The first lies below the corium, the second is

at the level between that of the sweat glands and sebaceous
glands, and the third extends from beneath the epidermis to

above the second plexus and is a deep network of many fine

blood vessels. Both the hair follicles and sweat glands are
supplied with blood from the second plexus. Hair follicles

and sebaceous glands also receive their supply-from the third
plexus. From the third plexus, being subepidermal, the

terminal arterioles give rise to numerous twisted capillary
loops lying in the dermal papillae. The papillae of the
follicle receive their rich blood supply from the deeper
Eirterial plexus (Odland, 1966). According to Zweifach (1959),
1:he terminal branching of the arteries has a regular pattern
<>f interconnecting links to form interarterial networks. The

Eidjacent link in these meshes gives off cross branchings that

Eire connected by arterial capillaries. The capillary branches

underneath the epidermis come directly from the inter-

anastomosing arterial arcades. The hair follicles and

54
straight portion of the ducts of eccrine sweat glands in
man are accompanied by parallel longitudinal vessels that

are interconnected by arteriolar and capillary cross-shunt

and anastomoses (Montagna, 1962). The follicles of the skin

of the horse are completely encircled by vessels and the
dermal papillae contain 3-4 vessels (Smith, 1888). The
hair follicles of cattle are also very rich in blood supply
(Hook and Walton, 1965). The epidermis is nourished by

nutrients from the vessels of the dermal papillae and sub-

epidermal capillary system. The nutrients reach the cells

by diffusion.

Venous blood drains via different channels from the
superficial part of the dermis and reaches a venous plexus
at the lower dermis at the level of the cutaneous arterial
plexus (Odland , 1966) .

Arterio-venous_anastomoses: The presence of arterio-
venous anastomoses in the dermis is of great importance to
the histologist for its peculiar arrangement and structural
development. These vessels short circuit, or act as a shunt,
"between arterioles and veins. They are of considerable
:importance functionally in the regulation of body temperature,
Ibroviding a shunt to diminish flow through extensive areas
<>f peripheral capillary beds, thereby decreasing loss of
fleat by convection and radiation from the peripheral vessels
( Odland, 1966). The arterio-venous anastomoses usually show

E1 long tortuous vessel which has an end artery Just before

tlhe controlling metarteriole. In the course, there is a

55
glomus body arranged circularly (Burton, 1966) which contains
special contractile epitheloid cells (Burton, 1966, and
COpenhaver, 1964). The shunt empties into the venous system
by a long tortuous vessel (Burton, 1966). Arterio-venous
anastomoses in large numbers have been reported in the ear
of the rabbit (Burton, 1966), in the human fingers and toes
(Montagna, 1962) and in the external ears of the Rhesus
monkey, cat, dog, guinea pig, horse, sheep, goat, and pig
(Danial and Marjorie, 1956). Mention has been made of the
presence of arterio-venous anastomoses in large numbers in
the dermis of the coronary border, and hoof of the horse.
They are also present in the papillae of the horse hoof
(Clara, 1956).

Recently other anatomical peculiarities have been
reported in the arteries leaving the main uterine arteries
in rats and other species. These arteries have an arterial
cushion, so that blood is withdrawn from the axis rather
than near the wall (Burton, 1966). Forman and Moffat (1961),

(<eited by Burton 1966) concluded that in arteries with a
cushion, hemoglobin concentration is greater than the main
airtery and for those without a cushion less than the parent

Eirtery. Such anatomical pecularities were reported in the

Eirteries of the kidney also (Burton, 1966).

SPhe Hair

The hairs are dead, horny, elastic structures, composed

Chompletely of packed keratinized cells, which grow out of.

56
epithelial pockets, the hair follicles. In fur bearing
animals, the hairs serve as an important organ of protection
by insulation and tactile perception (Odland, 1966). The
hairs traverse the dermis to varying depths (Copenhaver,
1964). The proximal part, or root, is set obliquely in the
skin. The hair root is attached to an underlying dermal
papilla by means of its knoblike, hollow proximal end called
the hair bulb (Trautmann and Fiebiger, 1957). The morphologic
characteristics of the hair vary from one species to another
(Montagna, 1962), and even the angle and direction of the
hair vary with the species and body region (Trautmann and
Fiebiger, 1957). Hairs may be spiny, stiff, soft, long,
short, thick, thin, woolly, colored, or white (Montagna,
1962). The skin of domestic animals is covered with hairs,
except at the mucocutaneous Junctions, foot pads, and the
nose. The hairs are classified according to most authors
into (1) cover hairs, (2) wool or lanugo hairs in sheep or
undercoat hairs in the goat, carnivores and some breeds of
swine (Trautmann and Fiebiger, 1957), (3) special or tactile
liairs. Smith (1888) divided the hairs of the horse into
'three classes: (1) Temporary hairs, covering the general
lbody surface, (2) Permanent hairs--the hairs of the mane and
izail and (3) The tactile hairs or feelers. Sisson and
Cirossman (1953) divided the hairs of all domestic animals
JLnto two broad groups: (1) the ordinary coat hairs,-and
(L2) the special hairs which are found in different places--
(Ia) tactile hairs - about the lips, nostrils and eyes,

('b) eyelashes or cilia, (c) tragi of the external ear,

I A

57

(d) vibrissae of the nostrils. In addition in horses the
mane (guba) springs from the dorsal border of the neck and
adjacent part of the withers; the foretop (cirrus capitis)
from the anterior part of the mane; and the tail bears very
large and long hairs except the ventral part known as cirrus
caudae. Tufts of long hairs on the flexor surface of the
fetlock surround a horny growth which is termed "ergot"
(Chauveau, 1873). According to Smith (1888), the hairs of
the mane and tail are thicker and coarser in texture in cart
horses, whereas in the thoroughbreed horse they are compar-
atively fine. Breed differences in the development of these
special hairs vary widely (Sisson and Grossman, 1953, and
Chauveau, 1873). The hair types in different animals has
been reviewed by Sinha (1964) in his thesis on the micro-
scopic anatomy of the integument of Holstein cattle.

Arrangement of hairs: The arrangement of hairs and
their pattern of distribution varies greatly from species to
Species. In horses and cattle, hairs erupt on the skin singly
(Krolling and Grau, 1960). Hairs occur in groups in dogs,
cats and pigs. There are usually three hairs in.a group, of
Vshich one is the main hair and is longer than the other two.
CPhe hairs are arranged in groups of 3 in the pig (Smith and
C2alhoun, 1960, Ellenberger, 1906, and Marcarian and Calhoun,
JL966) and dogs (Webb and Calhoun, 1954, and Lovell and Getty,
3L957). In the pig each follicle contains one hair (Smith

81nd Calhoun, 1964). In the cat, 12-20 hairs occur in a group

Eind 2-5 groups around a large central guard hair. Each

58
Each lateral group usually contains 3 primary hairs surrounded
by 6-12 lanugo hairs (Strickland and Calhoun, 1963). Thus,
in carnivores a whole bundle of hairs projects from a common
follicular opening (Trautmann and Fiebiger, 1957).

Development of haigsandfihair follicles: Dry (1926)
suggested 3 stages in the development of hairs in mice. The
following terms may be used in discussing the development of
hairs in any mammal. The terms are: (l) anagen--active
proliferation period, (2) catagen--cessation of proliferation
and pigment deposition and (3) telogen--the resting period.

During the develOpment of the animal, the first indica-
tion of a follicle is an invagination of the basal cells of
the Malpighian layer of the surface epidermis. These cells
give rise to external root sheath cells. Some of the undif-
ferentiated melanoblasts are also included in the original
follicular invagination (Chase, 32 31. 1951). The follicular
slope probably results from unequal growth on the Opposite
sides of the primodium (Colin, 1943). The terminal portion
of the invaginated Malpighian layer becomes enlarged as the
result of greater mitotic activity and it becomes indented
from below by a cluster of connective tissue cells, the
potential hair papilla (Odland, 1966). Here the hair bulb
is established on the papilla. The bulb is the extension of
Malpighian cells, the site of mitotic activity in hair pro-
liferation (Chase, 33 51, 1951). According to Montagna (1962),
the papilla remains attached to a basal plate of dermal cells

by a narrow stalk. The pigment cells migrate to the upper

59

part of the bulb and the cells below the pigmented layer form
the matrix of the follicle. A cone of keratinized cells
appears in the center of the upper part of the bulb above the
apex of the dermal papilla. According to Odland (1966),
subsequent proliferation of the lowermost cells of the bulb
results in the outward extension of the keratinized cone.
This is the inner root sheath, which grows upward through
the solid epithelial column and forms a keratinized lining
for the hair canal through which the hair grows. The follicle
becomes a tube formed by the outer root sheath around a
keratin cord, the inner root sheath (Montagna, 1962). The
cells of the hair matrix proliferate, which results in the
outward displacement of the overlying matrix cells and forms
the keratinized hair shaft. The hair shaft thus formed,
extends through the tubular inner root sheath (Odland, 1966).
According to Montagna (1962) the tip of the hair is always
free of pigments and the first hairs formed have no medulla.
A glassy membrane is formed around the lower half of the
follicle, below the bulge. Two connective tissue enve10pes
also differentiate around the follicle. The cells of the
connective tissue between the bulge and the surface differen-
tiate in a band of smooth muscles, the arrectores pilorum
muscles.

figplacement and;£egeneration of hairs: Shedding of hair
takes place in most mammals at regularly recurring periods.
In man, there is constant loss and replacement of hairs

(Copenhaver, 1964). This is more obvious in animals that

60

live in the far north. The northern animals commonly grow

a new coat for each winter and lose it for each summer (Ham,
1961). The horse changes its coat twice a year, once in the
spring and again in autumn. In the spring a fine short coat
is developed and in autumn the fine hair coat is replaced by
long and often shaggy hairs (Smith, 1888). The ponies in
Britain shed their coat between the end of March and end of
May. This shedding of the winter coat may be delayed due to
a cold season. The short fine summer coat lasts from June
to August, the adult winter coat from September to May (Hook
and Walton, 1965).

According to Bloom and Fawcett (1962) and COpenhaver
(1964), when the growth of hair ceases, the multiplication
of the hair matrix slows down and finally stops. The base
of the shaft and the root gradually become thinner. The
elements which cover the summit of the papilla become cornified
and hair becomes club shaped. The root separates from the
papilla, moves towards the neck of the follicle, and the
hair either falls or is pulled out. The papilla become
smaller, atrophic or disappear.

It is supposed that subsequent hair generations result
from reactivation of dormant hair follicles stimulated by the
proliferative activity of the hair germs (Odland, 1966).

The_hi§tologic structuges of the hairs: 'The morpho-
logic characteristic of hairs vary from one species to another,
and even hairs of the same animal may differ markedly in

'Various regions of the body. Hairs show great variations in

61
diameter and shape. Most hairs are round and oval but others
are also flattened (Montagna, 1962). The hair is composed
entirely of epithelial cells (COpenhaver, 1964). It consists
of medulla, cortex, and cuticula.

The medulla is absent from the finer, shorter (Lanugo)
hairs and the human scalp hairs (Copenhaver, 1964, and
Trautmann and Fiebiger, 1957) and frequently fails to extend
the whole length of the hair (Copenhaver, 1964). The medulla,
when present, consists of one or more rows of cuboidal cells
with shrunken nuclei(0dland, 1966). According to Trautmann
and Fiebiger (1957) it is solid in the region of the root
but, contains air vacuoles in the shaft. The medullary cells
in the hairs of the horse are rectangular, separated by wide
lighter streaks and the borders appear rough. According to
Marshall (1902), a characteristic feature of equine hair is
the presence of a medulla. The medulla has the tendency to
disappear at irregular intervals on the hair shaft,leaving
air Spaces of all sizes. In thoroughbreds, ponies, and
American chestnuts, the medulla is circular and pigmented
(Glaister, 1931). The hair of the pig has practically no
medulla (Montagna and Yum, 1964).

The cortex makes up the main bulk of the hair and con-
sists of several layers of cells (COpenhaver, 1964). Towards
the bulb the cells become soft, shorter, and finally become
ovoid with spherical nuclei (Trautmann and Fiebiger, 1957,
and Odland, 1966). These cells are derived from the undifferen-
‘tiated cells of hair matrix (Odland, 1966). In the upper

Kmart of the root of the hair and in the shaft the cortex

62
consists of cornified, fusiform cells with elongated nuclear
remnants (Capenhaver, 1964, Trautmann and Fiebiger, 1957).
In colored hair the pigment granules are found in and between
the cells which are formed by melanocytes during the growth
stage of the hair cycle (Odland, 1966 and Copenhaver, 1964).
Air may accumulate in the intercellular Spaces and modify
the hair cycle (Copenhaver, 1964). According to Odland (1966),
electron microscopic studies show that the cytOplasm of the
fully keratinized cortical cells consists almost entirely of
low-density filaments enclosed by a dense interfilamentous
substance.

The cuticle consists of a single layer of elongated,
cornified, scaly cells, the free margin of which is serrated
and directed towards the tip of the hair (Montagna, 1962,
and Odland, 1966).‘ According to Montagna (1962), the cells
of the cuticle of the hair inside the follicle are interlocked
with those of the inner root sheath. The arrangement firmly
anchors the hair within the follicle.

Vincent (1913) considered the cuticle of the hair and
the inner root sheath of the follicle as the continuation of
the stratum corneum. The cortex corresponds to the stratum
germinativum and the medulla to the stratum cylindricum of

the epidermis. ’

The Structure of the Follicle:
The follicles are continuous with the epidermis. They

Eire slanted and the diameter increases at the base where

63
they are dilated to form the bulb. The bulb encloses the
soft loose dermal papilla. Montagna (1962) termed the part
of the follicle between the cpening of the duct of the seba-
ceous gland and the surface, the pilary canal. Although
differences occur, the structure of the follicle in all
mammals is similar (Montagna, 1962).

The follicle consists of the epithelial sheath which
is the direct continuation of the epidermis, and the connec-
tive tissue sheath modified from the dermal connective tissue.
The epithelial part of the follicle consists of the inner
root sheath and the outer root sheath, and the hair remains
in the center. The thickness of the outer root sheath is
preportional to the size of the follicle (Montagna, 1962).

According to Copenhaver (1964) and Trautmann and Fiebiger
(1957), the structure of the hair follicle described as
follows:

The inner root sheath: consists of three layers, the
cuticle of the root sheath, the Huxley's layer, and Henle's
layer. The cuticle is a membrane similar in structure to
the hair cuticle against which it lies. The free edges of
the cells are directed towards the papilla so that the two
cuticles interdigitate. The cells are nucleated in the deeper
part and non-nucleated near the surface. The Huxley's layer
is the middle layer, consisting of several rows of elongated
cells. The cells of this layer are nucleated almost the whole

length. They send processes to the Henle's layer (Montagna,

i1962). The Henle's layer consists of a single layer of

64
non-nucleated cells. They are clear and rectangular. The
cytoplasm contains longitudinal horny fibrils.

According to Montagna (1962), the cells of all three
layers of the inner root sheath acquire trichohyalin granules.
The cells in the cuticle do not acquire trichohyalin granules
until they are about halfway up the follicle. The inner
root sheath grows upwards with the hair from the papilla as
far as the cpening of the sebaceous gland, where it disinte-
grates.

The oute;;root sheath: The outer root sheath is the
continuation of the Malpighian layer of the epidermis. The
outermost cells, adjoining the glassy membrane are tall and
arranged in a single row. These cells in the lower part of
the follicle have minute basal cytoplasmic processes (Montagna,
1962). This layer is comparable to the stratum cylindricum
of the epidermis. The rest of the cells are more polygonal
in shape. They have spinous processes and resemble the cells
of the prickle cell layer.

The connective tissue sheath: This sheath encloses the
epithelial hair follicle. The layer is continuous with the
connective tissue element of the hair papilla (Odland, 1966).
The connective tissue sheath contains dense layers of col-
lagenous fibers investing the follicle. Dick (1947) observed
the reticular fibers arranged in a radiate fashion around
the follicle, as if holding the follicles in place. He also
described the elastic fibers which form a fine net around the

hEitirfollicles. This sheath is composed of an outer

65
longitudinal layer of poorly defined and loosely arranged
bundles of white fibrous tissue. The middle layer is thickest
and composed of fine connective tissue fibers, arranged circu-
larly. The inner layer is a homogenous narrow band, the glassy
membrane made up of argyrophilic reticular fibers composed of
PAS positive ground substance (Montagna, 1962, and Odland,
1966). In reality, this layer may be regarded as a basement
membrane (Odland, 1966). According to Montagna (1962), this
glassy or vitreous membrane is single-layered around the upper
part of the follicles and two-layered where it is thickest
around the lower third or half of the follicle. Only the
outer layer is continuous with the basement membrane of the
epidermis. In the lower half of the follicle the epithelial
cytOplasmic processes and fine fibrils make a heterogenous
irregular lamina.

Follicular folds: Follicular folds are circular folds,
appearing in a vertical section of the follicle as corruga-
tions of the inner root sheath of the hair follicle near the
opening of the sebaceous glands. They were observed in the
cat (Strickland and Calhoun 1963), in cattle (Goldsberry and
Calhoun 1959 and Sinha 1964), in swine (Smith and Calhoun
1964, Fowler and Calhoun 1964 and Marcarion and Calhoun 1966),
in goat(Sar and Calhoun 1966), and in sheep (Kozlowski 1966).
Montagna (1962) observed these folds in the follicles of the
rat, mouse and sheep. A similar loose, horizontal, reticula-
ted collar around the emerging hair in the under portion of

the pilosebeceous canal in the human follicle was observed by

Idontagna (1962).

66
Tactile Hal;

The structure of tactile hairs in the horse was described
by Smith (1888), in rats (Vincent, 1913), in pig (Smith and
Calhoun 1964, Fowler and Calhoun 1964 and Marcarian and Calhoun
1966), in goat (Sar and Calhoun 1966), in cattle (Sinha 1964)
and in sheep (Kozlowski 1966). The follicles of these hairs
are richly supplied with sensory nervés. Blood vessels
resembling erectile tissue form a system of cavernous sinuses

around them (Vincent, 1913, and Montagna, 1962).

Arrector Pili Muscle

This bundle of smooth muscle originates from the connec-
tive tissue sheath of the lower part of the follicle, arches
over the sebaceous gland and inserts in the upper dermis.
In the cat, dog and in the follicles of the tails of most
mammals, the arrector pili muscles are well developed
(Montagna, 1962). .According to Trautmann and Fiebiger (1957),
they are thin on the coarse hairs or bristles of the pig
and the hairs of the mane of the horse. They usually end
in an elastic tendon. The arrector pili are absent in tactile
hairs. They are thick in sheep and often double in the pig
(Trautmann and Fiebiger, 1957). The muscle is often perforated
by ducts of the tubular glands (Sinha 1964). Contraction of
this muscle causes the hair follicle with the hair to assume
a more vertical position than usual which forms a swelling
on the skin surface, ”the goose flesh" (Odland, 1966, and

Copenhaver, 1964).

67

Histochemistryiof HaigFollicles

From the staining and histochemical properties of the
follicles many things may be learned about hair follicle
activity. As has been observed RNA disappears from the cells
of the bulb at the same rate that the proteinous fibrils
accumulate, which indicates that RNA plays a role in the
synthesis of proteinous materials (Montagna, 1962). Similarly,
proliferative potentiality of the cells of different parts
of the follicles, and the hair formation potentiality of the
follicle in general may be ascertained by the Feulgen tech-
nique.

glycogen in the hg;£;folli21§§: Bolinger and McDonald
(1949) observed glycogen in the hair follicles of man, rabbit
and the phalanger. The globules of glycogen were Observed
at the level where the sebaceous glands connect with the hair
follicles. Alkaline phosphatase and glchgen occur in the
developing hair follicles but as the hair grows the phospha-
tase disappears while glycogen remains in the follicle. The
special relation between the phosphatase and glycogen cor-
relates well with the path of glucose entry into the tissue
(Johnson and Bevelander, 1946). Glycogen occurs as a regu-
lar constituent of the outer root sheath of active follicles
in the mouse (Hardy 1952). According to Montagna (1962),
active hair follicles are always rich in glycogen. In the
tactile hairs of the mouse Melarazno and Montagna (1953)
observed abundant glycogen only in the external sheath.

In the connective tissue sheath some glycogen is found inside

68
the cytOplasm of fibroblasts and extracellularly along the
fibers. The inner layer of the vitreous membrane is PAS
reactive but diastase resistant. The cells in the middle
third of the follicle contain so much glycogen that only a
flimsy framework of cytoplasm is left. The cells are
gradually depleted of glycogen as they become keratinized in
their inner margins. Storage of glycogen bears an inverse
relationship to mitotic activity. Mitotically active cells
and the cells which are rapidly synthesizing keratin do not

contain demonstrable glycogen.

The Sebaceous Glands

The sebaceous glands are pear-shaped appendages of the
hair follicle and cpen inside the pilosebaceous canal.
Frequently the size of the glands varies inversely to the
size of the associated hair follicle. (Trautmann and Fiebiger
1957, Montagna 1962, Copenhaver 1964 and Odland 1966). In
most of the mammals they are widely spread over the body.
Most animals have the largest glands in the mucocutaneous
junctions of the lips, eyelids, vulva and perianal regions
(Montagna, 1962, and Trautmann and Fiebiger, 1957 and Smith,
1888). The horse and dog have the largest sebaceous glands
and the pig has the smallest (Ellenberger, 1906). In ungulates,
as a rule, 2-6 glands cpen into one follicle (Trautmann and
Fiebiger, 1957, and KrSlling and Grau, 1960). This is more
marked in the hoof margin (Ellenberger, 1906). In regions

With long hair these glands are long and narrow (Smith, 1888,

69
and Trautmann and Fiebiger, 1957). In the horse, they are
found everywhere on the skin and usually occur in pairs (Smith,
1888). Sebaceous glands are especially well develOped in the
horse (Sisson and Grossman, 1953. and Smith, 1888). In horses
and dogs the glands are branched (Ellenberger, 1906) and
occur on the posterior side of the follicle (Krglling and
Grau, 1960). A sebaceous gland in domestic animals is a
bilobular alveolar gland (Rock and Walton, 1965), but multi-
lobular glands may also be present (Montagna, 1962, and Rock
and Walton, 1965). They are holocrine glands. Regardless of
their size, shape and location, their morphology is similar.
According to Montagna (1962), the acini (termed alveoli by
many) of each gland are attached to a common excretory duct
that consists of squamous epithelial cells, continuous with
the pilosebaceous canal (Copenhaver, 1964, and Montagna,
1962). In the glandular acini the cells show a centripetal
enlargement, those in the center being large; the cells of
the outer periphery are undifferentiated and resemble those,
of the epidermis (Montagna, 1962). The outer peripheral
cells are a continuation of the duct cells and the stratified
layer of the duct gradually diminishes to a single layer on
the acini (Odland, 1966). In the ordinary histologic prepara-
tion when no fat remains in the cell cytoplasm, the cytoplasm
appears as a finely reticulated meshwork of baSOphilic mate-
rials (Odland, 1966). The basophilia increases towards the
periphery. This basophilic substance is abolished by ribo-
nuclease which indicates that the material is ribonucleo-

protein (Montagna, 1962).

70

The secretion of the sebaceous gland is known as sebum,
which, in general, is believed to protect the skin against
ultraviolet light, bacteria, and other foreign bodies and
provide an insulating layer on the skin surface (Rock and
Walton, 1965). The epithelial cells of the alveoli differen-
tiate from the peripheral cells and are engorged with lipid
materials. The lipid substance gradually increases in the
cytoplasm of the cells, starting from the periphery to the
central part where the cells become much larger and are
gradually distended with lipid droplets. Secretion results
from the disintegration of the cells, a holocrine mode of
secretion. The sebum is thus the product of disintegration
of the cells themselves (Copenhaver, 1964). The signs of
disintegration of the cells such as nuclear pycnosis and
coalescence of the lipid droplets, appear in the part of the
alveolus nearest the duct (Odland, 1966). It is not only
the cells themselves which are found in different states of
maturation, but the alveoli themselves are also in different
state of maturation. The alveoli in the same glandular unit
may be found to contain lipids only in the central parts
while others have lipids throughout the alveolus. Therefore,
it was concluded by Montagna (1962) that the individual icinus
is doomed to destruction once differentiation begins. Mito-
tic activity is abundant in the cells lying on the basement
membrane close to the wall of the excretory duct. The new
cells thus produced are pushed to the secretory region (Bloom
and Fawcett, 1962). The mitosis may also occur directly in

the peripheral cells (Montagna, 1962). According to Montagna

71
(1962), the sebaceous acini are in constant state of change;
epithelial buds grow from the walls of the excretory ducts,
they deve10p sebaceous kernels in their centers and then
grow into new sebaceous units. Sebaceous kernels may deve10p
anywhere along the acini outside or inside, where there are
accumulations of undifferentiated cells.

The body of the gland is covered by a delicate connec-
tive tissue sheath (Ham 1961). Each acinus of the gland is
surrounded by a complex network of arterial capillaries which
adhere very closely to the basement membrane but never pene-
trate the acinus (Montagna and Nobeck, 1947). The sebaceous
paranchyma of tarsal glands in horse and other mammals are
surrounded by nerves rich in cholinesterase (Montagna and
Ellis, 1959) while, in general, sebaceous glands are not
surrounded by nerves (Montagna, 1962). Perhaps the secretory
activity is influenced by temperature, sex hormones and age
(Odland, 1966).

Histochemistry of sebaceous glands: The main consti-
tuents of the sebum, the lipid secretory product of sebaceous
glands, consists of cholesterol, cholesterol esters, and
phospholipids. The Golgi apparatus of the acinar cells is
most intimately associated with the synthesis of lipid drOp-
lets in sebaceous differentiation. Montagna (1962) concluded
that the sebaceous transformation occurs within the structures
that correspond to the Golgi apparatus, either by chemical
alterations of the substances of the membranes, or by a

segregation of materials in the cytoplasm. According to

72
Ahmed (1965), the Golgi region is the center of sebum forma-
tion. The sebum differs from surface lipids as the latter
contain free cholesterol and probably this originates from
cholesterol esters through the action of nonspecific esterases.
Using Sudan IV and Sudan black, Montagna and Nobeck (1947)
demonstrated lipid substances, both in sebum and acinar
cells of the rat. The large central cells which possess
abundant lipid draplets contain large quantities of unsatura-
ted glycerides, while the dying cells contain glycerides
and cholesterol esters. Traces of lipid solvents and resis-
tant lipid substances also are present. Alkaline phosphatase
activity is abundant in the acinar cells. Peripheral cells
contain more phosphatase than the central cells whereas RNA
is limited only to the peripheral cells.

The sebaceous glands of all mammals other than man and
higher primates do-not contain any glycogen (Montagna and
Ellis, 1959, and Parakkal, 53‘21. 1962). The human sebaceous
glands abound in glycogen (Montagna, 1962). In the sebaceous
gland of man glycogen is associated with lipid storage. Seba-
ceous transformation takes place by a conversion of carbo-
hydrate to lipids (Montagna, 1962, and Montagna, EE.§$° 1952).

Development of sebaceousgglands: The development of
sebaceous glands is intimately associated with the hair fol-
licle. According to Montagna (1962), two bulges on the same
side of the hair follicle develop even before the follicle
is fully differentiated. The upper bulge becomes the analge
of the sebaceous glands and the lower bulge is the point

I

Where arrector pili muscles are attached.

73
The Sweat Glands

There are two kinds of sweat glands in domestic animals,
designated as merocrine and apocrine.‘ The secretory tubules
of both kinds have basement membranes containing circular
reticular fibers (Trautmann and Fiebiger, 1957). The skin of
the horse is richly provided with sweat glands (Smith, 1888,

Sisson and Grossman, 1953) and the glands are apocrine type

only (Evans, et al., 1957). They are largest and most numer-
ous in the lateral wing of the nostrils, the flank, mammary
gland, and the free part of the penis (Sisson and Grossman,
1953). They are well develOped on the general body surface
(Smith, 1888) and form almost a continuous layer of glands in
the flank (Ellenberger, 1906). The glands are small and less
numerous in the mane, tail and limbs (Smith, 1888), and lie
deep into the dermis in the mane and tail (Ellenberger,

1906 and Smith, 1888).

The apocrine glands: These are predominant in the skin
of the entire body surface of all domestic mammals
(Schiefferdecker, 1917, and Trautmann and Fiebiger, 1957).
Usually one apocrine gland is associated with each hair
follicle, except the tactile hair follicles (Trautmann and
Fiebiger, 1957). In man, apocrine glands occur only in the
axilla, mons pubis, external auditory meatus, circumanal
region, areola, nipple, eyelids, labia minor, prepuce and
scrotum (Montagna, 1962). The apocrine glands develop as an
outgrowth of the hair follicle during embryonal life
(Trautmann and Fiebiger, 1957, Montagna, 1962, and Odland,
1966). The duct may be separated later (Odland, 1966) and
Inay cpen directly onto the surface of the epidermis (Montagna,

74

1962). The ducts, however, ordinarily cpen in the neck of
the pilosebaceous canal (Odland, 1966, Montagna, 1962, and
Trautmann and Fiebiger, 1957). The tubules of apocrine
glands are wider than the tubules of merocrine glands. In
sheep, pig, cat, and horse, the secretory tubule is glomi-
form, whereas in the ox, goat, and dog it is serpentine
(Trautmann and Fiebiger, 1957). The coils of each gland are
encircled by plexuses of capillaries (Ellis, _e_§_ al., 1958).

The merocrine or eccrine glands: These are predominant
in the skin of general body surfaces of man and many primates
and secrete a watery sweat (Odland, 1966, and Trautmann and
Fiebiger, 1957). Amongst the domestic animals this type of
Sland is seen only in the foot pad of the dog (Nielsen,
1953) and metacarpal pads of cats (Strickland and Calhoun,
1963). The secretory portion of the merocrine glands con-
81 sts of a narrow tube of uniform diameter, which is generally
coiled into a Spherical mess (Trautmann and Fiebiger, 1957).
The ducts cpen directly on the epidermis, pursuing a tortuous

Course through the dermis and epidermis (Odland, 1966).

H. l stological Feature

Apocrine gland: As already mentioned, apocrine glands
are the general type of glands found in the skin of all
ClOmestic animals. The hi stologic and the histochemical
preperties of these glands are quite different from that of _
the merocrine glands. They have extremely large lumina and
t31'le tubule may show sac-like diverticula (Hibbs, 1962, and

Trautmann and Fiebiger, 1957). Where the secretory tubules

75

pmass into the excretory duct they suddenly become narrow
(Trautmann and Fiebiger, 1957). .

The cells of the secretory tubules are irregularly
cnalumnar; the terminal portion of some cells is elongated
sand projects into the lumen (Montagna, 1962). Some of the
tnlbules have a very wide lumen with low columnar to squamous
types of cells (Trautmann and Fiebiger, 1957, and Montagna,
15962). The cytoplasmic membrane of the secretory cells adja-
cent to the lumen forms a brush border consisting of micro-
vdilli (Montagna, 1962, Hibbs, 1962, and Yamada, 1960). Takagi
sand Tagawa (1959) reported the presence of a brush border at
tnie luminal end of the cells of the apocrine gland of the
kuarse. The brush border disappears at the formation of the
apocrine secretory projection (Kurosumi, 1959). According
t“) Hibbs (1962), the secretory cells of the apocrine glands
lie on rows of longitudinally oriented myoepithelial cells.
Between the myoepithelial cells cytOplasmic projections from
tdfie secretory cells are in contact with the basement membrane.
49- large intercellular space occurs between the myoepithelial
Eirni secretory cells (Kurosumi, 1959, Montagna, 1962, and
Iiilbbs, 1962). The apical cytoplasm of microvilli contains
vEicuoles and granules close to the luminal membrane (Montagna,
1962). The epithelium of the duct is double layered and
(ltfboidal in man (Montagna, 1962) and cow (Sinha, 1964).

Compositigg_of the apocrine secretion: The apocrine

glands of domestic animals, except the horse, secrete watery

8West and the secretion is not fatty (Trautmann and Fiebiger,

76
1957). According to Montagna (1962), the apocrine sweat in
man is milky, viscid and pale gray and the secretion from
the gland is usually slow and scanty. The secretion of
the sweat glands of the horse is proteinous or albuminoid
(Trautmann and Fiebiger, 1957, and Krglling and Grau, 1960).
The albumen is the chief protein but traces of globulin are
present (Jirka and Katos, 1959). Evans, pp a1. (1957) claimed
that the secretory cells of the horse sweat glands contain
glycogen and produce a large amount of watery sweat while
Takagi and Tagawa (1959) did not find any glycogen in the
horse's sweat glands except a trace in the myoepithelial cells.
Evans, pp_gl. (1957) considered the sweat gland of the horse
an intermediate form of gland between the eccrine and apo-
crine glands. Similarly, Montagna (1962) designated horse
sweat gland as a different organ, though in broad terms it
is apocrine like other mammals.

Mode of segpetion: The application of histochemistry,
cytochemistry and electron microscopic study has revealed so
many facts about the apocrine glands that the earlier view
of necrobiotic secretion has been altogether rejected.
Schiefferdecker (1917 cited by Montagna 1962) proposed a
theory of necrobiotic secretion in which the apex of the cell
bulges out to form a bleb which subsequently breaks away and
disintegrates to form the secretion. According to Montagna
(1962), the cytoplasmic blebs that appear in the lumen in
histologic section are really attached to the subjacent cells.
The pinched off appearance is brought about by a dehydration

and shrinking of soft cytoplasm. Frozen section studies under

77
phase microscOpe show the apical ends intact with the cell

(Cepenhaver, 1964). Hibbs (1962) observed in both secretory
and resting glands, that the lumina are either empty or
contain an amorphous material. No cellular debris or see-
retory granules are encountered. Anatomical evidence is

' lacking as to the necrobiotic secretion of the horses apocrine
gland (Rock and Walton, 1965, and Takagi and Tagawa, 1959).

It is strongly believed that the apocrine secretion occurs

by a pinching off of the terminal globules of the microvilli
or by an exudation of liquid substance through them (Montagna,
1962, and Copenhaver, 1964). This still remains apocrine in
nature as the pinching off of the microvilli form the secre-
tion. The secretion by disruption of the apical part probably
may occur only under extreme conditions, if at all (Hibbs,
1962).

Merocrine or eccrine glands: These are simple tubules
that extend from the epidermis to the dermis. The lower
secretory part is coiled and the upper excretory part is
straight or tortuous. The secretory part of the gland is
lined by cuboidal to columnar type of epithelium (Trautmann
and Fiebiger, 1957, and C0penhaver, 1964), lies on the base-
ment membrane and is comparatively thinner than the basement
membrane of apocrine glands (Montagna, 1962). Inside the
basement membrane lies the spindle-shaped myoepithelial cells
whose cell bodies are wedged in grooves between the bases of
the secretory cells (Odland, 1966). The protoplasm of the

secretory cells contains granules and fat draplets (COpenhaver,

78
1964). Distinct intra- and inter-cellular secretory capillar-
ies are seen through which the secretory product is eliminated
(Odland, 1966, Copenhaver, 1964, and Montagna, 1962).

According to staining characteristics, two types of cells
are recognized in merocrine glands--clear cells and dark
cells (Deleson, et a1. 1958, cited by Montagna, 1962).

Under the electron microscope both clear and dark cells were
recognized in the sweat glands of the foot pad of the cat
(Munger, 1960). The cytOplasm of the clear cells extends
microvilli of uniform diameter into the luminal border and
shorter microvilli into the canaliculi (Montagna, 1962,
Munger, 1960, and Hibbs, 1958).

The transition between the secretory portion of the tube
and the duct is abrupt (Montagna, 1962). The duct is lined
with two layers of cuboidal cells (Odland, 1966, and Trautmann
and Fiebiger, 1957). The cells of the luminal surface of the .
duct show low microvilli under the electron microscope
(Montagna, 1962). The lumen of the sweat duct contains mucus
'which tends to form plugs in the spirals within the stratum
corneum of the epidermal sweat ducts. The dark cells of the
secretory coils are the source of mucus. The mucin granules

are present in the cytOplasm of these cells (Lee, 1960).

Nerve Supplx

Thick ribbon-like nerve fibers are distributed in the
connective tissue surrounding the secretory tubule of the
horse sweat gland. They ramify and give rise to fine,

tortuous fibers, which form a terminal reticulum on the outer

79
surface of the myoepithelium of the secretory tubule (Takagi
and Tagawa, 1961). The myoepithelial cells of the apocrine
glands are supplied by the adrenergic fiber of the autonomic
nervous system (Hurby and Shelley, 1954). The sweat gland
of horses respond to intradermal injection of adrenalin and
also show a response to acetylcholine (Evans 23,21. 1956).
The apocrine glands of the horse are adrenergic. In the horse,
increase in the blood adrenaline is the normal mechanism

which causes sweating (Evans pp 21. 1957).

MATERIALS AND METHODS

Source of Animals

Skin specimens from 35 areas of two stallions, two mares
and two geldings were obtained from freshly killed animals.
The animals were from two to seventeen years old. A few
additional blocks of tissue were collected from the vet-
erinary clinics and pathology laboratory of Michigan State
University for studying the effect of different fixatives
on the skin, histochemistry of skin and the nerve endings

and nerve fibers of the lips.

Technigue
As soon as the animals were killed by pistol shot, the

specimens of about 1 (one) square inch were collected and
fixed in FAA for general study (Lavdowsky's mixture, Guyer,
1949). After three days the specimens were trimmed to about
4 x 10 mm size for vertical section and 5 x 5 mm for horizon-
tal sections. They were left in FAA solution for 30 days.
The specimens were than transferred through three changes of
dioxane (Bucher and Blackely, 1936). The first and second
changes were for two hours each and the third change over-
night (about 15-20 hours). The tissue was infiltrated in
vacuum under 20 atm. pressure for 30 to 60 minutes. The
paraffin blocks were sectioned at 8 microns thickness.

The sections were stained with (1) Harris' hematoxylin

smm.eosin for general study (Malewitz and Smith modification,

80

81

1955), (2) Gomori's aldehyde fuchsin ( Gomori, 1950) for
elastic fibers, mast cells and keratohyalin granules;
(3) periodic acid, silver, orcein and aniline blue (Gurr,
1962) to study the relative distribution of the three fibrous
components of dermal connective tissue; (4) Bielschowski-
Gros method (Lillie, 1965) for nerve fibers and endings;
(5) Bauer-Feulgen reaction for glycogen (Gridley mannual,
1960); (6) periodic acid Schiff for PAS positive substances
of the skin; (7) Domici's Mast cell stain (Gridley manual,
1960); (8) Gomori's aldehyde fuchsin counterstained with
Janus green to render all other tissue green except elastic
fibers in dermis to observe their absolute density; (9) Bodian's
method for nerve fibers and nerve endings (Bodian, 1936).

A special fixative, "Petrunkevitch's cupric paranitro-
phenol”, (Gurr, 1962) was used to render the keratin, as
well as the dermal connective tissue components, loose enough,
to study the compactness of the dermal connective tissue.
The fixative was found very satisfactory for the purpose.
The epidermis and dermis were measured in microns by an

ocular micrometer.

Body areas from which the skin specimens were taken.

 

Head Pectoral limb
Upper 11p 0 o o o o o o 1 BraChlum o o o o o .34
Lower 11p 0 o o o o o .14 Axilla o o o o o o o 035
NOStrilS o o o o o o o 2 CheStHUt o o o o o o 033
Upper eyelid. o o o o o 3
‘Lower eyelif. . . . . . 4
Pelvic limb
Forehead. . . . . . . . 5
EXto ear Ckfifllo o o o o 6 Thigh (Lat.) o o o o 025
Pinna(at EVE”). o o . . 7 Thigh (Med) 0 o o o .26
Chino o o . o o o o-o_015 .Chestnut o o o o o o .27
Smeantirdi”r region. ’16 ErSOtc o o o o o o o .28
Coronary border. . . .29
Neck
Dorsal Neck (center) . . 8
u " (base) 0 o o 9
Lateral HECko o o o o 017
Ventral neck(center). .18
Brisket o o o o o o o 019

Thorax

Lateral neck(base). . .20
POSto thorax o o o o 021
Ventral thorax(Ant.). .32

" (Post). .31
580k. 0 o o o o o o o 010

Abdomen

Flank o o o o o o o o .22

Vent. abdomen . . . . .30

Prepuce(Genital region)lOA ,
Glutealgpegion

Group 0 o o o o o c all

Lat. gluteal region
Circumanal region . . .24

o
o
h)
V)

2.1.1.

ROCt 0f the tail. 0 o .12
Tip 0f the tail 0 o o .13

 

 

RESULT AND DISCUSSION

General Features

Skin thickness: In the discussion, the term "general
areas” refers to all areas except those designated, "special
areas". The "special areas" include: upper lip; lower lip;
nostrils; upper eyelid; lower eyelid; external ear canal;
tail; chestnut; ergot; coronary border; dorsal neck; and
prepuce as shown in the sketch.

The average skin thickness of the general areas was
3.8 mm, (about 4.1 mm in the stallion, 3.7 mm in the geldings
and 3.3 mm in the mare). The skin of the stallions was
thicker than both the geldings and mares (Table V). The total
skin thickness varied according to age, sex, and individual
from 1.6 to 6.4 mm, which compared favorably with Sisson and
Grossman (1953), Krglling and Grau (1960), and Ellenberger
(1906), where they stated the skin thickness varied from
l-5mm. 0n comparing the general skin of a l7-year old gelding
with a 2-year old gelding, marked difference in thickness
was observed (Table I). The thickest skin on the general
body areas was on the croup (5.5 mm). Next, in order of thick-
ness on the general skin areas was the back and gluteal region
(4.5 mm), flank (4.4 mm), brachium (4 mm), abdomen and thorax
(3.9 mm) and thigh (3.7 mm). This observation is in agreement
with Smith (1888). The thinnest skin was on the pinna (2.5 mm)
and submandibular region (2.6 mm). In agreement with Sisson
and Grossman (1953) and Smith (1888), the thickest skin on

84

85
the special areas was on the dorsal surface of the tail
(6.4 mm) and on the attachment of the mane (6 mm) (Table I).

The thickest epidermis was, in order of thickness, on
the upper lip (757 u), lower lip (278 u), circumanal region
(262 u) prepiece (245 mm) and chin (205 u) (Table II).

The thickest dermis in the general skin areas was on the
croup (5.2 mm), back (4.4 mm), gluteal region (4.4 mm), flank
(4.4 mm) brachium (4 mm), abdomen (3.8 mm) and in the thigh
and thorax (3.6 mm). 0f the special areas, the dermis was
thickest on the dorsal surface of the tail (6.3 mm), ergot
(6.1 mm), and at the attachment of the mane (5.6 mm) (Table
III). '1

Components of Skin
Blood supply: The skin was highly vascular. The blood

vessels were more numerous in the areas with a thick epidermis
or dermis. Particularly the vascularity of the upper lips,
lower lips, croup, coronary border and nostrils were very
striking. There were three well formed vascular plexuses in
the areas under investigation. First the branches of the
large vessels of the subcutis formed a plexus at the lower
part of the reticular layer. A second very prominant plexus
was formed in the region of the cutaneous glands and hair
roots. Branches from this plexus supplied the hair papillae,
sweat glands and part of the sebaceous glands. A third prom-
inant plexus was formed at the upper part of the papillary
corium. Numerous vessels from this plexus entered the dermal

papillae. Similar arrangements were reported for cattle by

86
Goodall and Young (1955) and in the horse by Smith (1888).

Arterio-venous anastomoses: These were observed in the
papillary corium of all areas having a thick epidermis or
dermis. These vessels were thick walled arterial vessels
with several layers of epitheloid cells on the wall (Plates
XXXIV, XXXV and XXXVI). Thus the lumen of the vessels was
narrow. They formed coils or convolutions and gave an
appearance of a glomus. These vessels were usually cut in
several sites in a cross section (Plate XXXVI). They were
observed mainly in the upper lip, lower lip, nostrils, upper
eyelid, croup, coronary border, external ear, brisket,
circumanal region, and thigh. Their presence in the coronary
border and papillae of horse's hoof was reported by Clara
(1956), in the horse's ear by Daniel and Marjorie (1956), and
in human finger and toes by Montagna (1962), Odland (1966),
and Capenhaver (1964). This type of modification of the
arterial vessels in the skin provides a shunt to diminish
flow through extensive areas of vascular beds to decrease
loss of heat by convection and radiation from the extensive
surface of the skin.

In addition to the arterio-venous anastomoses, a large
number of intra-arterial cushions were present in the
arteries, usually in the areas rich in anastomoses. These
were composed of epitheloid cells pushed towards the lumen
from the inner wall of the vessels. Due to this arrangement,
the lumen became a crescent-like slit (Plate XXXVII).

Another type of intra-arterial cushion was observed in

the papillary layer of the corium in the upper lips and thigho

87
In this form arterial vessel left the parent artery through
the summit of the cushion carrying blood from the center of
the lumen (Plate XXXVIII). Burton (1966) mentioned this type
of arterial modification in the uterus of the rat and other
species. According to Burton (1966), this arrangement allows
the distributing arteries to sample the blood from the center
of the parent vesse1.' Such vessels receive blood which is
rich in red blood cells: Since the red blood cells have the
tendency to accumulate in the axis of the vessels, such an
arrangement distributes blood rich in red blood cell to a
part of the skin where local demand for oxygen becomes

increased due to functional demand.

Hair

 

Classification, density and arrapgement: The following

classification of the hairs of the horse is based on Sisson
and Grossman (1953). The hairs are divided into (1) general
coat or cover hairs which cover the general body surface;

(2) coarse hairs of great length located on the dorsal

border of the neck and the adjacent part of the withers (mane),
the forehead (foretOp), the dorsal and lateral surfaces of

the tail, and surrounding the horny ergot on the flexor sur-
face of the fetlock; and (3) the special tactile hairs endowed
with specific function located on the lips and nostrils; and
(4) fine hairs located on the lips, nostrils, external ears
and circumanal region.(Plates XXVI and XXVIII). The coarse
hairs were deeply rooted in the dermis but never crossed the

whole width of the dermis.

88

The coat hairs on the general skin surface were usually
medullated with a few large hairs set deeply into the dermis
(Plate XVI). The hairs were closely distributed on the dorsal
and lateral surfaces of the trunk and limbs. They were scarce
on the posterior side of the abdomen and medial surface of
the thigh (Plate XVII). An individual cover hair with its
follicle, associated sebaceous glands and sweat gland or duct
occurred as a unit in the dermis and was covered by organized
collagenous fibers (Plates XVI and XVII). They were arranged
in a distinct line towards the surface and finally erupted
in the grooves of the surface epidermis to form rows with
adjacent lines (Plate II). At the coronary border there
were groups of hairs which were longer than the normal cover
hairs bordering the dorsal border of the hoof (Plate XXXIII).
All the hairs erupted on the surface at an acute angle.

Structures of hairs: The structures of the hairs and
hair follicles conformed to the description of mammalian hair
found in standard texts with the exception of the findings
listed below.

1. The medulla of the hair of the horse contained
rectangular cells lying side by side. The intercellular
spaces were wide with desmosomes connecting the cells with
each other (Plate XVIII). Trautmann and Fiebiger (1957)
described these cells as rectangular, separated by wide
space with rough borders.

2. The connective tissue part of the follicle was

very thick at its middle third and was strongly acidophilic.

89

3. The roots of the hairs were branched and the end
of each branch was bulbous. The bulbs fitted into the pits
of the inner wall of the external root sheath. This was
revealed by the Bielschowsky-Gros method.

Follicular folds: These folds were present only in
large hair follicles just below the cpening of the sebaceous
glands. These appeared as a modification of the inner
epithelial sheath of the follicles. The external sheath at
this point thinned out and formed a pocket. The folds in the
form of flaps extended obliquely upwards on the inner surface
of the pocket (Plate XIX). The folds were directedly propor-
tional in size with the sizes of the follicles. As a result
of this anatomical feature of the follicular folds in the
horse they might function to prevent inward movement of any
substances whether sebum or debris from the outside. This
might afford protection to the soft structures of the fol-
licles. This seems reasonable due to the fact that the
large follicles had larger folds with 8-10 flaps, the medium
sized follicle contained 2-5 flaps and the smaller follicles
lacked folds. However since large sebaceous glands were
associated with small follicles, the function of the fol-
licular folds in the horse could not be confined solely to
the accumulation or expulsion of sebum as proposed by
Strickland and Calhoun (1960) in the cat and Sinha (1964) in
cattle.

Tactile hairs: These were found on the upper lip, lower

 

lip, and nostrils. They were very strong, long, and deeply

90

set into the dermis. The follicles were very large and
contained the hair at the center. It was 1.7 mm in cross
section. The connective tissue sheath was very thick and
divided into inner and outer layers with sinus Spaces in
between. The sinuses were filled with blood and traversed
by connective tissue trabeculae. There were usually two
small sebaceous glands at the neck. No arrector pili muscle
or sweat gland was associated with tactile hairs. The
"ringwulst" and annular sinus (Strickland and Calhoun, 1963
and Trautmann and Fiebiger 1957) which are the characteristic
feature of the tactile hairs of carnivores and rodents were
absent. Skeletal muscle fibers were seen in the near vicinity
of the follicle. This was also reported by Sinha (1964) in
cattle and by Strickland and Calhoun (1960) in cat.

Arrector pili muscles: They were observed throughout
the whole skin. They were usually placed on the side of
the hair follicle forming an obtuse angle with the dermis
(Plate XLVIII). The muscle arched on the free side of the
sebaceous gland. The ends in some cases were divided into
2-6 branches (Plate XV). Both ends terminated in elastic
fibers. In the general skin, in many cases, a branch was
detached from the main muscle and passed to the duct of sweat
gland where the duct perforated it and passed vertically for
a considerable distance along the arrector pili muscle. They
were very long with the follicles of the coarse hairs and
were strong and wide on the back, croup and gluteal region

' (Plate XV).

91

Histochemistry_of hair follicles: The follicles were
tested for PAS positive substances and glycogen. The cells
of the epithelial root sheath were highly reactive to PAS.

The connective tissue sheath was mildly PAS positive and the
basement membrane was strongly reactive. The cuticle of
the hair showed a mild reaction (Plate XXI). The cortex was
PAS negative. The medullary cells contained strongly PAS
positive granules scattered within the cells. The border of
these cells, along with the intercellular spaces, were also
strongly PAS positive.

Glycogen was present in the cytoplasm of the external
root sheath in diffused form. The quantity varied greatly
from the upper to the lower end of the follicles. The glycogen
rich cells were present in the middle third of the follicle.
The glycogen diminished gradually towards the root as well as
toward the epidermal end, until the cells were completely free
of glycogen at both ends (Plate XX). On a regional basis,
most of the follicles of the upper and lower lips and nostrils
were rich in glycogen. The follicles of the general body
areas though containing glycogen were usually poor. Accord-
ing to Montagna (1962), the active follicles were always
rich in glycogen.

On comparing the quantity of glchgen and PAS positive
substances in the follicles and hair, it was found that the
cells of the outer root sheath contained, in addition to
glycogen, other non-glycogenic PAS positive substances.

The inner root sheath of the follicles and the cuticle and

92

medulla of the hair contained only non-glycogenic substances.

Epidermis

The free surface of the skin contained ridges, grooves
(plate II), cpening of sweat ducts and hair follicles (plate
XXVII). The skin was distinctly wavy on the lateral and
ventral side of the neck, thorax, abdomen and tail. In these
areas, the epidermis was thickened at the summit of the
ridges, perhaps due to adhesion of the two slopes at the
summit. The Slope of the ridges were usually composed of
thin epidermis. The hair erupted in the grooves (plate II)
and the epidermis on both sides of the hair contained a
thickened area at the opening of the hair follicle. The Skin
on the forehead was almost smooth and on the back, croup,
legs, arms, lips, nostrils and eyelids was moderately wavy.

In the areas with thick epidermis, as in lips, nostrils,
eyelids, and posterior abdomen, the epidermal pegs were both
simple and branched. The core of the ridges was filled with
dermis and the spaces between the epidermal pegs were filled
by the dermal papillae. The vertical extension of the dermal
papillae varied directly with the thickness of the epidermis.

The epidermis was composed of stratified squamous
epithelium that covered the entire body surface. The shapes
and sizes of the cells differed, depending upon their position
in the epidermis. The functionally active or viable cells

‘were of cuboidal, columnar, polyhedral, and fusiform shape.

‘Usually, the cells at the lower layer were columnar in the

93
thick epidermis and high cuboidal to cuboidal in moderate to
thin epidermis. The cells from the suprabasal layer to the
surface layer became gradually fusiform and flattened. The
surface layer were composed of dead, flattened, scaly or
horny cells.

The epidermis was mainly composed of three layers--the
stratum corneum, stratum granulosum and stratum germinativum.
A stratum lucidum, characteristic of certain areas of the
skin of man (Montagna, 1962) and carnivores (Strickland and
Calhoun, 1963, Webb and Calhoun, 1954), was not found in any
part of the skin of the horse.

§ppatum coppeum: This layer was composed of dead,
flattened, non-nucleated, keratinized, closely packed squamous
cells. Occasional dissociated cells resembling scales were
also observed. The cell boundaries were not distinct in the
superficial layer but were distinct near the stratum granulOSum.
The cells of the corneal layer retained some cellular integrity
in areas of preSsure and where horny structures were formed.
This observation is in agreement with Montagna (1962). They
remained cemented together and did not exfoliate (Plate VIII,
XXVII). The cells of the stratum corneum in thin epidermis,
as on the general body surface, were somewhat loose and their.
cellular boundaries were obscured (Plates I and III). This
layer was extremely thick and formed thick horny tissue in
the chestnut (Plates VIII and XXXII), coronary border and
ergot. The layer also was very thick in upper lip, lower
lip, nostril (Plate XXVII), circumanal region, ventral

abdomen and tip of the tail. The cells of this layer were

94
very closely adherred in less hairy skin as in lips, nostrils,
and circumanal region, but formed distinct threads in the
general hairy Skin. The arrangement of these threads gave a
net-like appearance, particularly on the dorsal surface of
the tail. Fine filamentous structures were observed in the
cells of some thick cornified areas. This observation agrees
with Sinha (1964). Intercellular Spaces were very distinct
in thick cornified layer (Plate VIII) but no desmosomes were
observed in any areas under investigation. The surface was
usually smooth in areas of thick epidermis but uneven in
areas with thin epidermis.

Stratum granulosum; Large fusiform cells, heavily
packed with highly basophilic granules were present here
(Plate VIII). The cells were arranged in rows and there
were many layers which formed a thick stratum corneum. In
areas with a thin epidermis and a thin cornified—layer, they
were usually well apart in a single row just below the
stratum corneum. In general, the thickness of this layer
directly varied with the thickness of the superficial cornified
layer. At the coronary border, chestnut and ergot up to
12-20 layers of cells were observed (Plate VIII). Two to
four cells occurred in the lips, nostrils, and circumanal
region (Plate VI). Prominent intercellular Spaces existed
with indistinct desmosomes (Plates 71, VIII). The cyto—
plasmic organelles, particularly tonofilaments, were not
recognized, perhaps due to heavy accumulation of keratohyalin
granules. The nuclei were shrunken, and in the upper layer

of thick epidermis they were pycnotic.

95

Stratum germinativum: This layer may be subdivided into
(1) stratum cylindricum or page; gel; 1a er, composed of a
single layer of columnar or high cuboidal epithelium. The
cells of this layer were directly in contact with the dermis.
The cytoplasmic basal processes, which were often branched,
penetrated the underlying basement membrane (Plates V and VII)
and anchored the epidermis with the dermis. The desmosomes
and tonofibrils within the cytoplasm of the cells were
distinctly observed down in this layer (Plates V and VII).
Favre (1950) stated that the process of keratinization actually
began, although imperfectly, in the basal layer of the stratum
germinativum. The nuclei were elongated and paralleled the
long axis of the cells. Mitotic figures generally were
observed in thick epidermis but were seen less frequently in
the thin epidermis. The cells were laden with melanin pig-
ments in the epidermis of the eyelids, lips, nostrils, and
prepuce. In the general skin, the cells contained appreciable
quantities of pigments. The pigments usually were diffused
in the cytOplasm of the cells of this layer.

In addition to normal basal cells, dendritic melanocytes
were observed intermittently with the cells of the basal
layer. In ordinary preparation these cells did not take the
hematoxylin and eosin stain. On the basis of this character-
istic they are also termed clear cells. The melanocytes were
usually located horizontally. The basal portion of many of
these cells extended into the basement membrane (Plate XV).
The vertical processes projected into about half of the

stratum germinativum. The processes contained melanin

 

 

96
pigments which formed a bead-like configuration. At the end
of the dendritic processes the pigments tended to form a drOp
(Plate VI). The cytOplasm of the melanocytes lacked tono-
fibrils and the desmosomes were absent. 1

(2) Stratum spinosum4_Spinous layer1 or_prickle cell
iaygg: This layer extends from the top of stratum cylindricum
to the bottom of the stratum granulosum. The cells were
polygonal in shape with centrally placed, spherical or oval
nuclei. In thick epidermisthis layer was composed of as
many as 27 cells in the lips and nostrils, and in thin
epidermis like the pinna as low as three cells. On the
general body surface the thickness varied from 3-9 cells.

The cells adherred to each other by distinct desmosomes and
the cytoplasm was traversed by numerous tonofibrils (Plates
V and VII). Characteristic supranuclear caps were formed by
the melanin pigments in this layer (Plate VI). The caps were
more distinct in the heavily pigmented epidermis and in the
cells of the upper part of this layer. The significance of
such cap formation is not known. Their presence in such a
position in the cells might protect the nucleus and other
vital cytOplasmic organelles from injurious effects of the
ultraviolet ray of the sun.

Pigmentation: Pigmentation was observed in all skin
areas under investigation. The pigment granules were brown
and occurred within the cells of all layers of epidermis.
They occurred in abundance in the cells of the basal layers

and gradually decreased from the basal layer to the stratum

corneum. Within the cell cytoplasm, they assumed a

97
characteristic supranuclear position from the stratum
spinosum up to stratum granulosum (Plate VI and XV). They
were observed in the stratum corneum in more pigmented skin
such as lips, eyelids, and nostrils and were absent in
general skin areas. These findings agree with Sinha (1964)
and Goldsberry and Calhoun (1959). Continuing from the basal
epidermal cells pigment was abundant in the cells of the
basal layer of the external root sheath of the hair follciles
down to the opening of the duct of the sebaceous glands. The
wall of the ducts of the sebaceous and sweat glands also
contained some pigments. The skin of the upper lip, lower
lip, nostrils, upper eyelid, lower eyelid, prepuce and
circumanal region were heavily pigmented. The skin of the
general body surface wasmoderately pigmented. No difference
was observed in pigmentation of the skin between the dorsal,
lateral and ventral surfaces as was reported in cattle by
Goldsberry and Calhoun (1959) and Sinha (1964). A few cells
contained pigment granules in the dermis of heavily pigmented
areas.

Keratinization (cornification): The epidermis of all

 

the skin areas investigated showed keratinization. The pro-
cess is thought to occur due to transformation of tonofibrils
and the gradual formation of keratohyalin granules in the
cytOplasm of the cells of the stratum germinativum and stratum
granulosum (Montagna 1962). The keratohyalin granules of

the stratum germinativum and stratum granulosum finally
changed to keratin at the junction between the stratum

granulosum and corneum. The process of keratinization was

98
.more vigorous in areas where the thick cornified layer was
formed, such as the chestnut, ergot and coronary border.
This was evident by the presence of a thick layer of stratum
granulosum, the cells of which were laden with well formed
keratohyalin granules. The cornification process was also
quite active in all areas with thick epidermis, such as lips
and nostrils.

The formation of keratohyalin granules was tested by
Gomori's aldehyde fuchsin and the keratohylin granules were
stained with this method. This was confirmed by staining
the chicken gizard, which contains known keratohyalin gran-
ules. With the help of this stain the keratohyalin granules
were observed in the cells of the suprabasal layer of the
epidermis. The numbers increased rapidly from this layer
and filled the cells completely at the upper germinal layer.
These granules packed the stratum granulosum so completely
that the intercellular spaces barely could be seen. The
stratum corneum did not take the stain as it was free from
keratohyalin granules (Plate VII). This finding revealed
that the keratohyalin granules start forming in the cells
of the suprabasal layer. This is in disagreement with the
statement of Sinha (1964) that keratohyalin granules occur

only in stratum granulosum.

Dermis (Corium)

The dermis contains three types of connective tissue
fibers. The arrangement and pOpulation of the fiberous

components were different in the different parts. According

99
to the thickness and arrangement of the connective tissue
fibers, the dermis is divided into two layers--Stratum
papillare and Stratum reticulare. The junction between these
two layers was very distinct in the general body skin (Plate
IX). Usually, the layers were widely separated when the
skin was fixed in Petrunkevitch's cupric paranitro-phenol
fixative (Gurr, 1962) or when the sections were allowed to
float longer than normal on warm water and stained as usual.
Under these treatments, the fiberous components of the stratum
papillare in general were disorganized and loose, while the
stratum reticulare remained absolutely compact. This condi-
tion of disorganization and looseness of fibers in the
stratum papillare and compactness in stratum reticulare was
due to the different organization of fibers in the respective
layers.

Spgatum_papillare: This layer extends from the dermo-
epidermal junction to the subglandular region. This was the
narrower of the two layers of dermis and consisted of fine
collagenous fibers interwoven with elastic and reticular
fibers (Plates XIV and XLVIII). AS in cattle (Sinha 1964),
where the skin was thick and the hairs were scanty, dermal
papillae of various sizes and shapes were present. The
papillae were generally long and sometimes bifurcated, as in
the lips (Plate XXVI), abdomen (Plate IV), nostril (Plate VI),.
external ear (Plate XXIX), circumanal region (Plate XXX),
eyelids and prepuce.. The papillae contained loOpS of cap-
illaries. The superficial surface was uneven and perfectly

united with the epidermis, while the bottom surface was almost

100

even and imperfectly united with the reticular layer (Plate
IX). This layer contained the hair follicles, sebaceous
glands, sweat glands and arrector pili muscle. The arrector
pili muscles were attached with the superficial papillary
layer and the lower third of hair follicles by strong elastic
tendons (Plates XLVIII and XV). Skeletal muscle fibers in
bundles of 3-4 fibers or singly were present in this layer
in the nostril (Plate XLVIII) and lips (Plate XXVI). They
too were attached to the subepidermal part Of the dermis by
even stronger and thicker elastic bundles (Plate XII). The
ducts of sweat glands which often Opened directly on the
epidermal surface bisected the layer at its upper half (Plate
XXVII). Numerous blood vessels of different sizes formed
vascular plexuses in the subepidermal region Of this layer.

Stpptum reticulare: This layer was composed of coarse
collagenous fibers arranged in bundles. The bundles were
organized both vertically and horizontally in a definite
manner in the upper part and abruptly became disorganized at
the reticulo-papillary interface (Plates IX, X and XI).
They were very compact in the middle part and formed alter-
nating bundles at angles to each other and paralleling the
surface. This finding agrees with those of Montagna (1962)
for man. In the general skin areas, a subreticular layer
' was organized horizontally with medium Sized collagenous
fibers interwoven with coarse elastic fibers. The reticular
layer was comparatively poor in elastic fibers in its upper

part, whereas the lower part was very rich in elastic fibers.

101
This finding slightly differs with Schdnberg (1926), who stated
that the reticular layer in the horse was poor in elastic
fibers. Very large arteries and veins were present in this
layer (Plate X). They formed large vascular plexuses at the
lower part. Large bundles of nerve fiber were observed in all
the layers Of the dermis and pacinian corpuscles were seen in

the reticular layer Of the skin of the back.

Cellular Components:

Fibroblasts: The predominant cells Of the dermis were
the fibroblasts. They were most numerous and more baSOphilic
in the papillary layer than in the reticular layer. The cyto-
plasm was homogenous and slightly acidophilic. The nuclei
were mainly oval shaped and were strongly baSOphilic.
Occasional cells were branched with fine fibers attached
with the processes. The fibroblasts in the reticular layer
were less numerous and were compressed between the coarse
collagenous fibers. In the reticular layer the cells appeared
more elongated than the cells of the papillary layer. Cell
membranes were not seen clearly.

Mast Cells: They were Observed in the whole dermis but
were more numerous in the papillary layer. They were present'
in large numbers on the wall of the blood vessels in the
papillary layer Of the dermis. They were also present in the
subepidermal region and around the sebaceous glands. The
cells were irregular in outline, with a centrally placed

round nucleus. The cytoplasmic granules were abundant and

102

uniformly distributed. NO difference could be found in the
cells of the reticular and the papillary layer as was
observed by Sinha (1964) in cattle. Their distribution was
almost uniform in the areas under study.

Macrophages: In addition to those two types of cells,
the papillary layer contained macrophages with a round usually
acentrically placed nucleus. In a few cases, the macrophages
contained foreign particles in their cytoplasm.

Dermal Chromatophorep: These cells were common in the
areas adjacent to heavily pigmented epidermis. The cells .
contained granules in the cytoplasm. They were also Observed
on the walls of some blood vessels in the upper part of the

papillary layer and on the duct of sebaceous glands.

Fibrous Components
The dermis was composed Of collagenous elastic and

reticular fibers. All three components were present in both
stratum papillare and stratum reticulare. The sizes,
orientation, and the population of the type of fibers were
different in the different parts of the dermis.

Collagenous Fibers: These were the predominant type
throughout the entire dermis. They were fine and irregularly
arranged in the papillary layer. In the subepidermal region
the fibers became very fine and were continuous with the
reticular fibers, as if the reticular fibers branched Off from
the collagenous fibers (Plate XIV). This Observation was in
agreement with Robb Smith (1945). These fibers were inter-
woven with the elastic and reticular fibers and were distinct

in the stratum papillare (Plates XII, XIV and XLVIII). In

103
the reticular layer the fibers were very coarse and formed
prominent bundles. The fibers of the lower part of the reti-
cular layer of general areas were oriented parallel to the
surface (Plate X).

Elastic Fiber: The Skin of the horse in general was very
rich in elastic fibers. In the papillary layer they formed
wide nets between the hair follicles, on the wall of the
follicles (Plate XIII), in the subepidermal region, and
between the sweat glands. The fibers in the interfollicular
and interglandular not were medium sized and on the walls Of
the follicles and in the subepidermal nets were fine. Though
characteristically the elastic fibers were not in bundles,
their aggregations on the capsules of sebaceous glands (Plate
XIII), at the point of attachment with the arrector pili
muscle and skeletal muscles, presented a bundle like appearance
(Plate XII, XIII and XV). Fine branches were given Off from
the medium fibered interfollicular network to the hair follicles
where they were organized in both longitudinal and transverse
patterns along the long axis of the hair follicles. At the
middle third Of the hair follicle the fibers formed a thick
capsule-like sheath along with a few collagen fibers (Plates XII
and XIII). The areector pili muscles were attached to the
follicle as well as‘to the subepidermal region by means Of
elastic fibers. At both the ends Of the muscle the elastic
fibers branched in several subdivisions, covering a wide area
(Plates XV and XLVIII). At the follicular end of the muscle

(origin) the fine fibers joined the follicular capsules. At

104

the subepidermal end (insersion) the branching fibers
continued subdividing until they penetrated the basement
membrane (Plates XV and XXXI). Some of the very fine fibers
came in actual contact with the basal epidermal cells. The
elastic fibers encircled the ends Of the skeletal muscle
fibers in the lips and nostrils and branched Off to form a
brush-like tuft. These fibers extended vertically and joined
the subepidermal nets (Plate XII}. The fibers from the sub-
epidermal nets also branched Off and became finer and finer
tojoin the basement membrane (Plate XXXI). Occasional thick
elastic fibers were Observed in the upper and middle parts
Of the reticular layer. At the lower part of the reticular
layer of the general Skin areas, elastic fibers were in
abundance and paralleled the surface at an angle with each
other. This finding agreed with those of Sinha (1964), who
Observed a similar arrangement in cattle.

Reticular Fipgpp: These fibers were Observed mainly in
_the papillary layer. The fibers were frequently branched in
an acute angle. They were closely associated with the base-
ment membrane. Fine filamentous fibers branched Off from the
fibers of the basement membrane to come in close contact with
the basal epidermal cells (Plate XIV). Some of them appeared
to enter the intercellular spaces. They were present around
the hair follicles in a similar pattern. Reticular fibers were
also present on the wall Of blood vessels and cutaneous glands.

They did not occur freely in the reticular layer.

105

The Basement Membrane

The basement membranes of the dermo-epidermal interface
and of the hair follicles were very distinctly PAS positive.*
The basement membrane, on which the epidermis rests, was
present in all the Skin areas under study. The membrane
was thick where the epidermis was thick and thin with thin
epidermis. The PAS positive basement membrane surrounding
the hair follicles at their middle third was the thickest
of any in the skin (Plate XXI). The basement membranes of
the sweat and sebaceous glands were very thin. The area
occupied by a basement membrane at the dermo-epidermal inter-
face and that surrounding the hair follicles contained a
thick network of reticular fiber. When out obliquely those
fibers remaining close to the basal layer of the epidermis
gave an illusion of small button-like ends, projecting
towards the epidermis or remaining in the intercellular
spaces (Plate XIV). This was also the Observation of Dick
(1947) and Odland (1958). When the sections which were
stained by the PAS method, periodic acid-silver-orcin-
aniline blue, and by aldehyde fuchsin were compared, it was
apparent that the basement membrane was composed of a net-
work Of reticular fibers, and the meshes were filled by PAS

positive mucopolysaccharides.

Dermo-epidermal gunction

The basement membrane is one of the most important
structures at the dermo-epidermal junction. The minute

structural organization of which has just been'described..

106
In the first place, delicate cytOplasmic processes of the
cells of the basal layer entered into the basement membrane
where the processes gave Off short collateral branches or
divided further to form fine rootlets (Plates V and VII).
Probably the processes entered into the reticular meshwork
of the basement membrane first and then gave Off branches or
divided to form a bunch of rootlets at the end of each process.
Secondly, the elastic fibers from the dermis entered into the
basement membrane where they further divided and their terminal
branches arborized with the already rich reticular mesh and
cytOplasmic processes (Plates XV and XXXI). Finally, the
reticular fibers were also continuous with the fine
collagen fibers of the dermis.

In summary, the penetration of the cytoplasmic processes
into the basement membrane, the enmeshing Of elastic and
reticular fibers and the attachment of the reticular fibers
to the dermal collagenous fibers, all combined to give strength

to this attachment.

The Sebaceous Glands

Distributipn and Morphology: The sebaceous glands were
normally very large in the horse. With the exception of the
large tarsal glands, they Occurred in all areas under invest-
igation and Opened directly into the neck of the hair
follicles by a short duct. Usually there were two glands
with each follicle (Plate XVI) but as many as six in some
follicles. This was in agreement with Trautmann and

Fiebiger (1957), and Krolling and Grau (1960). Smith (1888)

 

llll'.‘i‘ll I! III

II) Iii-III: 1' In! I

107
observed two glands in association with one follicle. They
.both Opened on the sides Of the follicles, one on each Side
(Plates XVI and XLVIII). In case there were more than two,
the glands Opened in a series, one above the other, on both
sides Of the follicle (Plates XXII and 111). They were very
large at all the mucocutaneous junctions, prepuce, upper lips,
external ear canal and at the margin Of the hoofs. The
glands of the external ear canal and circumanal region were
lobulated and covered by a well developed capsule (Plates
XXIX, XXX). At the margin Of hoof and in the upper lip,
they were highly branched and each branch was lobulated
(Plates XXV, XXX). The glands with coarse hair were
extremely long and paralleled the long axis of the hair
follicle. .

Histologic Structure: The sebaceous glands of general
body areas were composed of 2-8 lobes and the special areas
mentioned above were highly lobulated with as many as twenty
lobes. All the lobes of a gland opened into a common short
excretory duct, which in turn discharged the contents of the
gland into the hair follicle. The wall of the ducts was cont-
inued with the external root sheath of the follicle and
contained two to four layers of cells. In the body of the
gland the cells varied in size. The cells in the periphery
were strongly baSOphilic and were cuboidal Shaped with a
round nucleus. Gradually the cells became larger and poly-
hedral near the center of the gland. They appeared foamy
towards the ducts, contained pycnotic nuclei and then

ruptured and disintegrated. Cellular debris was present

108

in the duct.

The Sweat Glands

Distribution and Morphology: The sweat glands were
present in the skin of the general areas of the entire body
surface and were well developed. There was always one gland
associated with each follicle and set deep into the dermis
below the level of hair follicles. They formed a continuous
layer just below the hair follicles in the flank and abdomen.
Smith (1888) observed such a formation in the flank. The
sweat glands on the posterior ventral side of the abdomen
were comparatively large (Plate IV). In the skin below the
mane, tail and fetlock they were long tubules with a very
small coiled secretory portion. The glands were very large
in the skin of the circumanal region (Plate XXX), external
ear canal (Plate XXIX), prepuce, nostril and in the lower lip.
A gland free zone in the lower lip was present for about 1 cm
from the mucocutaneous junction. The sweat glands were also
absent in the upper lip, around the margin of the hoof, (about
5.5 mm) and around the ergot (about 5.5 mm).

The sweat glands were normally simple tubular and the
tubules formed sac-like diverticula in the large sweat glands
of special areas (Plate XXX). In agreement with Trautmann
and Fiebiger (1957), the sweat glands in the horse were
glomiform. Thesecretory part of the tubule was never
branched but the diverticula from two adjacent coils were
often found united. This is in agreement with Montagna (1962).

The coiled ducts were much narrower than the secretory tubules.

109
after leaving the secretory coils the ducts often pierced
the arrector pili muscle in the general body skin. They
Opened either into the upper part of the follicle or on the
surface just adjacent to the follicular Opening (Plates XXX,
XXVII, and XLVIII). About one half of the ducts Opened
directly on the surface.

Hiptologig:§pructure: The secretory cells were in a
single layer, rested on a thin layer of myoepithelial cells
which in turn were on a thick basement membrane. This was
in agreement with Montagna (1962). A loosely arranged
connective tissue capsule was present. The secretOry
epithelium was columnar or high cuboidal with a round
nucleus placed at the base. The wall Of the cell facing the
intercellular surface was smooth and provided a somewhat
wider intercellular space. Infoldings were present at the
base of the cells and the folds interdigitated with the
underlying myoepithelial cells (Plate XXIII). Intercellular
canaliculi were present at the basal part of the cells but the
intracellular canaliculi or secretory canal mentioned by
COpenhaver (1964) in eccrine glands was not present (Plate
XXIV). The lumen of the secretory tubules contained PAS
positive secretory substances.

Four different modifications of the secretory cells
were observed: 1. Some glands of the general and special
areas contained an irregular luminal surface due to an uneven

projection of the apical end of a few cells. These apical

projections gave a pinched-off appearance. With the use of

110
frozen section and phase micrOSOOpy, these apical ends were
found intact and were thought to be artifacts (Copenhaver,
1964). 2. In the general skin areas, the luminal border of
the secretory cells contained a terminal web (Hay, 1966) with
scattered microvilli (Plate XXIV). 3. The free edges of the
secretory cells of some large glands were provided with
terminal web (Hay 1966) and a brush border. The brush border
was in such a form that the entire liminal surface formed a
continuous brush border (Plate XXIII). Takagi and Tagawa
(1959) reported the presence of a brush border in the Sweat
glands of the horse. According to Montagna (1962), the
luminal surface which consisted of a brush border was composed
of microvilli. 4. In tubules with wider lumens, the cells
were low cuboidal to even squamous type.

The secretory ducts were lined by stratified cuboidal
epithelium. There were no myoepithelial cells on the walls
of the duct. ,

NO glycogen was present in the cytoplasm of the secretory
cells or the myoepithelial cells. According to Evans S£.§l
(1957) sweat glands of the horse as well as the eccrine
glands contained glycogen. Takagi and Tagawa (1959) did not
find any trace of glycogen in the sweat glands of horses
except in the myoepithelial cells. The secretory cells
contained fine PAS positive granules particularly accumulated
at the luminal ends. The terminal web in the luminal border
of the cells was also PAS positive. PAS positive granules

were abundant in the lumen and had a tendency to adhere to the

111
terminal web of the cell border.

Functionally the sweat glands of the horse belonged to
the apocrine group as revealed by histologic and histo-
chemical studies, but differed from the apocrine glands of man
and other domestic animals. The typical apocrine mode of
secretion was present. In addition it is possible that some
secretion Occurred by the exudation Of liquid materials
through microvilli or pinching-off of the ends of microvilli.
Furthermore for profuse sweating in the horse perhaps the
basal canaliculi add the watery part Of the sweat through
intercellular Spaces (Plate XXIV). These dual secretory
activities of the sweat glands Of the horse has made them
so efficient that they function like both an apocrine and
eccrine gland in one unit. This prOposal of the secretory mode
of the sweat glands of the horse is based on the findings of
this investigation and the thoughtful suggestions Of
Montagna (1962) and Copenhaver (1964).

SPECIAL AREAS

Group and adjacent areas: This area of the dermis
is modified in the horse and extends caudally from the middle
of the back to the posterior end of the croup and laterally
from the croup covering most of the gluteal region. This
area according to Schgnberg (1926) becomes shiny and reflects
images on tanning and he termed the area "Spiegel" (mirror).
Odoni (1951) reported this area as extremely thick in the
horse in comparison to donkey and mules.

A different histologic feature distinguished this area
from other parts of the dermis. The dermis was very compact
in all the animals and the collagenous fibers were oriented .
in three distinct patterns in the reticular layer (Plate X).
Fibers in the upper part were arranged both vertically and
horizontally, the the middle part in all directions and in
the lower part parallel to the surface. Though this was the
normal arrangement Of the fibers in the dermis of all general
areas the arrangement in this area was particularly striking.
The thickness Of the total dermis reached its peak in the
skin of croup (5.5 mm) and gradually diminished anteriorly
and laterally but increased towards the tail accompanied by
diminished compactness of the dermis (Table IV). The thick-
ness of the dermis of the croup compared favorably with the
findings Of Odoni (1951). The develOpment of the papillary

and reticular layers appeared prOportionate with the other

general skin areas. A separate measurement of the reticular

112

113
and the papillary layer Showed that the reticular layer was
the thicker part. This finding was in agreement with Odoni
(1951). The hairs were mostly of uniform size and the seba-
ceous glands were well developed. The arrector pili muscles
were well develOped usually with branching ends. The indi-
vidual muscle occupied a large area for its attachment at the
. dermo-epidermal junction (Plate XV). The sweat glands were
also well develOped. The elastic fibers were much more
numerous than in any other part of the general body skin,
probably for the diffuse attachment of the arrector pili
muscles.

External Ear: Both the epidermis and dermis were very
thin on both surfaces of the pinna. The thickness increased
gradually towards the external ear canal and at the base of
the ear the skin in general became thick. There were prom-
inent epidermal pegs and dermal papillae. The hairs were
very fine and scattered. The sebaceous glands were highly
developed and lobulated. The whole unit Of the gland sur-
rounds the neck of the associated hair follicle in most
cases. A compact, moderately thick connective tissue cap-
sule covered the glands and thus the lobules were packed
together. Delicate connective tissue septa extending from
both the surrounding capsule and the connective tissue sheath
of the follicles separated the sebaceous lobules. NO seba-
ceous glands Opened on the surface. The sweat glands were
extremely large and were located deep in the dermis. The

secretory tubules were highly tortuous and clustered around

11a
the root of the hair follicles (Plate XXIX). The canali-
cular system and the brush borders were similar to those in
the general body sweat glands. Prominent myoepithelial cells
were present in the secretory tubules only. The ducts
usually passed through the sebaceous glands and Opened on
the surface adjacent to the pore of the hair follicles. The
epidermal cells were pigmented. The pigmentation extended
over the neck of the follicle and on the wall of the ducts
of sebaceous glands. Numerous arterio-venous anastomoses
with a regular glomus formation were observed. This is in
agreement with Danial 33 al. (1956).

Circumanal region: In this area the epidermis was very
thick (260u) but the cornified layer was not as prominent as
that of other thick epidermal areas. The epidermal pegs and,
the dermal papillae were well developed. The hairs were
very fine whereas the associated sebaceous glands were large
and lobulated. The lobules were well apart and converged
to the neck of the follicle to join the common duct. The
dermis was in direct contact with the adjacent skeletal
muscle bundles. The sweat glands were mostly a large sac-
cular type (Plate XXX). A few small sized tubular glands
were also present. The secretory cells in the saccular type
of glands were low cuboidal to squamous. Evidence of a
canalicular system was not seen in these glands, whereas in
the tubular glands the canalicular system and brush border
were present. The lumen of the secretory tubules and sac-

cules contained abundant secretory materials. The ducts

115
often Opened directly on the surface (Plate XXX). Smooth
muscle bundles were very prominent in the papillary layer.
The epidermis was heavily pigmented and the hair follicles,
sebaceous ducts and even the dermis contained pigment gran-
ules. Few pigments were found on the wall of the secretory
tubules of the sweat glands.

The dorsal neck and_£he tail: The histologic feature
of the areas under cover Of the long, coarse hairs, was
similar. In both areas the hairs penetrated deep into the
dermis but never crossed the entire width. The skin surface
was wavy and covered by a thick cornified layer. The corni-
fied layer in both sites was formed Of threadlike cornified
cells and occurred in a pattern that gave the appearance of
a net when loosened. Most of the long coarse hairs were of
uniform size. In cases where the hairs were closely distribu-
ted, a very small Space was left in between for the passage
of the arrector pili muscle and the cutaneous glands. Two
elongated sebaceous glands were associated with each fol-
licle. The sweat glands were the smallest of the body.

They were composed of a small secretory part with a long
duct, tortuous below and straight towards the epidermis.
Usually the ducts were associated with the arrector pili
muscle and penetrated the muscle at several points giving
the idea that the muscle supports the sweat ducts. The
ducts Opened in the upper part of the neck of the follicles
or on the surface close to the follicular opening. The
arrector pili muscles were long and slender. xUsually there

was one branched muscle associated with each follicle. In

116
some cases two muscles were also Observed. The individual
muscle was up to 2 mm in length. The reticular layer of the
dermis of the root of the tail contained sheets Of collagenous
fibers oriented parallel to the surface while the other areas
on the neck or tail did not have such a layer. The fibers
Of the reticular layer were very irregularly oriented. The
skin in these areas was 6-6.3 mm in thickness and the epider-
mis was about 80 microns on an average.

Eyelids: The skin of the eyelids were moderately thick
(about 3.00 mm) whereas the epidermis was thicker (about 100
microns in both the lids) with branched epidermal pegs. The
epidermis was heavily pigmented with densely packed clear
cells in the basal layer. The pigments were also present in
the dermis. The skin was covered by fine hairs and provided
with large branched sebaceous glands. Sinha (l96h) also
observed a similar condition in cattle. The sweat glands-
were both tubular and saccular types with secretory materials
in the tubules. The eyelashes were coarse hairs rooted deep
into the dermis. The accompanying secabeous glands were
also large. The dermis was provided with bundles Of smooth
muscle fibers (Mullers muscles) scattered throughout.
Scattered skeletal muscle fibers were observed deep in the
dermis. Tarsal glands, the largest modified sebaceous glands
in the body, occurred in rows in both lids. They were covered
by a thick connective tissue capsule and the lobules were
separated by distinct connective tissue septa derived from

the capsule. The sebaceous lobules joined at the center of

117
the glands to Open into a commun duct. The duct Opened
on the conjunctival surface near to the junction of epider-
mis with the conjunctiva.

Chestnut and ergot: The chestnut of both the fore and
hind limbs and the ergot Of the fetlock had similar histol-
ogic features. The epidermis was extremely thick with a
very thick cornified layer (Plate XXXII) which formed the
horny tissue of the organ. With the exception Of the stratum
lucidum, the other layers of the epidermis were present. The
stratum cylindricum formed the basal layer and the border Of
the long dermal papillae. The Often branched dermal papillae
were highly vascular and the lOOps Of the capillaries extended
to the tip Of the papillae. The cells Of the interpapillary
zone were polyhedral and contained large keratohyalin gran-
ules in the upper part. Horny tubules were formed regularly
above the dermal papillae. The stratum granulosum was very
thick (Plate VIII). The cornified cells near the stratum
granulosum had distinct intercellular spaces without inter-
cellular bridges. The dermis below the horny structure of
the chestnut was one Of the thinnest (2.5 mm) in the body
while the dermis below the ergot was one of the thickest.

The dermis usually contained a few arterio-venous anastomoses.
‘ Arterio-venous anastomoses were Observed just below the der-
mal end of the papillae. Clara (1956) reported arterio-venous
anastomoses in the papillae of the hoof. The sebaceous glands
surrounding the ergot were highly develOped and branched. A

zone of about 5.5 mm which was free from sweat glands was,

118
present around the ergot.

Coronarygborder: This is a well demarcated border
between the hoof margin and the skin. It consisted Of horny
and hairy parts. The epidermis from the hairy part gradually
thickened near the junction with the horny part. The cor-
nified layer, epidermal pegs and the dermal papillae were
also increased in thickness and length towards the border
line. At the border line the cornified layer changed into a
massive compact horny tissue; the epidermal pegs became the
interpapillary epithelial tissue and the dermal papillae
became the regular papillary body. Horny tubules appeared
at this point. The border line was demarcated by coarse,
long hairs. The follicles Of these hairs were in groups of
3-5 (Plate XXXIII) but erupted on the surface singly. The
sebaceous glands in the hairy part were normally large and
lobulated. The sebaceous glands gradually became larger and
larger towards the border line where the glands were extremely
large and highly branched (Plate XXXIII). They were one or
two glands thick and occurred on the horny side of the border
line. Sweat glands were absent for about 6 mm dorsal to the
border line and the coarse hairs Of the border were without
associated sweat glands. In agreement with Schummer (1951,
cited by Clara, 1956) few arterio-venous anastomoses were
present.

Prepuce: The thick epidermis was about 245 microns.

The epidermal pegs were very deep and branched. The dermal

papillae accordingly were tall with loOps Of blood vessels.

119
The basal cells were heavily pigmented (Plate XXXI) with a
few clear cells in between. The dermo-epidermal junctiOn
was very rich in fine elastic fibers. These fibers were
derived from the branching of vertically oriented sub-
epidermal elastic fibers (Plate XXXI). The hairs were thinly
distributed accompanied by a large sebaceous gland. The
sweat glands were saccular and placed in a row. The arrector
pili muscles were well developed. Prominent arterio-venous
anastomoses were present in the dermis.

Nostrils: The epidermis Of the nostrils was very thick
(about 300 microns) and well develOped epidermal pegs were
present. The stratum granulosum and corneum were fairly
thick. The basal layer Of the stratum germinativum was
heavily pigmented while the cells of the upper layer were
moderately pigmented (Plate XXVII). The dermal papillae
contained capillaries. The hairs were fine but were in large
numbers with moderately large sebaceous glands. Very large
tactile hairs were present throughout. The arrector pili
muscle and sweat glands were medium sized. Most of the
sweat glands were tubular and their ducts Opened on the
surface close to the pore of the hair follicles (Plates XXVII,
XLVIII). A few saccular glands were also present deep in
the dermis. The upper part Of the dermis was very rich in
elastic fibers and just below the dermo-epidermal junction
formed a thick elastic net. The central core formed by the
<iermis of both sides contained large skeletal muscles, bun-

ciles of collagenous fibers as well as fat (Plates XXVIII).

120
Small bundles Of skeletal fibers were detached from the
central large bundles to insert in the sub-epidermal region
by means of elastic tendons (Plates XXVIII, XLVIII). Arterio-
venous anastomoses were present in the upper dermis with a

regular glomus formation.

The Lips
Upper lips The upper lip had the highest epidermal

thickness in the body (750 microns) with a thick stratum
corneum and stratum granulosum. Epidermal pegs were well
develOped and branched. Tall dermal papillae, sometimes
branched, filled the space between the epidermal pegs. The
basal cells were moderately pigmented and pigments were also
present in the cells of the upper layer.

The hairs were fine to medium sized and were distributed
fairly close to each other. Large numbers of tactile hairs
were present and scattered among the common hairs. They were
somewhat smaller than the tactile hairs of the nostrils.

Each follicle received at least two sebaceous glands. The
sebaceous glands were usually branched and very large near
the mucocutaneous junction(Plate XXV). There were no sweat
glands in the upper lip. The dermis merged with the under-
1Ping thick layer Of skeletal muscle. Small bundles or
Single skeletal muscle fibers detached from the underlying
Imascle and inserted in the upper part of the dermis‘in both
the lips by an extremely thick elastic tendon (Plate XII).
The upper lip was perhaps the richest in elastic fibers of

‘&11.the areas studied. Probably this rich elastic component

 

 

 

ill Illllllllllll‘lllllllllui|| III 1

121
in the upper lip aids the animal in using it as the organ
of prehension. The upper lip was rich in arterio-venous
anastomoses. A few arterial cushions were evenly distributed
in the upper part of the dermis. The arrector pili muscles
were rudimentary.

nger lip: In general, the histology of the lower lip
was similar to that of the upper lip and nostrils except for
the few differences noted below.

l.‘ The average thickness of the epidermis was only 278

microns.

2. The skeletal muscle fibers extended in all directions
in the upper part Of the dermis (Plate XXVI).

3. There were saccular type sweat glands which Often
were very large in the region about 1 cm posterior
to the mucocutaneous junction.

The presence Of a large number of arterial cushions in
both the upper and lower lip aids in distribution of red
cell-rich blood to the local tissue. With the higher concen-
tration of red blood cells the blood naturally becomes
oxygen rich. This coupled with the rich deposits of glycogen
in the follicular cells of the lip might enable the horse to
meet the excess energy requirement for active use Of the lips
as the organ Of prehension.

Nerve supply of the lipg; Innervation of the lips was
demonstrated by the Bielschowski Gross method, a modification
by Devenford (Lillie 1965). Three plexuses of nerve trunks

were recognized in the lip. That first plexus was Observed

122
below the dermal end of the hair follicles. The nerve trunks
in this plexus were very thick with medulated fibers.
Medium-sized nerve trunks leaving this plexus reached the
space formed between the lower part Of the sebaceous glands
and hair follicles. At this point the trunks divided in
most cases and gave Off 2-3 branches (Plate XLIV). One Of
these branches entered between the gland and follicle and
the other two passed over the capsule Of the sebaceous glands.
The interfolliculo-glandular branch in many cases gave Off
many branches and some of them ended in round or slightly
elongated formed organs in close approximation to the fol-
licle (Plate XLIV). The others innervated the hair follicles.
The second plexus was formed in the subepidermal corium, about
the level of the neck of the hair follicles (Plate XXXIX).
A few branches from this plexus innervated the neck Of the
hair follicles. Numerous organized and free endings were
observed in this part of the dermis. Fine nondmedullated‘
fibers formed networks just below the dermo-epidermal junc-
tion. Many Of the fibers extended horizontally along the
junction and others entered into the epidermis. Their course
could not be followed above 5-6 cell layers. The third
plexus was in the dermal papillae. Smith (1888) also
Observed a similar plexus in the dermal papillae of horsefs
lip. Fibers, both in bundles and free, occurred in the
papillae (Plate XXXIX). Many fine fibers extended vertically
on the edge of the dermal papillae, reached the tip of the

papillae and entered the germinal cell layer of the epidermis,

123
where their course could not be followed further. Nerve
fibers on the wall of blood vessels were also Observed in
large number.

Innepgation of hair follicles: The hairs of the mamma-
lian skin are important organs of sensory perception. As
an organ of tactile perception,their pattern of innervation
suited them very well. According to Winkelmann (1959),
.nerve fibers supplying the hair follicles varied from 5-12.
The larger follicles had large numbers of fibers and the
smaller ones less.

The follicles were reached by fibers from the part of
the dermis below the loweat part of the sebaceous glands.
Thick myelinated fibers reaching the follicles divided and
formed a very distinct network. In this network one set Of
fibers extended longitudinally along the axis of the fol-
licle and another set encircled the follicles. The fibers
were in very close touch with the cells of the external root
sheath. According to Winkelmann (1959), the fibers Of this
network were non-myelinated and the inner set was arranged
longitudinally and the outer circular. Some of these fibers
appeared to terminate in the follicles and others branched
repeatedly along the hair axis. The fibers of the latter
group further subdivided and formed bushy terminal ends in
close contact with the walls of the cells of outer root
sheath (Plate XLVII). This Observation is in agreement with
Weddell and Palli (l95h), Singer and Salpeter (1966),
COpenhaver (l96h), Dixon (196u), and Winkelmann (1959).

Some Of the fibers from the subdermal plexus also innervated

124
the hair follicles.

Innervation of tactile hair follicles was similar to
that of the general hairs. Three to seven large nerve trunk
from the lower part of the dermis supplied the hair (Plate
XLIII). They crossed the sinus of the follicle through the
trabeculae to the inner connective tissue sheath of the fol-
licle. They extended vertically over the sheath until they
reached the neck where they innervated the external root

sheath of the follicle.

Crganized Nerve Endings

A large variety of nerve endings was Observed in the
lips of the horse. It was very difficult to assign each of
them morphologically to specific modalities. Smith (1888)
observed small pacinian-like corposcles in the lip of the
horse and a similar Observation was confirmed by Sinha (1964)
in cattle. It is the belief of many authors, including
Montagna (1962), that the morphological differences in nerve
endings depends on their location in the skin. According to
Montagna (1962), the number of follicles and the number Of
organized end organs were in inverse ratio. They were more
in the skin with less hairs or without hair. As cited by
Montagna (1962), Winkelmann observed that the nerve nets
in the skin without hair may give rise to a Spherical
mass simulating mucocutaneous end organs.

The nerve endings that were Observed in the lips of the
horse may be classified into four main groups: (1) Lamellated

endings (Plate XLVI), (2) Capsulated endings (Plates XLV,

125
XLII. XLI, XL), (3) Non-capsulated end balls (Plate XLIV),
and (A) Fine free nerve endings of the upper dermal papillae
and epidermis.

gamellated endings: These endings were not very common.
They occurred in the upper dermis. The endings were pump-
kin shaped. They were covered by a thin capsule of one cell
thick. Within the capsule there was a lamella of one cell
thick consisting of epitheloid cells. The nerve fibers were
at the central axis (Plate XLVI). Only one nerve fiber
entered the narrow end of the organ. NO filamentous breakup
was observed.

Capsulated end organs: They were the most numerous in
the dermis. They were usually large below the level of seba-
ceous glands and became smaller towards the surface. At
least four different forms were Observed. (a) Well formed
oval shaped organs (Plate XLV). This type was covered by a
one cell thick capsule. One thick myelinated nerve fiber
entered the organ and divided rapidly to form fine networks.
The networks, at points within the organ condensed to form
numerous round or oval shaped bulbs. The bulbs and the net-
works contained a central core. (b) Small oval shaped Organs
(Plate XLI) were the most numerous. They were smaller near
the epidermis. The distribution Of fibers within the organ
was the same as described under (a). The capsule was very
thin. (c) Capsulated disk (XLII). These were few and usually
occurred just below the epidermis. The fiber entering the

organ broke up into filaments which formed a disk.

126

(d) Paired capsulated ending--only one of the organs Of this
type was noticed below the level of sebaceous glands. PHI
outer capsule more than one cell thick was continuous around
the neural elements. The capsule thickened at the dorsal
and ventral sides to give connection with a centrally placed
septum 2-3 cells thick (Plate XL). The neural elements were
like the capsulated organs discussed under Capsulated End
Organs.

Fpee nerve endings; These were present in the dermis
close to the epidermis. Fine fibers penetrated the basal
5 or 6 cells layers of the stratum germinativum. In the
dermal papillae they were present along the border. They
extended little further from the tip of the papillae to the
to the surface.

Non-capsulatedggll: This type was seen along the hair
follicles and in the upper part of the dermis. One to three
fibers branched and formed a glomus-like ball. The branched

fibers formed a ball at the end (Plate XLIV).

TABLE I

AVERAGE MEASUREMENT OF TOTAL SKIN THICKNESS IN MM

MF

15*

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Forehead

127

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133
TABLE IV

DERMAL THICKNESS OF GROUP IN MM AND ITS COMPARISON
WITH ADJACENT SKIN AREAS

 

 

Back Croup Gluteal

(Post. Thoracic) Region
Stallion
Pap. Layer ZOO 2.5 202
Ret. Layer 2.} 2.2 2.2
TOtal 4.3 5.1.), 501
Geldings
Pap. Layer 2.2 2.b 2.1
Bet. Layer 2.0 2.5 2.5
Total 5.2 5.9 4.6
Mares
Pap. Layer 200 2e5 1e8
Bet. Layer 2.1 2.6 1.2
Total 4.1 5.1 3.7

Av. of all animals h.6 5.5 h.5

134

TABLE V

SEX DIFFERENCES IN SKIN THICKNESS IN MM

Stallion*

Upper lip
Nostril
Upper eyelid
Lower eyelid
Forehead

External ear
Base of the ear
Dorsal neck mane
Dorsal neck mane
Back

Group

Root Of tail - D
Tip Of tail
Lower lip

Chin

Submaxillary region
Lateral neck - C
Ventral neck - C
Brisket

Lateral neck - B

Chest wall

Flank

Gluteal region
Circumanal region
Thigh (lateral)

Thigh (medial)

Abdomen (posterior)
Thorax (posterior-ventral)
Thorax (ventral-anterior)
Brachium

Average

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Plate I

General skin area.
1. Epidermie. 2. Pile-sebaceous canal. 3. Opening of

ebaceous duct. 4. Sebaceous d . r f llicle

. Hair ehowi cortex and medglI2.'7§ FEIIicu ar fold.
. Arrector pi 1 muscle. 9. Opening of sweat duet at the
neck of the-follicle.

Vertical section, B & E Stain. X240

136

 

Plate II

Lateral surface of the thigh.

1. Surface integumentary ridges. 2. Core of integumentary
ridges filled by dermis. 3. Integumentary valleys or grooves.
4. Two edges of the epidermis on the side of integumentary
ridges. 5. Hairs erupt in the valleys in rows. 6. Summit

of the ridges.

Horizontal section, B a E stain. X60

 

Plate III

Skin of the abdomen.
1. Epidermis. 2. Pile-sebaceous canal. 3. Three sebaceous
glands open in one follicle. 4. Thick arrector pili muscle.

Vertical section, H a E Stain. X250

138

 

Plato IV

Skin of the abdomen.

1. Epiderlis. 2. Hair follicle. 3. Sebaceous gland.
4. Large sweat gland deep in the dermis.
Note: very few hair follicles.

Vertical section, H & E Stain. x60

139

 

Plate V

Dermo-epidermal junction, nostril.
l. Suprabasal region of epidermis with greatly increased

keratohyalin granules. 2. Desmosomes. 3. Tonofibrils.
4. Epidermal basal processes with collateral branches.

5. Basement membrane. 6. Elastic fibers.
Vertical section, Gomor's aldehyde fuchsin stain. X2450

 

Plate VI

Skin of the nostril showing melanocytes and their
processes.

1. Melaneoyte. 2. Dendritic processes of melanocyte.
3. Beaded afipearanoe of melanin pigments in the
processes. . Melanin pigments forming a drop at the
end of the processes. 5. Melanin pigments in the basal
cells. 6. Dermal papillae. 7. Supranuclear cap.

Vertical section, B a E Stain. X610

141

 

Plate VII

The skin of the nostril.

1. Stratum corneum. 2. The epidermal basal processes.
3. Basement membrane.

(Note the gradual increase of keratohyalin granules
from the basal layer to the stratum granulosum).

Vertical section, Gomori's aldehyde fuchsin stain. x1000

142

 

Plate VIII

The chestnut, showing the junction of stratum corneum
and stratum granulosum.

l. Stratum corneum. 2. stratum granulosum.

Vertical section, B e E Stain. x250

[I'lll'l‘l'v‘I-III‘I'}

143

 

Plate Ix

The skin of the rsgion of the croup.
l. Papillary layer. 2. Reticulo-papillary junction.
3. Betioular layer.

Vertical section, B e E Stain. 150

144

 

Plate X

Beticular layer of the croup, showing the three different
arrangements of the collagen fibers.

1. Junction between reticular and papillary layer.

2. Vertically and horizontally arranged bundles of
collagenous fibers. 3. Thick irregularly arranged

fibers in the middle layer. 4. Fibers arranged parallel
to the surface. 5. Subcutis.

Vertical section, B s E Stain. x200

 

Plate XI

Upper reticular layer of the croup, showing the collagen
bundle arrangement and the detail of the individual fiber.
1. vertical fibers. 2. Horizontal fibers. 3. Longitudinally
arranged fibrils within the fibers.

vertical section, B s E Stain. x620

146

 

Plate XII

The chin (level 15), showing the attachment of individual
skeletal muscle fibers with the superficial dermis by a
strong bundle of elastic fibers.

1. Hair follicle. 2. Sweat duct. 3. Sebaceous gland.

4. cross and longitudinal elastic fiber network on the
follicular wall. 5. Arrector pili muscle. 6. Sweat gland.
7. Skeletal muscle fiber. 8. Elastic fiber bundle.

Vertical section, Gomori's aldehyde fuchsin stain. X250

 

Plate XIII

General skin, showing the arrangement of the elastic
fibers in the papillary layer and the attachment with
hair follicle. -

1. Network of elastic fibers. 2. Attachment of elastic
fibers with hair follicle. 3. Hair follicle. 4. Sebaceous
glands. 5. Capillaries.

Horizontal section, Gomori's aldehyde fuchsin stain. X310

148

 

Plate XIV

Upper lip, showing the reticular fibers.

Pine reticular fibers which have broken up and formed

a network at the dermo-epidermal interface. Fine fibers
are seen in close contact with the cells of the basal
layer.

1. Heticular fibers. 2. Region of basement membrane with
reticular mesh.

Vertical section, periodic acid, orcein, silver and
aniline blue.~X700

 

Plate XV

The skin of the croup, showing the branched arrector pili
muscle and its attachment with the epidermis by elastic
fibers.

1. Epidermis. 2. Region of basement membrane. 3. Branching
elastic fibers 4. Arrector pili muscle. 5. A clear cell.

Vertical section, H a E Stain. X510

150

 

Plate XVI

Skin of the back showing different sizes of hair
follicles. Large hair follicles scattered among

the .all hair follicles.

1. hall hair follicles with at least two sebaceous

lands. 2. Large hair follicles. 3. Duct of sweat gland
cross section). 4. Sebaceous glands. 5. Duct of sebaceous
GWIe

Horizontal section, H a E Stain. 150

151

 

Plate XVII
Skin of the abdomen, showing a thin hair distribution.

a I. s Stain. x65 ‘

Horizontal section,

152

 

Plate XVIII
Hair and hair follicle.

l. Hedulla. 2. Cortex. 3. Hair cuticle with serrations
directed upward. 4. Inner root sheath. 5. External root
sheath. 6. Connective tissue sheath.

Note the desmosomes in the medullary cells.

Vertical section, H a E Stain. X950

153

 

.
I

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Plate XIX

Hair follicle, showing follicular folds.

l. Follicular folds. 2. Inner root sheath. 3. External

goat sheath. 4. Sebaceous gland. 5. Arrector pili muscle.
e 3811‘s

Vertical section, H a E Stain. x250

154

 

Plate XX

Hair follicle, showing the strong reaction for glycogen
in the cytoplasm of the cells of external root sheath.

No cytoplasmic glycogen in upper or lower part of the
follicle.

1. Hair bulb. 2. Cells of external root sheath. 3. Connec-
tive tissue sheath. 4. Sweat gland.

Vertical section, Bauer-Peulgen reaction for glyOOgen. 1170

 

- -.. ’1’ '
Plate XXI

 

A--PAS Reaction. x690

 

B--Bsuer-Peulgen reaction for glycogen. X690
1. Inner root sheath. 2. External root sheath. 3. Basement
membrane. 4. Connective tissue sheath.

156

 

Plate XXII

Hair follicle (submandibular region). Opening of 3 sweat
glands in one follicle.

l. Connective tissue sheath. 2. External root sheath.

3. Hair. 4. Ducts of sebaceous glands. 5. Disintegrating
cells in sebaceous glands. 6. Duct of sweat gland.

7. Arrector pili muscle.

Vertical section, H s E Stain. X220

157

 

Plate XXIII

Apocrine sweat gland (circumanal region).
1. hierovilli. 2. Myoepithelial cell. 3. Secretory cells.
4. Basal epithelial folds. 5. Lumen.

Cross section, H & E Stain. X2500

158

 

Plate XXIV

Sweat gland (general body region).
1. Secretory cells. 2. hyoepithelial cells. 3. Basal inter-
cellular canaliculi. 4. Hicrovilli.

H & E Stain. X900

159

 

Plate XXV

Skin of the upper lip.

1. Hair follicle with fine hair. 2. Number of branched
sebaceous glands open into the follicle.

Hots. Absence of sweat gland.

Vertical section, H a E Stain. X50

160

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Plate XXVI
Skin of the lower lip.
1. Hair follicle with fine hair. 2. Sebaceous gland.
3. Skeletal muscle fibers. 4. Sweat gland.

Vertical section, H a E Stain. X65

161

 

Plate HVII

Skin of the nostril.

1. Stratum corneum. 2. Stratum granulosum. 3. Stratum
germinativum. 4. Basal layer of cells with melanin pigment.
5. Sweat duct Opening on the surface of epidermis.

6. Epidermal pegs. 7. Dermal papillae. 8. Sebaceous glands.

Vertical section, H a E Stain. X270

162

 

Plate XXVIII

Skin of the nostril, showing the arrangement of the
skeletal muscle in the upper dermis.

1. Skeletal muscle fibers attached at the upper dermis.
2. Sebaceous gland. 3. Sweat gland. 4. Thick bundle of
skeletal muscle in the deep dermis extending in 3 planes.

Vertical section, H s E Stain. X70

 

Plate XXIX

Skin of the external ear.

1. Epidermis. 2. Hair follicle. 3. Large sebaceous gland.
4. very large saccular sweat glands deep in the dermis.
5. Hair bulb with dermal papilla.

Vertical section, H a E Stain. X60

 

164

 

Plate XXX

5. Hair follicle. 6. Large

saccular sweat gland. 7. Opening of the duct of sweat

gland.

l. Epidermis. 2. Dermal papillae. 3. Epidermal pegs.

Skin of the circumanal region.
Vertical section, H s E Stain. X60

4. Lobulated sebaceous gland.

165

 

XXXI

showing branched elastic fibers

Plate
attached to the basement membrane.

Skin of the prepuce,

1. Basal cells containing melanin granules. 2. Elastic
fibers at dermo-epidermal junction. 3. Area occupied by

basement membrane. 4. East cells.

vertical section, Gomori's aldehyde fuchsin stain. X620

166

 

Plate MII

Chestnut (hind limb).

l. Dermal papillae. 2. Peripapillary epithelium. 3. Inter-
papillary epithelium. 4. Stratum granulosum. 5. Stratum

corneum. 6. Horn tubule.

Vertical section, H s E Stain. X75

        

65.21 a? up“,

Plate XXXIII

‘1

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its» I

Coronary border.

1. Duct of highly branched sebaceous glands. 2. Very
large branched sebaceous glands. 3. A row of hairs in the
border. 4. Arrow points towards the hoof.

Vertical section, H s E Stain. X60

168-

 

Plate XXXIV

Arterio-venous anastomoses (upper lip).
l. Epitheloid cells. 2. Central lumen. 3. Arteraole at
opening of the connecting channel. 4. Venule.

Cross section, H s E Stain. X960

 

Plate XXXV

Longitudinal section of an arterio-venous anastomoses
in the skin of the croup.

1. Hair follicle with hair. 2. Arterio-venous anastomosis.
3. Epitheloid cells on the wall. 4. Arteriole. 5. Venule.

H s E Stain. X660

 

Plate XXXVI

Glomus formation (lower lip).
l. Artery. 2. Venule. 3. Modified wall of coiled arteriole
in the glomus cut in several cross sections.

H a E Stain. X260

 

Plate xxxv1 I

Cross section of an artery containing arterial cushion
(upper lip).

l. Cushion. 2. Lumen of the vessel. 3. Epitheloid cells.
4. Smooth muscle cells in the wall of the vessel.

H & 8 Stain. X1060

172

 

Plate XXXVIII

Arterial cushion cut in cross section, showing arteriole
leaving the main artery from the raised central part of
cushion (thigh, lateral surface).

1. Cushion. 2. Arteriole leaving the main lumen through
the cushion. 3. Epitheloid cushion cells. 4. Endothelium
of the arteriole. 5. Nerve.

H 8: E Stain. X1350

173

 
 

 

  

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Plate XXXIX

Skin of the upper lip.
Plexus of nerve fibers in the superficial dermis and the

innervation of dermal papillae.

1. Thick nerve fiber bundle in the papillae. 2. Fine nerve
fibers in the papillae. 3. Nerve plexus in the upper part
of papillary layer of the dermis.

Vertical section, Bielschowski-Gros. X240

17a

 

Platen

Paired encapsulated nerve endings (upper lip).

1. Capsule one to three cells thick. 2. Connective tissue
septum separating the two end organs. 3. Septum Joining
the capsule at a thicker area. b. Branched nerve fibers
within the organ. 5. Bound or elliptical shaped endings
forming buttons.

Bielschowski-Gros. X1750

 

Plate XLI

Thin capsulated nerve ending~(upper lip).
1. Capsule. 2. Nerve fibers. 3. Knob-like endings.

Bielschowski-Gram X1000

 

Plate XLII

Disk-like capsulated nerve ending (upper lip).
l. Capsule. 2. Condensed neural elements forming disk.
3. Nerve fiber. h. Hair follicle.

Bielschowski-Gros. X1150

 

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Plate XLIII

Innervation of the sinus hair (upper lip).

1. Nerve fibers entering the follicle. 2. Nerve fibers
in the inner connective tissue sheath. 3. Sinus contain-
ing blood. 4. Nerve fibers entering outer root sheath.
5. Connective tissue trabeculi.

Vertical section, Bielschowski-Ores. X250

178

 

Plate XLIV

Nerve endings along hair follicle (upper lip).

1. Sebaceous gland. 2. Hair follicle. 3. Glomus-like
nerve endings along the hair follicle. b. Nerve fibers
bundle.

Bielschowski-Gros. X730

179

 

Plate XLV

Highly organized encapsulated nerve ending (upper lip).

1. Capsule. 2. Nerve fiber entering the organ. 3. End
buttons within capsule. E. Central space. 5. Cross section
of a pacinian-like small ending. 6. Epitheloid cell.

Cross and longitudinal section, Bielschowski-Gros. X2h00

180

 

Plate XLVI

Lamellated nerve endings (upper lip).

1. Longitudinal section of a small pacinian-like ending.

2. One cell thick capsule. 3. Epitheloid cells within
capsule forming lamella. 4. Central axis containing nerve
fiber. 5. Cross section of similar nerve ending. 6. Hyelina-
ted nerve fiber. 7. Hair follicle.

Cross and longitudinal sections, Bielschowski-Gros. X1000

181

fl

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Plate XLVII
Innervation of a part of hair follicle (upper lip).
1. Nucleus of follicular cell. 2. Bundle of nerve fibers.
3. Nerve fibers break up into a network.

Bielschowski-Gros. X2600

182

 

Plate XLVIII

Skin of the nostril.

l. Arrector pili muscle-~(a) Origin (b) Insertion.

2. Hair follicle. 3. Sebaceous gland. h. Elastic network
around the hair follicle. 5. Elastic network in the upper
papillary layer. 6. Skeletal muscle fiber in the papillary
layer. 7. Sweat gland. 8. Opening of the sweat duct.

Vertical section, Gomori's aldehyde fuchsin. X75

SUMMARY AND CONCLUSIONS

Six adult horses, including 2 stallions, 2 geldings and
2 mares were used for this investigation. The animals were
from 2 to 17 years old. The average skin thickness of general
body skin was 3.8 mm, ranging from 2.5 mm in the submandibular
region to 6.4 mm on the dorsal surface of the tail. Variations
in skin thickness were determined in relation to age and sex.
The epidermis consisted of3 layers: stratum germinativum,
stratum granulosum and stratum corneum; the stratum lucidum
was absent. The outer surface of the epidermis formed ridges
and grooves. The hair erupted on these grooves and formed
rows on the epidermal surface. Keratohyalin granules in the
stage of formation were observed in the cells of the supra-
basal layer of the epidermis and increased gradually until the
cytOplasm of the cells of the stratum granulosum was completely
filled. Basal epithelial processes anchored the epidermis
with the underlying tissue. In addition to the cells of the
germinal layer the basal layer contained melanocytes with
long beaded dendritic processes filled with melanin pigments.
The dermis consisted of two layers: the stratum papillare
and the stratum reticulare with well recognized demarcation.
A modified dermal region extended from the posterior part of
the dorsum to the end of the croup with lateral extentions
over the gluteal region. The maximum thickness of this part
of the dermis occurred on the croup (5.5 mm). The collagenous
fibers in the papillary layer were fine and loosely arranged

183

18“
but in the reticular layer were compact and formed a third
layer in the lower part. The papillary layer contained an
extensive elastic network. Elastic fibers from this network
established connection with dermo-epidermal Junction. Where
the epidermis was thick and had epidermal pegs the dermis
formed papillary bodies, rich in blood vessels.

The medulla of the hair contained two layers of rectan-
gular cells and were attached to adjacent cells by desmo-
some. The hairs were not in groups and the larger follicles
contained follicular folds. Tactile hairs of the ungulate
type were present in the upper lip, lower lip and in the
nostrils. Usually two sebaceous glands were associated
with each follicle. They were large in regions with fine
hairs, elongated with coarse, long hairs and smaller near
normal body haris. They were very large in the external ear
canal, circumanal region, prepuce, lower lip and extremely
large and branched in the hoof margin. The arrector pili
muscles were well developed on the dorsal and lateral sides.
Those associated with the coarse hairs of the mane and tail
were particularly long and slender.

Sweat glands were apocrine type and tubular on the gen~
eral body surface. Large saccular sweat glands occurred in
the external ear, circumanal region, prepuce and lower lip.
On the general body surface they were large on the flank and
abdomen. The secretory cells contained a brush border at the.
luminal side and canaliculi at the basal side near the base

of the intercellular spaces. Both a submicroscopic apocrine

185
and canalicular secretory mode has been prOposed.

Numerous arterio-venous anastomoses were present in the
upper and lower lips, nostrils, coronary border, external
ear, and the dorso-lateral side of the hind quarter.
Arterial cushions were present in the lips, and in the skin
of the thigh.

Four types of morphologically different nerve endings
were recognized in the lip: lamellated endings, capsulated
end organs, free nerve endings, and non-capsulated balls.

In addition, nerve nets‘were demonstrated on the hair follicle.

BIBLIOGRAPHY

Adachi, B. 1903. Haut pigment beim Menschen und bei den
Affene ZeltSChre f0 Morph. no Anthrop. 631’131e

Aeby, C. 1835. Die Herkuft des pigments im Epithel.
Zentralkl. f. med. wiss. 23:273-275.

Ahmed, N. N. 1965. Histochemical study of lipids in human
sebaceous glands. J. Histochem. Cytochem. 13:6.

Aoki, T., S. Kimura and M. Nada. 1959. On the responsive-
ness of the sweat glands in the horse. J. Invest.
Dermatol. 33zbhl-h43.

Baird, P. C., N. F. Lever and T. D. Spices. 1942. The
physiology of skin. Ann. Rev. Physiol. “3171-186.

Baker, J. B. 19h6. Histochemical recognition of lipine.
Quart. J. MicroscOp. Sci. 87:441-“70.

Bates, E. P. 1934. A quantitative study and interpreta-
tion of the occurrence of basophile (mast) cells in
the subcutaneous tissue of the albino rat. Anat. Rec.
613231-239 e

Bear, R. S. and H. S. Morgan. 1957. Composition of bands
and inter bands of collagen fibrils. Connective Tissue
Symposium. pp. 321-333. Charles C. Thomas, Springfield.

Berlin, L. B. and I. G. Pridvizhkin. 1965. Histochemical
study of glycogen in the skin of human embryo. Dolk.
Akad. Naud. SSSR. 1603213-215.

Bern, H. A. 1954. Histology and chemistry of keratin
formation. Nature 1743509-511.

Berry, J. 1953. Recherches sur l'origine des cellules de
Langerhans de l'epiderme. Biol. Med. 42:293-315.

Bertalanffy, F. D., V. Pusey and M. 0. Abbott. 1965. Mitotic
rates of rat.epidermis during growth, maturity, senility,
and regeneration. Arch. Dermatol. 92:91-102.

Billingham, B. E. and P. B. Medawar. 1953. A study of the
branched cells of the mammaliartepidermis with special
reference to the fate of their divisions products.
Phil. Trans. of Royal Soc. B2373151-171. '

Billingham, R. E. and w. K. Silvers. 1960. The melanocytes
of mammals. Quart. Rev. Biol. 35:1-1h.

186

187

Bloch, B. 1921. fiber die Entwicklung desflHaut und Haar
pigments beim menchlichen Embryo und uber das Erloeschen
der pigmentbildung im ergraunden Haar. Arch. f. Dermat.
Ue Syph. 135377-1080

______:fl. 1927. Das pigment in Jadassohn's Handbuck, der
Haut und geschlechtskrankheilen. Part 1:434,
Berlin, Springer.

Bloom, w. and D. w. Fawcett. 1962. The Textbook of Histology.
pp. 379-380. N. B. Saunders Co., Philadelphia.

Bodian, D. 1936. A new method for staining nerve fibers
and nerve endings in mounted paraffin section. Anat.
Rec. 65:89-98.

Bollinger, A. and N. D. McDonalds. 1949. Histological
investigation of glycogen in skin and hair. Aust. J.
Exp. Biol. 23233-238e

Bornstein, P. and A. B. A. Karl. 1965. Collagen structural
studies based on the cleavage of methionyl bonds.
Science 148:1353-1355.

Bradfield, J. B. G. 1951. Glycogen of vertebrate epidermis.
Nature 167:40-41.

Breathnach, A. S. 1963. A new concept of the relationship
between the Langerhans cells and the melanocytes. J.
Invest. Dermatol. 40:279-281.

Breathnach, Ac Se, Me Se Ce Blrbeck, and Jo De Everalle
1963. Observations bearing on the relationship between
Langerhans cells and melanocytes. Ann. N. Y. Acad.
Sci. 100:223-237. '

Bucher, C. J. and K. D. Blackley. 1936. The use of Dioxane
in the preparation of histological section by the
paraffin method. Arch. Pathol. 22:225.

Burton, A. C. 1966. Physiology and Biophysics of Circula-
tion. Yearbook Medical Publishers Inc., Chicago.

Calvery, H. 0., J. H. Draize and E. P. Lang. 1946.
Permeability of skin. Phys. Rev. 26:502-514.

Cauna, N.. 1959. The mode of termination of sensory nerve
and its significance. J. Comp. Neur. 113:169-210.

Chambers, R. and G. S. Benji. 1925. The physical relation-
ship of the cells in epithelial layer. Am. J. Anat.
35:385-402.

Charles, A. and F. G. Smiddy. 1957. The tonofilaments of
human epidermis. J. Invest. Dermatol. 29:327-337.

188

Chase, H. 8., H. Ranch and V. N. Smith. 1951. Critical
stages of hair development and pigmentation in the
mouse. PhySlOIe 20010 2&31-8e

Chauveau, A. 1873. Comparative Anatomy of Domestic Animals.
pp.793. w. R. Jenkin, New York.

Clara, M. 1956. Die arterio-venasen Anastomosen. 2. Aufl.
Wein, Springer, verlag in Vienna.

Colin, E. C. 1943. Hair direction in mammals; Embryogenesis
of hair follicles in the guinea pig. J. Morph. 72:191-
223.

COpenhaver, w. M. 1964. Bailey's Textbook of Histology.
William's and Wilkin's Co., Baltimore.

Danial, P. M. and M. L. P. Marjorie. 1956. Arteriovenous
anastomosis in external ear. Quart. J. Exp. Physiol.
41:107-123.

Danneeh R. and G. Cleffmann. 1954. Die Einwanderung der
pigmentzellen in die Haut and die Haar bei Nagatieren.
Biol. Zentral. 72:414-428.

Dick, J. C. 1947. Observation on the elastic tissue of the
skin with a note on the reticular layer at the Junction
of the dermis and epidermis. J. Anat. 81:201-211.

Dixon, A. D. 1961. The innervation of hair follicles in
' the mammalian 11p. Anat. Rec. 1408147-158.

Dozza, G. and L. Rampichini. 19632 Histological characteris-
tics of the mast cells for mm in various cutaneous
regions of ”Equus Asinus" and pamphoid reactivity of
the same regions to intracutaneous inoculation of 48/80
(histamin releaser). Arch. vet. Ital. 14:555-566.

Dry, P. w. 1926. The coat of the mouse. J. of Genetics
163287-340e

Edwards, E. A. and S. Q. Duntley. 1939. The pigments and
color of living human skin. Am. J. Anat. 65:1-33.

Ehrlich, P. 1879. Beitrage zur kenntnis der granulierten.
Bend-geweleszellen und der eosinOphilen leukocyten.
Arch. f. Anat. u. Physiol. (phys. abt.) p. 166.

Ehrmann, S. 1885. Untersuchungen fiber die Physiologic und
Pathologie des haut Pigments. Arch. f. Dermat. u. Syph.

17:507-532.

El-Bahrawy, A. A. 1922. fiber den Mongolen fleck bei EnrOpaern.
ArChe f0 Dermat. no Syph. la13171-192e

139

Elias, H. 1948. On the phylogeny of the mammalian hair.
(abstract) Anat. Rec. 101:675.

Ellenberger, w. 1906. Handbuch der vergleichenden mikro-
skogischen Anatomie der Haustiere. Paul Parey, Berlin.
131 Is

Ellis, H. A., w. Montagna and {L.Fauger. 1958. Histology
and cytochemistry of human skin. XIV. The blood supply
of cutaneous gland. J. Invest. Dermatol. 30:137-145.

Evans, C. L. and D. F. G. Smith. 1956. Sweating reSponse
in the horse. Proc. Roy. Soc. 145:61-83.

Evans, C. L., A. M. Nisbat and K. A. Ross. 1957. A histolog-
ical study of the sweat glands of normal and dry coated
horses. J. Comp. Path. Therap. 67:397-405.

Favre, M. 1950. Le chondriome de l'epiderme normal et des
epidermes pathologiques. Ortho et parakeratinizasions.
Ann. derm. Syphilig. 10:241-262.

Fazzari, I. 1951. Problema dello-strama. Bass Clin. Sci.
Into BlOCheme Itale 278355-361e

Felsher, Z. 1966. Studies of the rings of Henle associated
with collagenous bundles in human and calf skin. Anat.
Rec. 1548513-525.

Fowler, E. H. and M. L. Calhoun. 1964. The microscopic
anatomy of developing fetal pig skin. Am. J. Vet.
ReSe 258156-1640 .

Frey, M. 1896. Untersuchungen.fiber die Sinnesfunktioner der
megschlichen Haut. Abh. Bach's Ges (Akad) wiss. 40:175-
26 .

Glaister, J. 1931. A study of Hairs and hbols belonging to
the Mammals. pp. 91-92. MISR Press, Cairo.

Goldberry, S. and M. L. Calhoun. 1959. The comparative
histology of the skin of Hereford and Aberdeen Angus
cattle. Am. J. Vet. Res. 20:61-68.

Gomori, G. 1950. A1dehyde-fuchsin--A new stain for elastic
tissue. Ame Jo Cline Path. 203665-666e

Goodall, A. and 8. Young. 1955. The vascular supply of the
skin of Ayrshire calves and embryos. J. Agri. Sci.
“u:l-ue (AbStraCt in Veto BUlle 1955)e

Grassmann, w. 1955. Unsere heutige kenntnis des collagen.
Ledere 6:241.

190

Gridley, M. F. 1960. Dominici's mast cell stain. Manual
of Histologic and Special Staining Technics. pp. 108-
109. McGraw-Hill Book Co., Inc., New York.

. 1960. Bauer-Feulgen reaction for glycogen. Manual
of Histologic and Special Staining Technics. pp. 129-
130. McGraw-Hill Book Co., Inc., New York.

Gurr, E. 1962. Periodic acid-silver-<3rcein-analine blue
(for selective demonstration of connective tissue
fibers). Staining Animal Tissue. pp. 359-361. Leonard
Hill (Books) Limited, London, N. w.

. 1962. Petrunkevitch's cupric paranitrophenol
fixative. Staining Animal Tissue. pp. 483. Leonard
Hill (Books) Limited, London, N. W.

Guyer, M. F. 1949. Animal Micrology. University of Chicago
Press, Chicago.

Hadwen, S. 1931. The melanotoma of gray and white horses.
Can. Med. Assoc. J. 25:519-530.

Hall, D. A., R. Reed and R. E. Tumbridge. 1955. Structure
of elastic tissue. Nature 170:264-266.

Hall. D. A. 1957. Chemical and enzymatic reaction on
elastin. Connective Tissue Symposium. pp. 238-253
Charles C. Thomas, Springfield.

Ham, A. w. 1961. Histology. Lippincott Co., Philadelphia.

Hansen, G. A. 1957. On the structure and function of mast
cells. Connective Tissue Symposium. pp. 12-36. Charles
C. Thomas, Springfield.

Hardy, M. 1952. Histochemistry of hair follicles in mouse.
Am. Jo Anat. 903285-338e

Hay, Ee De 1966e EpithellMe HlStOIOSYe BeOe Greep, E’jltcr,
End 93., p. 74. MoGraw-Hill Book Co., Inc., New York.

Hibbs, R. G. 1958. The fine structure of human eccrine
sweat glands. Am. J. Anat. 103x201-218.

. 1962. Electron microscOpy of human apocrine sweat
glands. J. Invest. Dermatol. 38:77-84.

Holmes, R. A. 1960. The Microscopic Anatomy of the Skin
of Mus norvegicus albinus. M. S. Thesis, Michigan State
University.

191

Holmgren, H. and 0. Wilander. 1937. Beitrag zur kenntnis
der Chemie und Funktion der Ehrlichschen Mastzellen.
Z. Microskop. Anat. Forsch. 42:242-278.

HOpkins, F. M. 1949. Analysis of some phases of melanoblast
migration in the barred Plymouth Rock embryos. Physiol.
ZOOle 2281-22e

Bu, Fe, Re Jo Sterleo, He Pinkus and Re Po Fasnaugh. 1957e
Human melanocytes in tissue culture. J. Invest.
Dermatol. 28:15-32.

Hu, F. and R. Cardell. 1962. Observations on the ultrastruc-
ture of human skin. Henry Ford Hosp. Med. Bull. 10:63-87.

Hurley, H. J. and w. B. Shelby. 1954. The role of myoepithe-
lial cell of the human apocrine sweat gland. J. Invest.
Dermatol. 22:143-156.

Jarrett, A. 1958. Structure du collagene et du tissue de la
peau non-fixee. Bull. Soc. Franc. Dermatol. et Syph.
658376-380e

Jirka, M. and J. Katas. 1959. Some observation of the
chemical composition of horse sweat. J. Physiol. 147:
7 '77-

Johnson, P. L. and G. Bevelander. 1946. Glycogen and phos-
phatase in the developing hair follicle. Anat. Rec.
95:193-200-

Kawata, S. 1943. fiber die Nervenversorgung im Pferdehufe,
insbesondere seiner Endapparate. Jap. J. Vet. Sci.
53479-497.

Keddie, F. and D. Sakai. 1965. Morphology of the horny cells
of the superficial stratum-corneum, cell membrane and
melanin granules. J. Invest. Dermatol. 44:135-138.

Kozlowski, G. P. 1966. The Microscopic Anatomy of the
Integument of Sheep. M. S. Thesis, Michigan State
University.

Kr81ling, 0. and H. Grau. 1960. Lehrbuch der Histologie und
vngleichenden Microskopichen Anatomy der Haustiere.

pa “53e

Kuntz, A. and J. Hamilton. 1938. Afferent innervation of
the skin. Anat. Rec. 71:387-400.

Kurusumi, K. 1959. E. M. studies on the human axillary

agzcrine sweat glands. Arch. Histol. Japan. 16:523-
5 e

192

Lee, M. M. C. 1960. Histology and histochemistry of human
apocrine sweat glands with special reference to their
defense mechanism. Anat. Rec. 136:97-105.

Leydig, F. 1876. fiber die allgemeinen Bedeckungen der
Amphibian. Arch. f. Mikr. Anat. 12:119-241.

Lillie, R. D. 1952. Histochemistry of connective tissue.
Lab. Invest. 1330e

. 1965. Bielschowski-Gros Method Histopathologic
Technic and Practical Histochemistry. 3rd. ed. pp. 612-
613. McGraw-Hill Book Co., Inc., New York.

Lovell, J. E. and B. Getty. 1957. The follicle, epidermis,
dermis and skin glands of dog. Am. J. Vet. Res. 18:
873-8850

Ludford, H. J. 1924. Cell organization during keratinization
in normal and malignant growth. Quart. J. Micro. Sci.
69329-58e

Malewitz, Te De and Be Me Smith. 1955e A nuclear stain
employing dilute Harris hematoxylin. Stain Tech. 30:
311“312e

Mallory, F. B. and F. Porter, Jr. 1927. Reticulin. Am.
Je Path. 38595‘525e

Marcarian, H. Q. and M. L. Calhoun. 1966. Microscopic
anatomy of the integument of adult swine. Am. J. Vet.
Res. 27:765-772. ' '

Marshall, P. H. A. 1902. On hairs in the equidae. Vet. J.
57:34-42.

Masson, P. 1948. Pigment cells in man: Biology of melanomas.
Spec. Puble Ne Ye Acad. SCle 6815'51e

Matoltsy, A. G. and M. Matoltsy. 1962. A study of
morphology and chemical preperties of keratohyaline
granules. J. Invest. Dermatol. 38:237-247.

McGuire, J. S. and A. B. Lerner. 1963. Effect of trico-
SOpeptide "ACTH" and alpha melanocyte stimulating hor-
mone on the skin color of man. Ann. N. Y. Acad. Sci.
1008622-625e

Medawar, P. B. 1941. Sheets of pure epidermal epithelium
from human skin. Nature 148x783.

Meirowski, E. and G. Behr. 1948. Some aspects of the physiol-
ogy and pathology of cornification. J. Invest. Dermatol.
10:343-378.

Melaragno, H. P. and w. Montagna. 1953. The tactile hair
follicles in the mouse. Anat. Rec. 115:129-142.

193

Menefee, M. G. 1955. The differentiation of keratin con-
taining cells in the epidermis of embryo mice. Anat.
ReCe 1223181”192e

Mercer, E. H. 1961. Keratin and keratinization. Pergamon
Press, New York.

Messenger, J. H. 1890. The vibrissae of certain mammals.
J. Comp. Neur. 10:399-402.

Meyer, K., P. Hoffman and A. Linker. 1957. The acid muco-
polysaccharides of connective tissue. Connective
Tissue Symposium. pp. 86-96. Charles C. Thomas,
Springfield.

Mikhail, G. R. and A. M. Milinska. 1965. Mast cell popula-
tion in human skin. J. Invest. Dermatol. 43:249-254.

Montagna, w. 1950. Perinuclear sudanOphil bodies in mammalian
epidermis. Quart. J. Micro. Sci. 91:205-208.

. 1962. The Structure and Function of Skin. Academic
Press, New York.

Montagna, W., H. B. Chase, and H. P. Melaragno. 1951.
Histology and cytochemistry of human skin. 1. Meta-
chromasis in the mons pubis. J. Nat. Cancer Inst. 12:

591-597 e

Montagna, w., H. B. Chase, J. D. Malone and H. P. Melaragno.
1952. Histology and cytochemistry of human skin.
11. The distribution of glycogen in the epidermis,
hair follicles, sebaceous glands and eccrine sweat
glands. Anat. Rec. 114:231-248. '

Montagna, w. and H. A. Ellis. 1957. Histology and cyto-
chemistry of human skin. XII. Cholinesterases in the
hair follicles of the scalp. J. Invest. Dermatol. 29:
151-157.

. 1959. Cholinergic innervation of the meibomian
gland. Anat. Rec. 135:121-127.

Montagna, w. and C. H. Nobeck. 1947. Histochemistry of
sebaceous glands of rat. Am. J. Anat. 81:39-58.

Montagna, w. and J. S. Yun. 1964. The skin of the domestic
pig. J. Invest. Dermatol. 42:11-21.

Hunger, B. L. 1960. An electron microscope study of the
sweat glands of cat foot pad. Anat. Rec. 136:248-253.

~.\l

L ‘4 ‘e’
4’

Nicholas, A. M. 1963. The mast cells. Ann. N. Y. Acad.
Sci. 103:246-252.

Nielsen, S. w. 1953. Glands cf the canine skin. Am.J.
Veto ReSe 14:448-454.

Odland, G. F. 1950. The morphology of the attachment
between the dermis and the epidermis. Anat. Rec.
1083399°407.

. 1958. The fine structure and interrelationship
of cells in the human epidermis. J. BiOphy. Biochem.
Cytol. 48529-538e

. 1966. Skin. Histology. H. 0. Greep, Editor.
2nd ed., pp. 420-445. McGraw-Hill Book Co., Inc., New York.

Odoni, E. 1951. Der Spiegel des Pferdes, eine modifizierte
Hautpartie in der Kruppen und oberschenkelgegend.
Buchdrudkerei A3. Baden. zfirich.‘

Okun, M. H. 1965. Histogenesis of melanocytes. J. Invest.
Dermatol. 44:285-299.

Orekhovitch, V. N. and V. 0. Sphikiter. 1957. Procollagen
as biolOgical precursors of collagen and the physico-
chemical nature of this protein. Connective Tissue
Symposium. pp. 281-293. Charles C. Thomas, Springfield.

Parakkal, P., w. Montagna, and R. A. Ellis. 1962. The skin
of primates. XI. The skin of white brewed gibbon.
Anate ReCe 1433171-179e

Pease, D. C. 1958. The basement membrane. The structure
of histological order and complexity. Fourth Internatinn
conference on Electron Microscopy. Springer -
Berlin, Germany.

Porter, K. R. 1966. Mesenchyma and connective tissue.
Histology. H.O. Greep, Editor. 2nd. ed. p. 99.
Mc-Graw-iill Book Co., Inc., New York. .

Pullar, P. 1964. Keratin formation in tissue culture. IQ:
Progress in the Biological Science in Relation to Dermato-
logy. Cambridge University Press, New York. 2:213-224.

Habl, H. 1896. Pigment und pigmentzellen der Haut der
Wizbeltiere. Ergebn. d. Anat. u. Entwickl. 6:439-470.

Heed, H. 1957. The architecture of the collagen fibrils.
Connective Tissue Symposium. pp. 299-307. Charles C.
Thomas, Springfield.

Beneto, G. S. 1963. Mechanism of migration of the melanocytes
from the hair f0111013e Anne Ne Ye Acad. 301e 1003
249-253. ‘

195

Riley, J. F. 1959. The Mast Cells. E. and S. Livingston
Ltd., Edinburgh, England.

Robb-Smith, A. H. T. 1945. Histology of the reticular tissue
in the Sklne Brits Made Bu11e 33172-175e

. 1957. What is reticulin? Connective Tissue
Symposium. pp. 177-184. Charles C. Thomas, Springfield.

Rock, A. J. and G. S. Walton. 1965. Comparative Physiology
and Pathology of Skin. Blackwell Scientific Publishers,
Oxford.

Ross, J. B. 1965. The dermo-epidermal interface in health
and disease. Brit. J. Dermatol. 77:77-79.

Rothman, S. 1954. Physiology and Biochemistry of the Skin.
University of Chicago Press, Chicago.

Howles, M. E. 1940. The development of melanophore from
embryonic mouse tissues grown in the coelom of chicken
embryOSe PrOCe Nate Acade SCle Ue Se 268673-680e

. 1947. Origin of pigment cell from the neural
crest in the mouse embryo. Physiol. Zool. 20:248-266.

. 1953. Origin of mammalian pigment cell and its
role in the pigmentation of hair. In: Pigment Cell
Growth. pp. 1-12. Academic Press, New York.

Sandifer, T. N. 1966. Equine Infectious Anemia. Washington
Notebook. Veterinary Economics. June, p. 20.

Sar, M. and M. L. Calhoun. 1966. Microscopic anatomy of
the integument of the common American goat. Am. J.
Vet. ReSe 27:444-456.

Sch8nberg, F. 1926. "Der Ross Spiegel.” Eine Eigentum-
1ichkeit des integumentum Pelvis beim Pferde. Berliner
Tierarztliche wochenschrift. Vol. 42.

Selby, C. C. 1957. An electron microscopic study of the

epidermis of mammalian skin. J. Invest. Dermatol. 29:.
131-135e '

Shane, G. P. 1939. The role of embryonic ectoderm and
mesoderm in pigment production in amphibianS. J. Exp.
ZOOle 88193‘215e

. 1944. The embryology - "Vertebrate pigment cells"
Part II (Bird)e Quart. BBVe BlOle 19398‘117e

196

Singer. M. and M. M. Salpeter. 1966. ”Nervous Tissue."
Histology. 8.3. Cree , editor. 2nd ed. McCraw-Hill
Book Co., Inc. New York

Sinha, H. D. 1964. Microscopic Anatomy of the Integument
of Holstein Cattle. M. S. Thesis, Michigan State
University.

. 1965. Eye Adnexa of Sheep and Goat. Ph. D.
Thesis, Michigan State University.

Sisson, S., and J. D. Grossman. 1953. The Anatomy of the
Domestic Animals. W. B. Saunders Co., Philadelphia.

Smith, F. 1888. The histology of the skin of horse. Vet.
J. 26: 333-340.

Smith, J. L. and M. L. Calhoun. 1964. The microscOpic
anatomy of the integument of newborn swine. Am. J.
Vet. ReSe 253165-173e

Straile, W. E. 1960. Sensory hair follicles in mammalian
skin.“ "The Tylotrich follicles." Am. J. Anat. 106:
133-1 1.

. 1961. The morphology of Tylotrich follicles in
the skin of the rabbit. Am. J. Anat. 109:1-13.

Strickland, J. H. and M. L. Calhoun. 1960. The microscOpic
anatomy of the skin and external ear of Felis domesticus.

Ame Je Veto ReSe 21:845-850.

. 1963. The integumentary system of cat. Am. J.
Vet. ReSe 2481018-1029e

Szelingrowski, E. 1962. Contribution to the histology of
the skin of horse. (Collection of vet. med. Warsaw,
Poland). Folia Morphol. (Warsaw) 13:531-535. (English
Summary).

Takagi, S. and M. Tagawa. 1959. A cytological and cyto-
chemical study of the sweat gland of the horse. Jap.

J. Physiol. 98153-159.

. 1961. Nerve fibers supplying the horse sweat
gIand. Jap. J. Physiol. 11:158-161.

Thuringer, J. 1928. Studies on cell division in the human
epidermis. Anat. Rec. 40:1-13.

'Trautmann, A. and J. Ftebiger. 1957. Fundamentals of
Histology of Domestic Animals. (Translated and revised
by R. E. Habel and E. L. Biberstein). Comstock Publish-

ing Associates, Ithaca, New York.

197

Vincent, S. B. 1913. The tactile hair of the white rat.
J. Comp. Neur. 23:1-38.

Waddington, C. H. 1956. Principals of Embryology, "Origin
of Melanocytes." Allen and Unwin, London. p. 256.

Waldeyer, W. 1875. fiber Bindgewebezellen. Arch. f. Mikr.
Anat. 113176-178e

Wall, P. D. 1964. Two transmission systems for the skin
sensation. lg: Sensory Communication. Massachusetts
Inst. Tech. Press, Cambridge, Mass. pp. 475-496.

Waterston, D. 1933. Observations on sensation. The sensory
function of the skin for touch and pain. J. Physiol.
77:251-257e

Webb, A. J. and M. L. Calhoun. 1954. The microsc0pic

Eggtomy of mongrel dogs. Am. J. Vet. Res. 15:274-

Weddell, G. 1941. The pattern of cutaneous innervation in
relation to cutaneous sensibility. J. Anat. 75:444-

453.

Weddell, G. and D. C. Sinclair. 1952. The anatomy of
organized nerve endings in human skin. J. Anat. 86:
496-503-

Weddell, G. and W. Pallie. 1954. The value of spreading
factors in the demonstration of tissue neural elements.
Quart. Je MlCTOe SCle 958389-397e

Weddell, G., E. Palmer and W. Pallie. 1955. Nerve endings
in mammalian skin. Biol. Rev. 30:159-195.

'Weissenfels, N. 1956. Licht phasenkontrast und elektro-
nenmikroskOpische untersuchungen uber die enstellung
der prOpigment granule in Melanoblastenkulturen.
Zeitschr. fe zellforSChe 45860-73e

Wellier, B. H. 1953. Basic mechanism in the differentiation
of pigment cells. J. Embryol. EXper. Morphol. 1:297-
299-

Winkelmann, R. K. 1957. The sensory end organ of the hair-
less skin of the cat. J. Invest. Dermatol. 29:374-354.

. 1959. The innervation of a hair follicle. Ann.
Ne Ye Acad. SCle 83:400-497.

 

. 1960a. Nerve endings in the normal and Pathological
. Skin. Charles C. Thomas, Springfield.

 

198

Winkelmann, R. K. 1960b. Similarities in cutaneous nerve
end organs. "Advances in Biology of Skin." Vol. 1.
Cutaneous Innervation. Pergamon Press, New York.

Winkelmann, R. K. and R. W. Schmidt. 1959. Cholinesterase
in the skin of the rat, dog, cat, guinea pig and rabbit.
Je InveSte DermatOIe 33:185’190e

Yamada, H. 1960. Electron microscopic observations of the
secretory processes of the axillary apocrine glands.
Acta Pathol. Jap. 10:173-187.

Yoichiro, S. 1965. Histochemical demonstration of phosphory-
lase a, b and phosphorylase kinase in normal human skin.
JOhukO Je EXP. Med. 853160-170e

Zelickson, A. S. 1965. The Langerhans cells. J. Invest.
Dermatol. 44:203-205.

Zimmermann, A. A. and S. W. Becker. 1955. Further studies
on melanocytes and melanogenesis in the human fetuses
and newborn. J. Invest. Dermatol. 25:103-112.

. 1959. Melanoblasts and Melanocytes in Fetal
Negro Skin. University of Illinois Press, Urbana, Ill.

Zweifach, B. W. 1959. The structural aSpects and hemo-
dynamics of microcirculation in the skin. In "The
Human Integument". Am. Assoc. Advance. 301., Washington
D. C.

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