‘AgA-‘n .u—1-lLAL" .A ' . .c. o .. < A -.. ‘ ‘ ' . t‘.‘ ' -, “RV , ‘2'.“ 53' V THE DEVELOPMENTAL HISTOLOGY OF THE INTEGUMENT OF FETAL PIGS Thesis {or {‘he Degree of M. 5. ME REGAN STATE UREVERSETY Eciward H. Fowler 1962 LIBRARY Michigan State University THE DEVELOPMENTAL HISTOLOGY OF THE INTEGUMENT OF FETAL PIGS BY Edward H. Fowler A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Anatomy 1962 ACKNOWLEDGEMENTS The author's sincere appreciation is extended to all those who have made this thesis possible. Dr. M. Lois Calhoun, Professor and Head of the Department of Anatomy is extended special thanks for her encouragement, guidance and criticisms throughout this investigation. Other members of the Anatomy Department to whom gratitude is expressed are Drs. Madan B. Bharadwaj, Esther M. Smith, Thomas Bell, Jr. and Thomas W. Jenkins for their assistance in preparing the manuscript, Mrs. Esther B. Colby for her suggestions and assistance, and Mr. Andrew Moore for his contributions in the preparation of the photomicrographs. Thanks are also due the Animal Husbandry Department and specially Dr. Duane E. Ullrey for the acquisition of the animals used in this study, and the Department of Surgery and Medicine for performing the cesarean sections. Words cannot express the indebtedness the author feels toward his wife whose constant encouragement and assistance have made this theSis a reality. ii TABLE OF CONTENTS Page INTRODUCTION . . . . . . . . . . . . . . . . . . . . 1 REVIEW OF THE LITERATURE . . . . . . . . . . . . . . 2 Epidermis 3 Dermis or Corium 5 Hair 8 Glands 13 MATERIALS AND METHODS . . . . . . . . . . . . . . . . 17 RESULTS AND DISCUSSION . . . . . . . . . . . . . . . 21 Epidermis 21 Dermis 23 Hair 27 Glands 32 Sebaceous Glands 32 Sweat Glands 33 Mammary Glands 34 SPECIAL STRUCTURES . . . . . . . . . . . . . . . . . 35 Eyelid 35 External Bar 36 Digits 36 SUMMARY AND CONCLUSION . . . .'. . . . . . . . . . . 38 LITERATURE CITED . . . . . . . . . . . . . . . . . . 42 iii LIST OF PLATES PLATEzFIGURE I. Areas from which skin specimens were taken . O O O O O O O O O O O O O O 0 II. 1. First formed hair follicles from the jowl of a 35-day-old fetus . . . . . . . 2. Hair follicles on the jowl of a 50-day- old fetus . . . . . . . . . . . . . III. 1. Sinus hair follicle on the jowl of a 72- day-old fetus . . . . . . . . . . . . 2. Compound tubular glands from the jowl IV. 1. First two hair follicles on the upper lip of a 53-day-old fetus . . . . . 2. Eruption of two hairs through the epidermis of the snout of a 95-day- old fetus . . . . . . . . . . . . V. l. Eyelids of a 53-day-old fetus showing first hair follicles . . . . . . . 2. Eyelids of a 72-day-old fetus with hair follicles and glands . . . . . . . . VI. 1. Early coiled tubular sweat gland duct formation in a 53-day-old fetus, vertical section . . . . . . . . . 2. Sweat gland duct in a 53-day—old fetus, horizontal section . . . . . . . . . VII. 1. Hair follicle and associated structures in a 72-day-old fetus . . . . . . . 2. Concentric rings of fibroblasts around early hair follicles in a 50-day- old fetus . . . iv Page 19 47 47 48 48 49 49 50 50 51 51 52 52 PLATE:FIGURE VIII. XI. XII. XIII. l. Sebaceous gland cells completely filling early hair follicles in a 72—day-old fetus . . . . . . . . . . . . . . . Sebaceous gland around a hair follicle in a 95-day-old fetus Sebaceous gland formation in a sinus hair follicle of a 95-day-old fetus Sebaceous gland around regular hair follicle in a 95-day-old fetus Interlacing connective tissue fibers between sinus hair follicles in a 72-daY-Old fetus o o o o o o o o o Interlacing connective tissue fibers between sinus hair follicles in a 95-day-old fetus Groups of hair follicles and gland ducts in a 72-day-old fetus . . . . . . . An atypical infiltration of connective tissue cells around a group of hair follicles in a 95-day—old fetus . . An arrector pili muscle wrapping around a hair follicle in a 72-dayqold fetus . An arrector pili muscle connecting two hair follicles in a 95—day-old fetus Horizontal sections of hair follicles in a 95-day-old fetus . . . . . . . n " Sweat gland duct with a sac 72-day-old fetus . . . . . attached in a Page 53 53 54 54 55 55 56 56 57 57 58 58 PLATE:FIGURE Page XIV. 1. Vascular supply to early sinus hair follicles in a 72-day—old fetus . . . 59 2. Vascular supply to sinus hair follicles in a 9S-day-old fetus . . . . . . . . 59 XV. 1. Deep dermal arterial pdexus in a 72-day— old fetus . . . . . . . . . . . . . . 60 2. "Follicular folds" in a hair follicle from a 95-day-old fetus . . . . . . . 6O XVI. 1. Rear hoof of a 53-day—old fetus showing laminae and epidermis . . . . . . . . 61 2. Front hoof of a 50-day-old fetus showing laminae . . . . . . . . . . . . . . . 61 XVII. 1. Developing skeletal muscle in a 50-day- old fetus . . . . . . . . . . . . . . 62 2. Two ducts in the nipple of a 95—day- old male fetus . . . . . . . . . . . 62 XVIII. l. Nerve entering the blood sinus of a sinus hair follicle in a 95-day-old fetus . 63 2. Nerve entering sinus hair follicle on the jowl of a 95-day-old fetus . . . . . 63 XIX. 1. Elastic tissue fibers in the dermis of a 95-day—old fetus . . . . . . . . . 64 2. Junction of the rectum andthe anus in a 95-day-old fetus . . . . . . . . . . 64 XX. 1. Compound tubular sweat gland encircling a sinus hair follicle in a 95-day- old fetus . . . . . . . . . . . . . . 65 2. Junction of a coiled tubular sweat gland duct and a hair follicle in a 72- day-old fetus . . . . . . . . . . . . 65 vi PLATE:FIGURE Page XXI. 1. "Blood island" in the dermis of a 35-day- old fetus . . . . . . . . . . . . . . . 66 2. Another "blood island" in the 35-day-old fetus . . . . . . . . . . . . . . . . . 66 vii INTRODUCTION Very little work has been done on the histology of the integument of fetal mammals. Only recently has the integument of immature and adult mammals been studied in any detail. The skin of the human Species has been the most widely studied and there is still much to be learned. This thesis merely touches the highlights of the information to be gained in this field. It is the hope of the author that further study will be directed along these paths and eventually the most obvious tissue of all mammals, the integument, will have been fully explored. In the Department of Anatomy, Michigan State University, theses have been written describing the histology of the skin of mongrel dogs, cats, white rats, cattle and new-born swine. Work is being conducted at this time on the skin of goats and adult swine. These are major contributions in this field. REVIEW OF THE LITERATURE The general microscopic anatomy of mammalian skin has been studied extensively and in many cases specific details have been described. More attention has been directed toward the skin of the human species than of any other mammal to which end Montagna (1956) devoted an entire book. Smith (1960) provided an excellent review of the literature and described the histological picture of the skin of newborn swine. The developmental anatomy of mammalian skin has been considered briefly in textbooks of embryology (Patten, 1946; Arey, 1959). Hanson (1947) discussed the development of the epidermis in the rat and mouse. Diomidowa (1954) considered the embryonic development of sheep's skin. Grenburg (1957) made a comparative histological study of epidermal development in vertebrates and man, and Lyne and Heideman (1959) concentrated on the pre-natal development of the skin and hair in cattle. Other authors concerned with developmental anatomy have considered more specific areas of the skin such as cutaneous hematopoiesis (Popoff and Popoff. 1958), mitotic rates (Eichenlaub and Osbourn, 1951), fat in the subcutaneous tissue (Alexandrowicz, 1949), mast cells (Arvy, 1957), basement membrane (Becker and Zimmerman, 1957; and Bocharov, 1957), keratinization (Menefee, 1955; Matoltsy, 1958; and Achten, 1959), blood (Goodall and Yang, 1954), glands (Balinsky, 1950; and Yasuda, §t_gl, 1957) and hair and wool follicles (Colin, 1943; Balinsky, 1950; Diomidowa, 1955; Hardy and Lyne, 1956; Stephenson, 1958; and Dolnick, 1959). Epidermis Patten (1946) and Arey (1959) both provided an excellent general description of the development of the skin in human fetuses. The epidermis arises from embryonic ectoderm and is first noticed as a single sheet of cuboidal cells which Hanson (1947) referred to as the stratum germinativum. A second layer is added and the outer layer is termed the periderm which is flatter than the underlying cuboidal layer. Gradually the epidermis becomes three-layered with the stratum intermedium being interposed between the basal and periderm layers (Patten, 1946; and Arey, 1959). As the fetus increases in age the lower or basal layer continues to divide and the epidermis becomes highly stratified. The deepest stratum (basal cells) and the next most superficial stratum (prickle cells) constitute the stratum germinativum (Patten, 1946; and Arey, 1959). Copenhaver and Johnson (1958) referred to the basal cells as the stratum cylindricum and the prickle cells as the stratum spinosum. These two layers contain the actively dividing cells of the epidermis. Eichenlaub and Osbourn (1951), by random sample count, found more mitotic figures in the prickle cells than in the basal cells of pig embryos. Directly above the stratum germinativum is the thin stratum granulosum containing keratohyalin granules. This stratum and the next most superficial one, the stratum lucidum which contains a fluid eleidin, are found only in the plantar and palmar epidermis of the human fetus. Smith (1960) was unable to find the stratum lucidum in his study of newborn swine, but found the stratum granulosum in the snout and interdigital area. The layer of cells directly above the stratum lucidum, when present, and directly beneath the periderm is termed the stratum corneum. Arey (1959) stated that the process of cornification is not well understood but first occurs in the peripheral cytoplasm of the cells and causes a loss of trans- parency of the epidermis. A wax-like substance which is considered to be transformed eleidin is located more centrally in the cytoplasm of these cells. The periderm remains intact until the hairs emerge arui loosen or break it. This layer lying over the hairs truen becomes known as the epitrichium. Desquamated epitrichial arui epidermal cells mingle with cast-off lanugo hairs and seibaceous secretion to form the vernix caseosa which acts as; a protective covering for the epidermis against a nuacerating influence exerted by the amniotic fluid (Arey, 1959). Grenburg (1957) described three distinct periods of eupidermis formation. The first is characterized by a (distinct epithelial differentiation in various areas of the lmody and head. The most advanced areas are located at the Inid-line of the body in the region of the spine and abdomen, and at the cranial region of the head. In the second period there is a differentiation of epithelium laterally from.the spine and abdomen. During the third period the epithelium takes on the appearance of a many-layered definitive epithelium of a distinctive post-embryonic period. Dermis or Corium Underlying the epidermis is a fibrous layer of nlesodermal origin termed the dermis or corium. In the human fetus_this layer contains collagenous fibers, which arise first, elastic fibers, which arise someWhat later, blood vessels, nerves, hair follicles and glands. The dermis is subdivided into two layers, the papillary layer (Dempsey, 1948) and the deeper reticular layer (Schaeffer, 1953; Copenhaver and Johnson, 1958). The fibers of the papillary layer are finer and more closely arranged than those of the reticular layer. Wilson (1941) found the reticular layer of swine skin to be composed almost entirely of adipose tissue. Smith (1960) found it difficult to differentiate between these two layers in new- born swine as the fibers were interwoven. Smith also was unable to demonstrate elastic tiSsue in the stratum papillare or around the hair bulbs, sebaceous glands, or sweat glands. Mancini, gt_al. (1959) found elastic tissue first appears in rat skin at 12 post-natal days of age and increases until adult age. In the early stages of the formation of the skin, the plane of union between the epithelium and the dermal connective tissue is smooth. As the epithelium thickens, however, its lower surface beComes irregular, exhibiting ridges and hollows into which the connective tissue pushes. Connective tissue projections of this type are called dermal papillae whereas the epidermal projections are termed rete pegs. The dermal papillae contain the terminal loops of the capillaries nourishing the skin and some of them contain Meissner's corpuscles which are the sensory nerve end-organs of touch (Patten, 1946). Trautmann and Fiebiger (1957) described small dermal papillae in the areas where hair is abundant and large papillae in hairless areas. Smith (1960) found this relationship true only in about one-half of the obser- vations, except in the snout where it was always true. The loose fatty subcutaneous layer is located beneath the dermis. Wilson (1941) referred to the three layers of skin as the epidermis, dermis and superficial fascia. Most authors (Schaeffer, 1953; Stiles, 1956; Sisson and Grossman, 1956; Maximow and Bloom, 1957; Ham, 1957; Trautmann and Fiebiger, 1957; and Copenhaver and Johnson, 1958) refer to the superificial fascia, subcutis, hypodermis, or tela subcutanea as the same area, but not necessarily as a distinct layer. When the superficial fascia is exceptionally fatty it is called the panniculus adiposus (Sisson and Grossman, 1956; and Copenhaver and Johnson, 1958). Montagna (1956) described a flat sheet of skeletal muscle below the panniculus adiposus which he termed the panniculus carnosus. This, he stated, is well developed in most mammals, but is vestigial in man. Smith (1960) described a connective tissue band composed of dense collagenous connective tissue fibers and elastic fibers which divides the panniculus adiposus into a super- ficial and a deep layer. Alexandrowicz (1949) reported an accumulation of fat in the subcutaneous tissue of pig embryos which occurs first in the vicinity of the blood vessels in the muscular tissue and later gradually invades the skin. The first fat in the subcutaneous tissue appears simultaneously with the fat in the sebaceous glands. In the beginning of the accumulation there is more fat in the abdominal region of the body. After 90-100 days this difference disappears and toward the end of embryonic life there is more fat in the dorsal region of the embryo. Goodall and Yang (1954) and Ryder (1955) described three plexuses of blood vessels in the skin. The first plexus lies below the corium; the second is at a level between the sweat glands and the sebaceous glands; and the third is located between the epidermis and the second plexus. Fair. Hair grouping differs among different mammals. In man, hair is arranged in groups of 3-5, while in swine there are usually three hairs in a group, of which the main hair is the largest (Schaeffer, 1953). Medawar (1953) described hairs of the snout of pigs arising from the center of epidermal hills surrounded by an orbit of four or five "mucous" gland openings. Pig bristles having multiple medullae were described by Pinkus (1951). Smith (1960) described variations in the groupings of hair follicles which range from single follicles to groups of four to six, with each group being separated by dense collagenous fibers. Smith (1960) found the structure of the hair follicle in agreement with the description given by Trautmann and Fiebiger (1957), and Copenhaver and Johnson (1958) except that "follicular folds," previously described by Strickland (1958); Goldsberry and Calhoun (1959), and Holmes (1960), are also present. Smith found the "folds" both inferior and superior to the sebaceous gland opening which differs from the previous descriptions where they had only been reported on the inferior aspect. Arey (1959) indicated the first places hair appears in the human fetus are the eyebrows, lips and chin. Patten (1946) mentioned these areas but also includes the eyelids and scalp. Arey (1959) described hair development as follows: The first evidence of a future hair in the human fetus is a crowding and elongation of a cluster of germinative cells in the epidermis. Active proliferation soon produces a cylindrical, epithelial peg 10 which projects down into the corium. A continuation of the basal columnar cells of the epidermis forms the outer wall of the epithelial peg which surrounds a central mass of polyhedral cells. Investing the "peg" is mesenchyme which condenses into a mound-like papilla at the base and will later become the connective tissue sheath. As the hair peg pushes deeper into the corium, its base enlarges into the bulb which becomes molded like an inverted cup over the papilla. The basal epidermal cells lying next to the papilla proliferate and give rise to an axial core which will become the inner epithelial sheath and shaft. Peripheral cells on the sides of the original downgrowth comprise the outer epithelial sheath. The young hair cone continues to grow upward pushing through the central cells of the solid, primodial follicle, and is molded into shape by the organizing inner sheath. When the shaft reaches the epidermis it follows along a hair canal and erupts at the surface. Above the level of the bulb, the cells of the hair shaft cornify and differentiate into an outer cuticle, middle cortex, and central (inconstant) medulla. Two swellings of the outer epidermal sheath appear on the side of the follicle forming an obtuse angle with the stratum germinativum. The more proximal of these swellings becomes the sebaceous gland and 11 the distal swelling is the epithelial bed contributing to the growth of the periodically regenerating hair follicle. Phasenchymal tissue near the epithelial bed transforms into snnooth muscle fibers of the arrector pili muscle which arttaches to the side of the follicle. Dolnick (1959) found the earliest hair buds appear CH1 the face of the mink when the epidermis is two to three cxells in thickness. She also observed some dermal fetal feat cells which form a perifollicular envelope and appear tc> be related to the development of the hair follicle. In tflie mink the hair follicles are in clusters with the hair 5fl1afts all emerging through a common follicular neck similar tr) the structure in cats found by Strickland (1958). Lyne and Heideman (1959), recognized three stages of fuair follicle development in cattle fetuses, first-, later—, Eund last-formed follicles. The first-formed follicles, the liargest in the fetus, are characterized by the formation of ar1 "ental" swelling to which the arrector pili muscle is attached and medullated hairs are formed. Later-formed fOllicles form pairs which show a common epidermal hair canal, one of the pair being larger, and these follicles form non-medullated hairs. The last-formed follicles include scme of the smaller of the paired follicles in addition to 12 solitary ones. Only non-medullated hairs are formed by these follicles during the first hair cycle. In the guinea pig the first hair follicles are evenly :spaced. These isolated follicles then act as organizers of :satellite follicles, resulting in groups of seven to nine, eurranged in transverse rows (Colin, 1943). Straile (1960, 1961) described an atypical tactile liair which occurs in rats, rabbits, mice and guinea pigs termed tflie tylotrich follicle. This follicle is surrounded by an suinulus of blood vessels and nerves and has a thidker cnonnective tissue sheath above the level of the sebaceous gland. In sheep primary follicle development is initiated iiirst on the head and limbs and throughout subsequent develop- nuent those regions are most advanced. Secondary follicle iJiitiation is more rapid on the wool-bearing areas of the 1modyy and higher ratios of secondary to primary follicles arne recorded from these regions (Stephenson, 1958). Hardy and Lyne (1956) described branching follicles which developed from the secondary follicles. Dixon (1961) described the innervation of the hair fOllicles in the mammalian lip as follows: Vibrissal hairs are innervated by extremely thick nerve bundles which pass 13 obliquely through the fibrous follicle from below to spread out over the epithelial root sheath. A supplementary supply to the neck of the follicle is derived from the cutaneous nerve plexus. Smaller hair follicles are innervated by nerve bundles which descend from the cutaneous plexus toward the neck of the follicle just below the sebaceous glands. Nerve fibers are distributed on the epithelial root sheath in circular or longitudinal fashion and end between the cell layers or as expansions on the surface of the root sheath. Dixon explained these differences embryo- logically on the basis of relative time of development of the various structures. Vibrissal hair follicles have reached an advanced stage of differentiation at the time when main nerve fiber bundles are extending toward the skin surface and are thus able to receive direct innervation via their deep aspect from these nerve bundles. Small hair follicles differentiate at a later stage and must necessarily "invaginate" the cutaneous plexus from which they are then innervated. Glands Sebaceous Glands Smith (1960) reported rudimentary sebaceous glands in newborn pigs. David (1932), Sisson and Grossman (1956), 14 and Trautmann and Fiebiger (1957) also noted this fact. Smith (1960) described simple alveolar sebaceous glands filled with stratified epithelial cells associated with the hair follicles. These glands form rossette patterns around the upper third of the tactile hairs of the snout. According to Arey (1959) sebaceous glands begin as swellings on the outer epithelial sheaths of the hair follicles which gradually become lobulated, flask-shaped sacs whose lumina arise by fatty degeneration of the central cells. This is a holocrine type gland because of this disintegration of the gland cells themselves. Cells in the neck of the sac are the regenerating elements. Sweat Glands Smith (1960) found sweat glands associated with hair follicles in all areas of the body. The majority of the tubular Skin glands of domestic animals are of the apocrine type (Trautmann and Fiebiger, 1957). Smith (1960) found a coiled tubular type gland in all areas but the snout, where they are compound tubular. This agrees with the description given by Sisson and Grossman (1956). Trautmann and Fiebiger (1957) named the body or secretory portion of the swine sweat gland the glomiform gland. 15 Yasuda, §t_al, (1957) described the development of the sweat glands of cattle embryos. The primordia of sweat glands are first found on the forehead and sole and then on the abdomen. Most of these open into hair follicles near the epidermis. The formation of a glandular lumen takes place in cells of the transitional region from the root to the median portion of the ampulla as a longitudinal narrow cleft. The first glandular lumen appears on the forehead. Apparently the lumen forms by necrosis of central cells in the apical region and not by pressure of accumulated secretion. The excretory tubular portion consists of an inner layer of short columnar cells and an outer layer of flat cells. The carpal organ, a modified sweat gland, and the vinterdigital glands were described by Sisson and Grossman (1956), Trautmann and Fiebiger (1957), and Kurosumi and Kitamura (1958). Smith (1960) was unable to demonstrate either of these two types of glands in newborn pigs. Mammary Glands Balinsky (1950) studied the development of the mammary gland in the mouse and rabbit. Three periods of development were observed: (1) rudiments of the mammary glands are formed by an aggregation of epidermal cells; 16 (2) growth is retarded and differentiation limited to a rounding-off of the mammary buds; and (3) the glands become heterogeneous in comparison with the body as a whole. The mitotic indices of these areas indicate that formation of the mammary-gland‘rudiments is due not to local proliferation, but_to morphogenic movements (the migration or streaming of cells toward certain centers). Arey (1959) described an ectodermal thickening which makes a distinct linear elevation that extends from axilla to groin in the hog. This elevation is called the mammary ridge, or milk line. The originally elevated primordium flattens, cornifies, and becomes hollowed. About the time of birth, or later, these sunken areas elevate to become the nipple. MATERIALS AND METHODS Source of Animals Purebred Yorkshire fetuses were obtained by cesarean section from sows supplied by the Animal Husbandry Department of Michigan State University. The fetuses for this study represent four age groups, namely 30-35 days, 50-53 days, 72-74 days and 95 days of age. In the four fetuses in the youngest age group sex could not be determined grossly. Two males and two females were studied in each of the 50-53 and 95-day-old groups and two females only in the 72-74-day-old age group. Technique Immediately upon removal from the sow by cesarean .section the pigs were weighed, measured and put in a mixture of commercial formalin, 95% ethyl alcohol, glacial acetic acid, and distilled water (Lavdowsky's mixture, Guyer, 1949). The fetuses were removed from this mixture after forty-eight hours and placed in 70% ethyl alcohol. Skin specimens representing twenty-eight areas were taken from the fetuses (Pl. I). 17 m U 0 w P '11 0 Z 2 H N O 18 PLATE I AREAS FROM WHICH SKIN SPECIMENS WERE TAKEN Lip--upper . . Snout . . . . . . . . . . . Nose--dorsal . . . . . . . . . Eyelid--medial canthus . . Head-~frontal region jowl region . . . . . . Ear--external auditory meatus Neck--dorsal region ventral region . . . . . Thorax--dorsal region . . . . lateral region . . . . ventral region Abdomen--lateral region ventral region . . . Dorso-lumbar region . . Tail--proximal . . . . . . . . Anus . . . . . . . . . . . . . External genitalia (scrotum or Inguinal region . . . . . . . Hind limb--thigh . . . . . . . lateral tarsus . . lateral metatarsus Interdigital region Front limb--axilla . . . . . lateral humerus lateral carpus . . medial carpus . . Mammary gland . . . . . . . . vulva) and digits Section No. N [\J KO U'I\I U1 0‘0 .5 00 N l-’ N 00) F‘H «>0 H id rd #4 H UltPLAJNH F‘F‘NIH m-q~qo\ NNNN £100wa M 00 19 PLATE I 20 After fixation, the specimens were dehydrated and infiltrated by the butyl-alcohol-mush method of Johnson, gt 31. (1943). The tissues were embedded in "Tissuemat"l and horizontal and vertical sections were cut at 6 microns and stained with Harris' hematoxylin and eosin (Malewitz and Smith modification, 1955), Weigert's and Van Gieson's connective tissue stain, and Mallory's Triple Stain (Crossmon's modification, 1937). l . . . . . Fischer Sc1ent1fic Company, Pittsburgh, Pennsylvania. RESULTS AND DISCUSSION Epidermis In the 30—35-day-old fetus the epidermis was composed of 2-5 cell layers being thicker on the snout, jowl and digits than on the rest of the body (Pl. II, Fig. 1). The most superficial layer, the periderm, was composed of flattened, elongated cells with flattened nuclei. The beggi- lgyg; contained tall cuboidal to columnar cells with large active nuclei and prominent nucleoli. The cells of the middle layers, the stratum intermedium, were round to polygonal and also contained large nuclei. By 50-53 days there were 5-10 epidermal cell layers (Pl. IV, Fig. l). The periderm and basal layer were still prominent. Just superficial to the basal layer, or stratum cylindricum, was the stratum spinosum, both of which made up the stratum germinativum. The cells directly under the periderm had begun to flatten out to form the stratum corneum in thicker areas such as the snout and jowl. In all areas except the digits the basement membrane was parallel with the superficial surface of the epidermis (Pl. II, IV and VI, Figs. 1). The "rete pegs" seen in the digits were the beginning of the hoof laminae (Pl. XVI, 21 22 Figs. 1 and 2). The lack of rete pegs and hair follicles in many areas was believed to be responsible for the epidermis sloughing off. This occurred frequently during handling at the time of cesarean section. The cells of the entire epidermis were enlarged at this stage of development and contained little visibly stained cytoplasm (Pl. IV and V, Figs. 1). The enlarged cells were believed due to a type of "physiological edema" caused by the amniotic fluid in contact with the epidermis. Histochemical studies, which were beyond the scope of this work, might reveal a substance present in these cells which was dissolved during the fixation process. The intercellular substance of the stratum spinosum stained deeply in contrast to the cytoplasm of the cells and was possibly the start of the intercellular bridges. The epidermis of the 72—74—day-old group increased by 1-2 layers and the stratum corneum was well developed. Rete pegs had developed in the snout, but as yet were not very advanced. Hair canals had begun to form in advance of the emerging hair shaft, and ducts from the compound tubular and coiled tubular sweat glands were observed in the epidermis in close proximity to the hair canals (Pl. V, Fig. 2). The only hairs emerging from the epidermis at 23 this stage were the sinus hairs on the jowl (Pl. III, Fig. 1).- In the 95-day-old group the epidermis was fully developed on the snout, but was only partially developed on the lateral body wall. The stratum corneum was 2-3 cell layers thick and most of the cells had lost their edematous appearance. The stratum granulosum was apparent on the snout and was composed of 2-3 layers. very prominent rete pegs and dermal papillae were present and the sinus hair shafts on the snout had penetrated the epidermis and pushed some of the stratum corneum superficial to them (Pl.IV, Fig. 2). In the dorsal cervical area the epitrichium was observed lying over some hair shafts (Pl. IX. Fig. 2). It was 2—3 layers thick and lighter staining than the under- lying stratum corneum. No vernix caseosa was observed. Dermis The dermis of the 30-35-day-old fetus was composed mainly of mesenchymal connective tissue. Small blood vessels were present as well as "blood islands" containing nucleated erythrocytes (Pl. XXI, Figs. 1 and 2). These "blood islands" appeared to agree with the description of cutaneous hematopoiesis by Popoff and Popoff (1958); however, it was suggested these areas might have been hemorrhagic foci caused 24 by the state of anoxia which occurred directly following cesarean section. In the 50-53-day-old group the dermis had become more vascular and new capillaries and arterioles were characteristically lined by large endothelial cells. Large nerve trunks could be distinguished deep in the dermis. Two layers of the dermis were evident in the snout and on the abdomen, the more superficial stratum papillare made up of closely associated connective tissue fibers and the deeper stratum reticulare composed of a looser arrangement of connective tissue fibers. Many young fibroblasts were present and the hair follicles were surrounded by concentric rings of these cells which evidently formed the collagenous connective tissue fibers of the dermal root sheath (Pl. VII, Fig. 2). Collagenous connective tissue fibers were inter- laced between the hair follicles to form the network described by Smith (1960) (P1. X, Fig. 1). In the dorsal cervical, lateral thoracic, and abdominal areas the dermal layers were not yet formed. In the jowl there were many fibro- blasts loCated directly beneath the epidermis and connective tissue was forming beneath these cells. The panniculus adiposus, or fat layer, was well formed in the abdominal and axillary areas. Fat was scattered 25 throughout the dermis as well as being located in the fat layer, a finding observed by Alexandrowicz (1949). There appeared to be considerably more fat in the ventral than in the dorsal or lateral body areas. No collagenous connective tissue septum separating the panniculus adiposus into a superficial and deep layer was observed at this stage. The panniculus carnosus, or muscle layer, was visible beneath the panniculus adiposus. Developing skeletal muscle fibers, characterized by an atypical nuclear arrangement were observed throughout the body. The nuclei were arranged perpendicular to the long axis of the muscle fiber and in close proximity to each other (Pl. XVII, Fig. 1). In the 72-74-day-old fetuses the vascularity of the dermis had increased and blood vessels could be seen approaching the hair follicles (Pl. XIV, Fig. l) and the epidermis. Two of the three blood vessel networks previously described by Goodall and Yang (1954) and Ryder (1955) were formed. The deep dermal (Pl. XV, Fig. l) and mid-dermal networks were formed whereas the subepidermal plexus was not, although a few large vessels were apparent in this area. From the mid-dermal plexus branching vessels supplied the sinus hair follicles. Nerve fibers were observed innervating the sinus hairs in the snout. Large branches penetrated the heavy 26 dermal sheath and divided into smaller branches that coursed through the blood sinus and then proceeded up the outer root sheath to supply the rest of the follicle (Pl. XVIII, Figs. 1 and 2). Nerves were also associated with the connective tissue trabeculae of the blood sinus. The interlacing of collagenous connective tissue between the hair follicles was most advanced in the snout where the follicles were best developed. Each sinus hair follicle was separated from the others by these connective tissue fibers (Pl. X, Figs. 1 and 2) whereas in other areas the follicles were divided into groups of 3-5 (Pl. XI, Fig. 1). On the dorsal, lateral, and ventral areas of the body the fibroblastic proliferation had increased and the two layers of the dermis were apparent. The skeletal musculature was still forming and the smooth muScle fibers of the arrector pili muscles were visible. These muscles were attached to the hair follicle below the level of the sebaceous gland (Pl. VII, Fig. l) and on cross section could sometimes be seen completely encircling the hair follicle like a sling (Pl. XII, Figs. 1 and 2 and P1. XIII, Fig. 1). In the 95-day-old fetuses the dermis contained all of the components described by Smith (1960). Development was further advanced on the snout than on the dorsal and lateral 27 surfaces. All three plexuses of blood vessels were present. Elastic tissue fibers could be seen throughout the stratum reticulare in the ventral body areas and limbs in this age group (P1. XIX, Fig. 1). More strands were visible in conjunction with the heavy collagenous fibers separating the panniculus adiposus into a superficial and deep layer. The only elastic tissue seen in the younger fetuses was in the intima of the larger arteries. Hair In the 30-35-day-old fetuses two very prominent hair follicles were forming on the jowl (Pl. II, Fig. l), and on the snout small epidermal downgrowths indicated the beginning of other hair follicles. The epithelium around the follicles on the jowl was raised above the surrounding tissue. In the 50—53-dayéold group hair follicle formation had advanced considerably. Areas such as the snout, face, neck, back, ventral thorax and limbs had many follicles, while areas suCh as the lateral body wall and ventral abdomen showed no follicular activity. The hair follicle first appeared as an invagination of the stratum cylindricum of the epidermis (Pl. II, Fig. 2). As the cells pushed downward into the dermis the "peg" became 28 larger and was composed of a layer of cuboidal to columnar cells peripherally continuous with the stratum cylindricum, and many polyhedral cells in the center (Pl. VII and XI, Figs. 1). The cells were characterized by a large nucleus with prominent nucleoli. Fibroblasts were concentrically arranged around the column of cells and contributed to the collagen formation of the dermal or connective tissue sheath (Pl. VII, Fig. 2). In the snout the hair bulb of the follicles had formed, but as yet no invagination of connective tissue into the bulb had occurred. The two major types of hair were recognizable. The sinus hairs, found on the snout, jowl and eYE1ids, could be distinguished from the regular hairs on the rest of the body by their more advanced formation and increased thickness of the connective tissue sheath (Pl. II, Fig. 2). Early formation of each type of hair was similar. Sinus hairs did not form in groups as did the regular hairs. N0 hair shaft formation was evident in any of the hair follicles at this stage of development. In the 72-74-day—old group the blood vessels had begun to surround the sinus hairs and form the blood sinuses (Pl. XIV, Figs. 1 and 2). A single branching blood vessel SuPplied many hairs. 29 Sebaceous glands that had appeared only as swellings from the hair follicle at 50 days were fully formed and appeared functional (Pl. VII, Fig. 1). Other sebaceous glands were still in the process of formation (P1. VIII, Fig. 1). The regular hairs, which began their formation later than the sinus hairs, occurred in groups of 3-5 follicles (Pl. XI, Fig. l). The central hairs of the groups were more developed than the others. Frequently in the columnar cells of the peripheral layer the nuclei were all lined up distally leaving the cytoplasm toward the basement membrane (Pl. VII and XII, Figs. 1). This cytoplasm did not stain readily and this "layer" could possibly be the fetal version of the glassy layer of the hair follicle located between the dermal root sheath and the outer epithelial root sheath in adult follicles. In all areas except the snout the hairs formed on an angle with the epidermis (P1. VI, Fig. 1). In the snout the follicles occurred directly perpendicular to the epidermis (Pl. IV, Fig. 1). In many of the follicles the hair shaft had begun to form, especially in the sinus hairs. This was evidenced by an extension of cells from the bulb of the follicle in a 3O spear-like fashion forcing its way through the follicular cells. As the hair shaft increased in length the central cells appeared to lose their nuclei and migrate toward the surface as a part of the medulla. Peripheral to these "dead" cells the cortex and cuticle could be distinguished. The inner and outer epithelial root sheaths were apparent in those cells peripheral to the shaft. Nucleated cells were observed in both Henle's and Huxley's layers (Pl. XIII, Fig. l). Sweat glands had formed over the entire body and their ducts extended parallel and in close proximity to the hair follicle. Although the duct appeared to open into the hair canal as it passed through the epidermis, this could not be- proven in this study (Pl. V, Fig. 2; Pl. VI, Fig. l and P1. XX, Fig. 2). The duct was generally located along the hair follicle on the same side as the sebaceous gland. In a few cross sections of hair follicles, sweat glands could be seen completely surrounding the follicle (Pl. XX, Fig. 1). This could possibly have been at a level where a portion of the compound tubular gland wound around the hair follicle. By 95 days of fetal life many of the hairs were fully formed and had emerged from the surface. In some areas where hair was late forming such as the lateral body wall, the follicles were still in the process of formation. In all 31 areas there were some follicles not as far advanced as others. A few groups of hair follicles were infiltrated by many connective tissue cells (Pl. XI, Fig. 2). The blood sinus of the sinus hairs was well developed and in some areas was observed between two layers of the dermal sheath while in others it was between the dermal root sheath and the outer epithelial root sheath. As mentioned earlier, nerves were observed penetrating the dermal sheath (P1. XVIII, Figs. 1 and 2). In the regular hair follicles "follicular folds" were present (Pl. XV, Fig. 2). The folds observed were inferior to the sebaceous gland opening as described in cattle by Goldsberry (1955). On cross section of the hair follicles, all of the layers except the glassy layer could be readily distinguished (Pl. XIII, Fig. 1). In many instances medullae were lacking. In a number of instances, two medullae were present in the same follicle observed on cross section. This finding was not mentioned by Smith (1960) as occurring in the newborn swine; however, Pinkus (1951) reported the occurrence of multiple medullae in pig bristles. The tylotrich follicles previously described by Straile (1960) were not observed in this study. 32 Glands Sebaceous Glands Sebaceous glands were first apparent as swellings at the junction of the proximal and middle thirds of the hair follicles. These were first noticed on follicles in the snout and jowl of the 50-53-day-old fetuses (Pl. II, Fig. 2). The formation of these glands is apparently connected with the age of the follicle rather than the age of the fetus as they begin forming only in those follicles that have reached a certain length. In the 72-74-day-old group many of the sebaceous glands had already formed and appeared to contain functional cells (Pl. VII, Fig. 1). These cells were very large and exhibited foamy cytoplasm and a large nucleus. In some cross sections of the hair follicles these cells filled the entire follicle indicating the possible transformation of the polyhedral cells of the hair peg into glandular cells (Pl. VIII, Fig. 1). When the hair shaft separated the cells of the hair follicle, the glandular cells and in turn the cells of the outer epithelial root sheath were forced laterally (Pl. IX, Fig. 1). In many cross sections of hair follicles there were 1-7 areas of sebaceous gland cells around a central core 33 (Pl. IX, Fig. 2). These could either be 1-7 different develop- ing glands or one gland with 1-7 different lobes. According to Smith (1960) the sebaceous glands were rudimentary simple alveolar glands filled with stratified epithelial cells. In many areas of this study such as the sacral region and limbs the glands were not well developed. However, in other areas these glands appeared large and functional (Pl. VII, Fig. l and P1. VIII, Fig. 2). Sweat Glands Sweat glands in swine skin are of two principle types, coiled tubular and compound tubular (Trautmann and Fiebiger, 1957). The coiled tubular glands were found in all areas except the snout, whereas compound tubular glands were found in the snout and jowl (Pl. III, Fig. 2 and P1. XX, Fig. l). Coiled tubular glands were first observed in the 50-53- day-old fetuses in the eyelid and on the dorsal cervical regiOn. The main portion of the gland was located in the dermis at the level of the base of the hair follicle. In these young fetuses the duct of the gland was nearly the same size as the hair follicle itself (Pl. VI, Figs. 1 and 2). The body of the compound tubular glands was located deeper in the dermis near the panniculus adiposus. 34 The majority of sweat glands were first observed in the 72-74-day—old group. By this age all of the compound tubular glands had formed in the snout and the coiled tubular glands had formed over the rest of the body. Coiled tubular glands were less prevalent on the limbs and sacral region. The exact communication between the sweat gland duct and the hair follicle could not be determined in the present study. Carpal or interdigital glands were not observed in any of the fetal pigs in this study. Circumanal glands were readily observed in the 95-day- old fetuses (Pl. XIX, Fig. 2). MammaryrGlands In the 50-53-day-old group the only evidence of mammary gland formation was a "swollen" area on the surface Which corresponded to the mammary ridge described by Arey (1959). Increased fibroblastic activity was noticed in the dermis in this region. In the 724day-old fetuses there was increased collagen formation in the raised area, and hair was forming on each side but not immediately in the area. In the 95-day-old group the hair had fully formed at the sides of the mammary ridge and two ducts had formed in the nipple area (P1. XVII, Fig. 2), but no other glandular 35 activity was noticed. No sex differences were observed. Special Structures Eyelid In the 50-53-day-old fetuses the eyelids were fused together and the largest hair follicles had started to form on the dorsal lid at the fused area (Pl. V, Fig. 1). These follicles were the primordia of the eyelashes. The other follicles were not as far advanced. In the 72-day-old group the hairs were further developed, but the lids were still fused together (Pl. V, Fig. 2). The sebaceous and sweat glands were similar to those over the remainder of the body. Sinus hairs were observed both on the dorsal and ventral eyelids in approxi- mately the area of junction with the regular skin. No mention of these hairs was made by Trautmann and Fiebiger (1957) or Smith (1960). Many blood vessels, nerves and skeletal muscle fibers were observed in both eyelids, and in the 95-day-old fetuses the heavy connective tissue described by Trautmann and Fiebiger (1957) was observed. No tarsal glands were apparent. In the 95-day-old fetuses the eyelids were no longer fused. 36 External Ear In the 30-35—day-old fetuses the ear canal was observed. It consisted of a small canal lined with 1-2 layers of epithelium. No other structures were apparent. In the 50-53-day-old group the auricle had formed and the cartilage was forming. Hair follicle formation was observed. The epidermis over the auricle was thicker than that covering the head. Considerable collagenous connective tissue was forming in the main part and base of the auricle. In the 72-day-old fetus the hair follicles were fully formed, but the shafts had not yet formed. Sebaceous and coiled tubular glands had both formed. The skin covering the auricle and base of the auricle was fully formed in the 95-day-old group. No arrector pili muscles were observed attached to the hairs. Digits In the 30-35-day-old fetuses the area of the digits was characterized by an increased number of epidermal layers. By 50-53 days long rete pegs had formed which were the precursors of hoof laminae (Pl. XVI, Figs. 1 and 2). The epidermis had increased in thickness and the outer layers had begun to flatten. By 72 days the laminae had increased considerably in length and were only a few cells in thickness. By 95 days 37 the length had further increased and the epidermis became very thick. No distinct cornification was observed at this stage. SUMMARY AND CONCLUSIONS The skin of fourteen fetal Yorkshire pigs representing four different age groups was studied histologically. There was about 20 days difference in age between the groups of fetuses, namely 30-35, 50-53, 72 and 95 days of age respectively. The normal gestation period for a sow is 114 days. The epidermis in the 30-35-day-old fetus was 2-5 cell layers thick increasing by 2-3 layers every 20 days. The epidermal cells of the young pigs appeared edematous, perhaps due to contact with the amniotic fluid. In the older groups these cells gradually lost this appearance as the epidermis became thicker and more developed. The stratum germinativum was apparent by 50-53 days, the stratum corneum by 72 days and the stratum granulosum by 95 days. The stratum lucidum could not be observed in this study. The dermis in the 30-35-day—old fetuses was composed of mesenchymal connective tissue with "blood islands" scattered throughout. Fibroblasts appeared early and had begun forming collagenous connective tissue by 50-53 days of age. Blood vessels and nerves increased in numbers and were very nearly all formed by 72 days of age. Elastic tissue fibers were quite prevalent in the ventral body areas of the 38 39 95-day-old fetuses, but were absent in the earlier age groups. The two layers of the dermis, the superficial stratum papillare and the deeper stratum reticulare, were recognized at 50-53 days of age on the snout and abdomen. The panniculus adiposus was apparent in the 50-53-day- old fetuses and was more prevalent in the ventral body areas. By 95 days of age this layer was divided into a superficial and deep layer by a broad band of collagenous connective tissue fibers. The development of the panniculus carnosus was characterized by formation of skeletal muscle with nuclei arranged perpendicularly to the long axis of the muscle fiber. The earliest developing hair follicles were observed on the jowl and snout in the 30-35-day-old fetuses. The follicles on the jowl were further advanced. By 50-53 days of age follicles were located over most of the body in various stages of development. The sinus hairs could only be distinguished from the regular hairs at this stage by the greater development and thicker connective tissue sheath. Sebaceous gland swellings were visible at this stage, but very little glandular material was apparent. In the 72-74- day-old group of fetuses the hairs had developed far enough 40 for hair shafts to be apparent. These shafts had not penetrated the surface except on the jowl. The sinus hairs could be distinguished from the regular hairs, and nerves were seen penetrating the dermal root sheath of the sinus hairs. Sinus hairs were apparent on the snout, jowl and eyelids. By 95 days of age all of the characteristics of hairs were apparent including the "follicular folds," and new follicles were still forming. The sebaceous glands appeared functional in the 72-74— day—old group of fetuses. The glandular cells were large with prominent nuclei and foamy cytoplasm which filled the entire follicle before the developing hair shaft separated these cells. Sebaceous glands were present in all sections studied although they were more prevalent in the ventral body areas than in the dorsal. Sweat glands of the coiled tubular type were observed over the entire body, except on the snout where there were compound tubular glands. The body of the coiled tubular gland was located parallel with the base of the hair follicle and was first observed in the 50-53-day-old group in the dorsal cervical area and the eyelid. The compound tubular glands located on the snout and jowl were not present until 72-74 days of age. The bodies of these glands were located very deep in the dermis near the panniculus adiposus. 41 Mammary glands were observed only as a raised area or "mammary ridge." In the 95-day-old fetus two ducts were observed in the mammary gland regardless of sex. The eyelids remained fused until 95 days of age. Large hairs developed on the dorsal lid and sinus hairs were observed on both lids away from the lid margins. The eyelids were characterized by considerable muscular and connective tissue structure. The development of the skin covering the auricle showed no differences from the other body areas. Hoof laminae were first noticed in the 50-53-day-old fetuses as small epidermal projections. In the older age groups they were longer and were thicker on the pectoral than on the pelvic digit. LITERATURE CITED Achten, G. 1959. Recherches sur la kératinisation de la cellule épidermique chez l'homme et le rat. Arch. Biol. 70:1-119. Abstracted in Biological Abstracts 35:5083. 1960. ‘ Alexandrowicz, St. 1949. The accumulation of fat in the subcutaneous tissue of pig embryos. Bull. Internat. Acad. Polonaise Sci. et Lettr. Cl. Sci. Math. et Nat. Ser. B: Sci. Nat. (2) [2001.] 1949 (4/6):26l-275. Abstracted in Biological Abstracts 24:2240. 1950. Arey, L. B. 1959. Developmental Anatomy. W. B. Saunders Company, Philadelphia. Arvy, L. 1957. Histogénese et répartition des labrocytes chez le rat. Compt. Rend. Assoc. Anat. 43:165-170. Abstracted in Biological Abstracts 33:3110. 1959. Balinsky, B. J. 1950. On the developmental processes in mammary glands and other epidermal structures. Trans. Roy. Soc. Edinburgh 62:1-31. Becker, S. W., Jr. and A. A. Zimmermann. 1957. Development of the basement membrane in human skin. Jour. Invest. Dermatol. 28:195-198. Bocharov, Y. S. 1957 [On the morpho-physiological inter- action of the epidermis and dermis during the development of the skin of man.] Biul. Mosk. O-va. Ispyt. Prirody 62:106-109. Abstracted in Biological Abstracts 35:1831. 1960. Colin, E. C. 1943. Hair direction in mammals; embryogenesis of hair follicles in the guinea pig. J. Morph. 72:191—223. Copenhaver, W. M. and D. D. Johnson. 1958. Bailey's Textbook of Histology. Williams and Wilkins Company, Baltimore. Crossmon, G. 1937.. A modification of Mallory's connective tissue stain with a discussion of the principles involved. Anat. Rec. 69:33-38. 42 43 David, L. T. 1932. Histology of the skin of the Mexican hairless swine (Sus scrofa). Amer. Jour. Anat. 50:283-292. Dempsey, M. 1948. The structure of the skin and leather manufacture. Jour. Roy. Microscop. Soc. 67:21-26. Diomidowa, N. A. 1954. Die Embryonale Entwicklung der Schafhaut. Acta Agron. Acad. Sci. Hungarieae 4:387-403. Abstracted in Biological Abstracts. 33:2776. 1959. 1955. Novye dannye o stokakh obrazovaniya sherstyanykh volokon u ovets. [New data concerning the time of formation of wool fibers in sheep.] Zhivotnovodstvo. 1955:64-69. Referat. Zhur., Biol., 1956, No. 61367. (Translation). Abstracted in Biological Abstracts 32:1072. 1958. Dixon, A. D. 1961. The innervation of hair follicles in the mammalian lip. Anat. Rec. 140:147-158. Dolnick, E. H. 1959. Histogenesis of hair in the mink and its relationship to dermal fetal fat cells. J. Morph. 105:1-31. Eichenlaub, F. J. and R. A. Osbourn. 1951. Studies in the histogenesis of the epidermis. Arch. Dermatol. and Syphilol. 64:700-712. Goldsberry, S. and M. L. Calhoun. 1959. The comparative histology of the skin of Hereford and Aberdeen Angus cattle. Am. Jour. Vet. Res. 20:61-68. Goodall, A. M. and S. H. Yang. 1954. The vascular supply of the skin of Ayrshire calves and embryos. Jour. Agric. Sci. 44:1-4. Grenburg, T. F. 1957. Sravnitelno-gistologicheskoe izuchenie razvitiya epidermisa pozvonochnykh zhivotnykh i cheloveka. [A comparative-histological study of epidermis development in vertebrates and humans.] Arkh. Anat. Gistol. i Embriol. 34:79-89. Abstracted in Biological Abstracts 35:1831. 1960. 44 Guyer, M. F. 1949. Animal Micrology. University of Chicago Press, Chicago. Hanson, J. 1947. The histogenesis of the epidermis in the rat and mouse. Jour. Anat. 81:174-197. Ham, A. W. 1957. Histology. J. B. Lippincott Company, Philadelphia. Hardy, M. H. and A. G. Lyne. 1956. The pre-natal development of wool follicles in merino sheep. Austral. Jour. Biol. Sci. 9:423-441. Holmes, R. A. 1960. The Microscopic Anatomy of the Skin of Mus Norvegicus Albinus. Thesis. Michigan State University. Johnson, E., F. N. Andrews and C. L. Shrewsbury. 1943. The preparation of muscular tissue for histological study. J. Anim. Sc. 2:244-250. Kurosumi, K. and T. Kitamura. 1958. Occurrence of foldings of plasmal membrane (B—cytomembrane) in cells of pigs carpal organ as revealed by electron microscopy. Nature 181:489. Lyne, A. G. and M. J. Heideman. 1959. The pre-natal development of skin and hair in cattle (Bos Taurus L.). Austral. Jour. Biol. Sci. 12:72-95. Malewitz, T. D. and E. M. Smith. 1955. A nuclear stain employing dilute Harris hematoxylin. Stain Tech. 30:311. Mancini, R. E., O. Vilar, E. Stein and H. Fiorini. 1959. Estudio histoquimico en el desarrollo postnatat del tejido conectivo cutaneo. Rev. Soc. Argentina Biol. 35:196-204. Abstracted in Biological Abstracts 35:4353. 1960. Maximow, A. A. and W. A. Bloom. 1957. A Textbodk of Histology. W. B. Saunders Company, Philadelphia. Matoltsy, A. G. 1958. Keratinization of embryonic skin. Jour. Invest. Dermatol. 31:343-346. VL' ‘UVA‘ vflv nov. ave. 45 Nbdawar, P. B. 1953. The micro-anatomy of the mammalian epidermis. Quart. Jour. Microscop. Sci. 94:481-506. Menefee, M. G. 1955. The differentiation of keratin- containing cells in the epidermis of embryo mice. Anat. Rec. 122:181-192. Montagna, W. 1956. The Structure and Function of Skin. Academic Press, New YOrk. Patten, B. M. 1946. Human Embryology. The Blakiston Company, Philadelphia. Pinkus, H 1951. Multiple hairs (Flemming-Giovannini). Report of two cases of pili multigemini and discussion of some other anomalies of the pilary complex. Jour. Invest. Dermatol. 17:291-301. Popoff, L. and N. Popoff. 1958. L'hémopdiése cutanée au cours de la vie intfa-utérine. Ann. Dermatol. et Syphiligraph. 85:157-167. Ryder, M. L. 1955. Studies on the nutrition of wool follicles in sheep: The anatomy of the general blood supply to the skin. Jour. Agric. Sci. 45:311-326. Schaeffer, J. P. 1953. Morris' Human Anatomy. The Blakiston Company, New York. Sisson, S. and J. D. Grossman. 1956. Anatomy of Domestic Animals. W. B. Saunders Company, Philadelphia. Smith, J. L. 1960. The Microscopic Anatomy of the Integument of New Born Swine. Thesis. Nfichigan State University. Stephenson, S. K. 1958. Wool follicle development in the New Zealand Romney and N-type sheep. ll. Follicle population density during fetal development. Austral. Jour. Agric. Res. 9:138-160. Stiles, K A. 1956. Handbook of Histology. The Blakiston Company, Philadelphia. Straile, W. E. 1960. Sensory hair follicles in mammalian' skin: the tylotrich follicle. Am. J. Anat. 106:133-148. 46 1961. The morphology of tylotrich follicles in the skin of the rabbit. Am. J. Anat. 109:1-13. Strickland, J. 1958. The Microscopic Anatomy of the Skin and External Bar of Felis Domesticus. Thesis. Michigan State University. Trautmann, A. and J. Fiebiger. 1957. Fundamentals of Wilson: Yasuda, Histology of Domestic Animals (Translated and Revised by R. E. Habel and E. L. Biberstein). Cornell University Press, Ithaca, New York. (1960 German Edition-Lehrbuch Der Histologie und Vergleichenden Mikroskopishen Anatomie der Haustiere, Revised and Enlarged by Otto Krolling and Hugo Grau, Paul Parey, Berlin.) J. A. 1941. Modern Practice in Leather Manufacture. Reinhold Publishing Corporation, New York. K. T., T. Kaga, T. Goto, T. Kashimura, H. Furusawa and K. Kobayashi. 1957. A study of the development of the sweat glands of cattle embryos. Keio Jour. Med. 6:159-218. Figure 1. Figure 2. 47 PLATE II A vertical section through the jowl region of a 35-day-old fetus showing the two first hair follicles to be formed. Notice the appearance of the epidermis. H. and E. Stain. 310X A vertical section through the jowl region of a 50-day-old fetus showing the raised area of epidermis and the three hair follicles present in this area. Notice the further development and thicker connective tissue sheath of the middle hair follicle. H. and E. Stain. 77X .. o'...n’..¢.ao to , to A o. on o .. . .4 . 48 PLATE III Figure l. A vertical section through the jowl region of a 72-day-old fetal pig showing the sinus hair follicle with its shaft. Mallory's Trichrome Stain. 64X Figure 2. A horizontal section of the jowl region of a 72- day-old fetus showing the compound tubular sweat glands of this region. Mallory's Trichrome Stain. 123x Figure 1. Figure 2. 49 PLATE IV A vertical section of the upper lip of a 53-day- old fetus showing the first two hair follicles to develop in this area. Notice the small blood vessel near the larger of the two follicles. H. and E. Stain. 208X A vertical section through the snout of a 95-day- old fetus showing the eruption of two hair shafts. Notice the difference in the epidermis between this Figure and Figure 1. Mallory's Trichrome Stain. 125x 50 PLATE V Figure l. A vertical section through the fused eyelids of a 53-day-old fetus showing the first few hair follicles. The largest hair follicle present is on the dorsal lid. Mallory's Trichrome Stain. 105X Figure 2. A vertical section through the fused eyelids of a 72-day-old fetus showing the marked increase in development of hair follicles and glands. The dorsal eyelid is on the left-hand side. Mallory's Trichrome Stain. 92X 1. Sebaceous gland 2. Sweat gland duct 0" ‘ I \.5 Figure 1. Figure 2. 51 PLATE VI A vertical section of the dorsal cervical area of a 53-day-old fetus showing the early development of the coiled tubular sweat gland duct and its relationship to the early hair follicle. Notice the angle of the hair follicle with the superficial surface of the epidermis. Mallory's Trichrome Stain. 193x 1. Hair follicle 2. Sweat gland duct A horizontal section of the same area as Figure 1 from the same fetus showing the relationship of the gland duct with the hair follicle in cross- section. Notice the lack of a lumen in the sweat gland duct. Mallory's Trichrome Stain. 222x .0 ' ' ‘ . ‘ , -0. o -~J-“d4 (“jOUO '7 . Vi wv & o'i' V . vol "I..- . 4 ‘ . N f! “ 52 PLATE VII Figure l. A vertical section through the dorsal cervical area of a 72-day-old fetus showing the development of the hair follicle and its associated structures. Mallory's Trichrome Stain. 155x 1. Sebaceous gland 2. Arrector pili muscle 3. Columnar cells surrounding hair follicle 4. Sweat gland duct Figure 2. An oblique section through the snout of a 50-day— old fetus showing the concentric rings of fibro- blasts around the hair follicles. Mallory's Trichrome Stain. 222X all .‘Ju ‘Niuvwmx . \ {ref}? _, s . ,,, . «$12 .. 53 PLATE VIII Figure l. A horizontal section through the snout of a 72-day- old fetus showing the sebaceous gland cells completely filling the hair follicle. Notice the sweat gland ducts close to the follicles. Refer to page 32 for an explanation of this Figure. Mallory's Trichrome Stain. 377X Figure 2. An oblique section through the snout of a 95-day-old fetus showing the relationship of the sebaceous gland around the hair follicle. H. and E. Stain. 225x . ..., -- -...f 3..-. {franfdnhflfl . 1 id}: .. .o . ”It 1” V. . . I. ., I a - .,. n..-i.l\..‘ll--- ‘7... .L..||u(\ 1 .1 » . If.” ’0 ’5. .n‘ , , I‘ll. \0 - . W-, , .. ., , 1.. [In ngflwwwtth . Figure 1. Figure 2. 54 PLATE IX A horizontal section through the snout of a 95-day- old fetus showing the relationship of the sebaceous gland and the sinus hair follicle. Mallory's Trichrome Stain. 105x An oblique section through the dorsal cervical area of a 95-day-old fetus showing the relation- ship of the sebaceous gland to a regular hair follicle. Notice there are many clusters of sebaceous gland cells around the follicle. Mallory's Trichrome Stain. 425x 1. Epitrichium 2. Part of hair shaft - S Y u a \J 55 PLATE X Figure 1. A horizontal section through the snout of a 72- day-old fetus showing the interlacing of connective tissue fibers between the hair follicles. Mallory's Trichrome Stain. 106X Figure 2. A horizontal section through the snout of a 95- day-old fetus showing the further development of the interlacing connective tissue fibers and the prominent sinus hair follicle. H. and E. Stain. 101X v. sns _“ Ig'; ‘ vii-”'5‘ 1W:- ' .f.."é.‘ "' .. ( .L ..‘ .3! o ‘V‘p ‘ ‘ ' f V ‘ 0' ' \ w «“2 ,. as: i}. «x. ‘ _ ‘ ' 9“: _\ “.‘M~'r;; - a “i C ‘ ”,4, "4 Figure 1. Figure 2. 56 PLATE XI A horizontal section through the dorsal surface of the nose of a 72-day—old fetus showing the groups of hair follicles and gland ducts with the connective tissue between. H. and E. Stain. 99X A horizontal section through the dorsal surface of the nose of a 95-day-old fetus showing an atypical infiltration of connective tissue cells around a group of hair follicles and gland ducts. Mallory's Trichrome Stain. 198x 57 PLATE XII Figure l. A horizontal section from the dorsal cervical area of a 72-day-old fetus displaying how the arrector pili muscle was observed completely surrounding the hair follicle. Notice the muscle on either side of the sweat gland duct (1). Mallory's Trichrome Stain. 294x Figure 2. A horizontal section from the dorsal surface of the nose of a 95-day-old fetus showing the arrector pili muscle connecting two follicles. Mallory's Trichrome Stain. 190X Figure 1. Figure 2. 58 PLATE XIII A horizontal section taken from the dorsal surface of the head of a 95-day-old fetus showing the cross- sectional appearance of four hair follicles and an arrector pili muscle connecting two of them. Mallory's Trichrome Stain. 290X An oblique section through the dorsal surface of the nose of a 72-day-old fetus showing a hair follicle, an arrector pili muscle and a gland duct with a small sac-like structure attached. Mallory's Trichrome Stain. 120X 59 PLATE XIV Figure 1. A horizontal section through the snout of a 72-day- old fetus showing a large blood vessel and its branches around the hair follicles. H. and E. Stain. 115X Figure 2. A horizontal section through the snout of a 95- day-old fetus showing a similar arrangement as in Figure 1. Mallory's Trichrome Stain. ll3X s ... Ow w J . \x . 4 _ i ..1. .. ,.. - . . 14‘ 6 31. b« . .li... r .g I ”‘Ol'iy. .rl rillvdiuhwlwnfl 6O PLATE XV Figure l. A horizontal section from the lateral humeral area of a 72-day-old fetus showing the deep dermal arterial plexus of blood vessels. H. and E. Stain. ll6X Figure 2. A vertical section from the dorsal surface of the head of a 95—day-old fetus showing an oblique section through the follicular folds. H. and E. Stain. 365X Figure 1. Figure 2. 61 PLATE XVI A horizontal section through the pelvic digit of a 53-day-old fetus showing the laminae and epidermis. H. and E. Stain. 63X 1. Third phalanx 2. Laminar corium 3. Laminae 4. Epithelium A horizontal section through the pectoral digit of a 50-day-old fetus showing the laminae and epidermis. Mallory's Trichrome Stain. 88X 62 PLATE XVII Figure l. A vertical section from the ventral abdominal area of a 50-day-old fetus showing developing skeletal muscle fibers. Mallory's Trichrome Stain. 123X Figure 2. A horizontal section through a nipple of a 95-day- old male fetus showing two ducts in the center. H. and E. Stain. 46X 1.. ‘fiL‘l.’ v9 ... I... 0.1. .. .sv ._ . . enomwdou$ (\ A . o 5 av». .. .3. w. A“... Figure 1. Figure 2. 63 PLATE XVIII A vertical section through the snout of a 95-day-“ old fetus showing two nerves penetrating the dermal sheath of a sinus hair follicle and extending up into the blood sinus. See arrows. Mallory's Trichrome Stain. 226X A vertical section through the sinus hair on the jowl of a 95-day-old fetus showing the entrance of the nerve fiber through the dermal sheath. See arrows. Mallory's Trichrome Stain. 97X 64 PLATE XIX Figure l. A horizontal section from the axillary region of a 95-day-old fetus showing some of the elastic tissue fibers present in the dermis. (Elastic tissue is black). Weigert's and Van Gieson's Stain. 200X Figure 2. A vertical section through the junction of the rectum and the anus of a 95-day-old fetus showing the Circumanal glands. Mallory's Trichrome Stain. 88X Figure 1. Figure 2. 65 PLATE XX A horizontal section from the snout of a 95-day- old fetus showing fragments of a compound tubular sweat gland positioned around a sinus hair follicle and a duct leading away. H. and E. Stain. 84X A vertical section from the dorsal thoracic area of a 72-day-old fetus showing the junction of a' coiled tubular sweat gland duct with the hair follicle. H. and E. Stain. 68X 66 PLATE XXI Figure 1. One of the areas of blood cells ("blood islands") observed in the 35-day-old fetus in the dermis throughout the body. Mallory's Trichrome Stain. 400X Figure 2. Another "blood island" observed in the 35-day- old fetal pig. Mallory's Trichrome Stain. 400X “LIB." ”TIT/[[1711 [[[Ifl[lfl[[ [11111111 [7!“