THE COMPARATIVE MICROSCOPIC ANATOMY OF THE TRACHEA OF THE PIG AND HORSE By Mara K* Henderson A THESIS Submitted to the College of Veterinary Medicine of Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Anatomy 1964 ProQuest Number: 10008722 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10008722 Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346 / r To My Mother ACKNOWLEDGMENTS My sincere thanks and appreciation are extended to Dr- M* Lois Calhoun, Professor and Chairman of the Depart­ ment of Anatomy, for her patience, guidance, during the course of this investigation* and cooperation X am deeply grateful to Mrs* Janet Sharon, who made the photomicrographs possible, and to Dr* Esther M« Smith, Associate Professor, of Anatomy, Department for her interest and technical assistance. Special thanks are extended to Dr* C. W. Titkemeyer, Associate Professor, Department of Anatomy, and the other members of the staff of the Anatomy Department, constructive criticisms of this manuscript. thanks are due to Mr* Igolima T. D. Amachree, ing assistance, suggestions, for their My heartfelt for his untir­ and encouragement throughout the preparation of this thesis. Thanks are also due to Dr. Richard Witter, Misses Betty Mason, Effie Lou Ball, this investigation* and Dr* Pauline Ho, who started X am grateful to the staff of Michigan State University*s slaughter house, and the Lang Feed Com­ pany, Romulus, Michigan, for their kind assistance in col­ lecting the trachea specimens of the pig and horse respec­ tively * Last but not least, Government for its support, I am indebted to the Liberian thus making my study in the United States of America possible. TABLE OF CONTENTS Page * * * * * .................... vi LIST OF TABLES * . . . . . LIST OF PLATES. INTRODUCTION. ........... vii . . . . . . . . . . . . REVIEW OF LITERATURE. .................. . . . . . . . MUCOSAL EPITHELIUM. 2 . . . . . . 3 TUNICA PROPRIA, SUBMUCOSA AND ELASTIC FIBERS. G LA N D S 0 0 0 1 . . 0 . . . . . . 0 . . . . . . . . 6 B . CARTILAGE AND PERICHONDRIUM . . . . . 10 MUSCLE. . . . . . . . . . . . . . . . . . . . . 10 ............. 11 VESSELS AND NERVES. MATERIALS AND METHODS . . . . . . . . . . . RESULTS AND DISCUSSION. MU C O S A O O O O . . . . . . . O 12 . . . . . . . . . O ® . . 9 . 14 0 0 0 . . . . . 1 4 . Epithsliuin a o o o o . o o . o . o o . o * . Basement Membrane Lamina Propria. SUBMUCOSA GXiANDS. . . . . CARTILAGE . - . . . . . . . . . . . . . . . . 1 7 . . . . . . . . . . . 14 . . . . . . . o . o . . . . . . . . 17 . . . . . 19 . . . . . 1 9 o o o . o o o . o o . o o o o . . ..22 PERICHONDRIUM . . . . . . . . . . . 23 ANNULAR LIGAMENT. . . . . . . . . . . . . . . . 23 TRACHEALIS MU SCLE . . . . o . BLOOD AND LYMPHATIC VESSELS o . o . . . . . . 2 4 . . . . . . . . . . 25 Page NERVES .............................................. 26 ADVENTITIA. . . . . . . . COMPARATIVE HISTOLOGY . . . . . . SUMMARY AND CONCLUSIONS LITERATURE CITED. 26 28 ............................ 33 ........................................ v 38 LIST OF TABLES Table 1. 2. 3. Page Measurements of the microscopic structures of the trachea of the p i g ........................... . 30 Measurements of the microscopic structures of the trachea of the h o r s e ......... .. . 31 Comparison dimensions at dorsal, the upper, of the mean values of the structural of the tracheas of the pig and horse lateral, and ventral regions and at middle and lower levels............. . 32 LIST OF PLATES Plate Page 1. Comparative histology; horse and pig trachea; cross section, ventral region, middle level . . . 39 2. Epithelial cell types; pig trachea; cross section, ventral region, lower lev e l ............... 40 3* Transitional-type epithelium; horse trachea; cross section, lateral region, lower level. . . . 41 4. Basement membrane; horse trachea; cross section, lateral region, middle level ........................42 5. Circular elastic fibers in superficial lamina propria; pig trachea; cross section, lateral region, upper level ............................... 43 6. Circular elastic fibers underlying elastic membrane; horse trachea; cross section, lateral region, upper l e v e l ......... . ..................... 44 7. Branched tubular glands; horse trachea; cross section, ventral region, upper level............... 45 8. Mixed cross 9. 10. 11. 12. 13. sero-mucous gland acini; horse trachea; section, dorsal region, middle level . . . . 46 Duct of mixed sero-mucous gland; pig trachea; cross section, dorsal region, middle level. . . . 47 Gland distribution; pig trachea; cross section, ventral, lateral, and dorsal regions, middle level . . . . . . . . . . . . . . . . . 48 Gland acini outside muscle fibers; pig trachea; cross section, dorsal region, upper level . . . . 49 Gland structure in relation to intercartilagenous area; pig trachea; longitudinal section, ventral region, upper level . . . 50 Appearance of cartilage rings; horse and pig tracheas; longitudinal section, dorsal, lateral and ventral regions, middle level . . . . . . . . 51 vii Page Plate 14. Cartilage degeneration; horse trachea; cross section, ventral region, lower l e v e l ............... 52 15o Relative thickness of perichondrium; pig trachea; cross section, ventral region, lower level* . * * 53 16. Elastic character of annular ligament; horse trachea; cross section, dorsal region, middle level . . . . . . . . . . 54 Indirect muscle attachment; horse trachea; cross section, dorsal region, lower level ............. 55 17. 18. Direct muscle attachment; horse trachea; cross section, lateral region, upper l e vel............... 56 19. Comparison of muscle-gland distribution; horse and pig trachea; cross section, dorsal region, .................................57 lower level 20. Blood and lymphatic vessel; pig trachea; cross section, lateral region, middle level ........... viii 58 INTRODUCTION Realizing the need of veterinary anatomists to es­ tablish general histological criteria by which a given organ or its counterpart peculiar to various species can be dif­ ferentiated, histologists are actively engaging in research and comparing the results with those of earlier German in­ vestigations * Other avenues of veterinary medicine are under con­ sideration* For example, animals are potentially important as possible donors of tissue transplants to the human spe­ cies* The production and distribution of food especially animal protein, today* is one of the major concerns in the world If such developments are to attain significance, the normal anatomy of experimental animals must be carefully examined* This can be accomplished only if students of veterinary and human medicine investigate the normal his­ tology and cytology of domestic and laboratory animals* The normal morphology of individual species must be thor­ oughly mastered before physiological and pathological con­ ditions can be differentiated* This study is an attempt to supplement existing information by comparing the microscopic anatomy of the pig and horse trachea* 1 REVIEW OF LITERATURE Few textbooks, if any, contain a detailed microscopic study of the trachea of any domestic animal. As a result, teachers and researchers have to rely on the more complete material available on the human trachea or whatever scat­ tered information is found on the various domestic animals. Today anatomists are trying to bridge this gap both by ac­ tively engaging in histological research and comparing the results with earlier German investigations. Microscopic anatomy of the trachea of the following species have been reported: 1913); pig, horse, and cat (Paul, dog and cat (Niewenhuis, 1961); mink (Stowe and Calhoun, 1962); and ruminants (Miller, 1963). Since the investigations by Niewenhuis Miller (1961) and (1963) contain a thorough review of the literature available on the trachea, the citations here are limited to publications on the trachea of the pig and horse. How­ ever, reference will be made to other animals where compara­ tive or morphological differences are noted. The tracheal wall of domestic animals is a composite of four main layers, namely: a) mucosa, b) submucosa, c) fibro-elastic membrane containing and connecting the carti­ lage rings, and d) muscularis present only dorsally. This has been substantiated by the investigations of many authors including Trautmann and Fiebiger 2 (1957) and Niewenhuis (1961). 3 Miller (1963) does not consider the trachealis muscle as a separate layer. Trautmann and Fiebiger (1957) included the submucosa as a part of the mucosa. (1962) Stowe and Calhoun found the annular ligament (known as the fibroelastic membrane) in the ranch mink void of elastic fibers. Paul (1913) listed the following components of the tracheal wall of the pig, horse, and cat: a) mucosal epi­ thelium, b) basement membrane, c) tunica propria, submucosa, and elastic tissue (referred to as subepithelial layers), d) glands, e) muscle, and f) cartilage. MUCOSAL EPITHELIUM Paul (1913) classified the epithelium of the tra­ cheal mucosa in the pig and horse as stratified, ciliated cylindrical. Chauveau Prior to P a u l ’s investigation, Verson (1868), (1873), Frankenhauser (1879), Ellenberger and Baum (1908), and Sussdorf (1911) had used this classification to describe the tracheal epithelium of domestic animals (Paul, 1913). The epithelium in the horse (Paul continued) is five to ten nuclear layers in depth compared to three layers in swine, and consists of the following cell layers: ciliated cylindrical cells, 2) intercells, 1) 3) basal cells, 4) goblet cells, and 5) leucocytes. According to Paul (1913), the ciliated cells form the superficial layer of the tracheal mucosa. These cells have a clear cuticular border which is not as wide as that 4 found in swine. shaped. The cells vary from cylindrical to cone- The cytoplasm is granular and homogenous. Its oval shaped nucleus is located near the cell base. The basal border of the ciliated cell is smooth in some areas or indented in others or even formed “foot-likew processes in some sections. The intercells or wedge cells of Paul (1913) are spindle shaped and lie lateral and distal to the ciliated cells with their nucleus toward the cell base. cited Miller Miller (1963) (1932) who noted that these intermediate cells form a layer of cells. However, according to Paul (1913), these cells do not form a distinct layer. Paul angular, (1913) reported two layers of irregular, poly- spherical nucleated basal cells beneath the cylin­ drical cells. swine. They are larger in the horse than in the He noted that Verson (1868), Krause (1876), and Frankenhauser (1879) described two layers of basal cells in large animals and one layer in small animals. Between the cylindrical cells, isolated or in groups, Paul (1913) found goblet cells in the dorsal and ventral regions of the pig and horse tracheal mucosa. Niewenhuis (1961) reported goblet cells in the dorsal part of the dog and cat tracheal epithelium. Miller (1963) found goblet cells decreasing in number from the ventral to lateral to dorsal region in the goat, sheep, and cow. Paul (1913) noted more numerous goblet cells in the horse than in the pig. Usually, goblet cells are more abundant toward the 5 lung and larynx* Paul (1913) and Trautmann and Fiebiger (195 7) ob­ served lymphocytes in the tracheal epithelium of the pig and horse* Paul noted that Bauersachs (1910) described leucocytes in all layers of the sheep tracheal mucosa. Miller (1963) observed frequent lymphatic invasion of the epithelium in the trachea of the goat, sheep and cow. Andrew (1959) recalled Mihalik*s (1935) study on the origin of cilia in the respiratory epithelium of cats, dogs, and rabbits as follows: "In the basal replacing cells, the centrosomes multiply in number and group themselves about an intracellular vesicle which appears in the cytoplasm. Cilia then grow out into the lumen of this vesicle. as the basal cells extend to the surface, Later, the ciliated ves­ icles open out, giving rise to the free ciliated border." In Andrew*s opinion, this seems to be a more efficient and rapid method of cilia replacement on the free surface than would be possible from migrating centrosomes. Paul (1913) identified islands of flat "squamous" epithelium in the dorsal region of the ciliated cells in the swine trachea. Patches of this transitional type epi­ thelium were reported in the dorsal region in dogs and cats (Niewenhuis, 1961); goat, sheep and cow (Miller, 1963). Paul reported that the transition from flat to cylindrical epithelium is abrupt. The basement membrane in the pig and horse lies be­ tween the epithelium and the connective tissue of the lamina 6 propria (Paul, 1913). In the horse it is clear, homogenous, irregular in thickness and at times filled with leucocytes• In the pig, Paul continued, the basement membrane toward the lamina propria contains small cells with elongated nuclei lying parallel to the nuclei of the connective tissue. the epithelium, the basement membrane is indistinct. Toward In some animals the basement membrane is rudimentary (Trautmann and Fiebiger, 1957); indistinct (Niewenhuis, 1961); faint but discernible, more refractile than cellular (Miller, TUNICA PROPRIA, Paul 1963). SUBMUCOSA AND ELASTIC FIBERS (1913) described a clear fiber containing matrix of elastic elements and lymphoid cells between the basement membrane and the submucosa in the swine. He recalled Frank- enhauser's (1879) finding that the mucosa rests on connective tissue which contains elastic fibers and mixed lymphoid cells. In larger animals, Frankenhauser continues, an inner layer of lymphoid cells, a middle layer of strong elastic fibers and an outer layer of connective tissue can be dif­ ferentiated. are fused. In smaller animals the inner and middle layers Ellenberger (1911) confirmed Frankenhauser*s report that in cows and rats lymphoid cells are abundant. In swine the lamina propria contains fine fibers and lymphocytes. A fibro-elastic layer of collagenous and longitudinal elastic fibers separates it from the underlying submucosa (Trautmann and Fiebiger, Ebner (1902), cited by Paul 1957). (1913), compared the 7 elastic layer which divides the mucosa from the submucosa, to the muscularis mucosa of the intestine* In the horse Paul (1913) described three subepithel- ial layers beneath the basement membrane: of elastic fibers, 1) a middle layer 2) a layer of glands, and 3) a subepi- thelial layer which blends into the submucosa- The super­ ficial subepithelial layer, or "stratum granulosura," con­ tains connective tissue cells, cell elements, blood vessels, and many lymphatic cells including some lymph nodules* middle elastic layer is thick- The Its superficial fibers are thin but become stronger and are arranged in bundles in its deeper part- Between these bundles are small gland acini and many blood vessels. Przewosky, according to Paul (1913), reported an inner circular and an outer longitudinal elastic layer where the fibers anastomose forming large bundles. Niewenhuis (1961) noted two layers of longitudinal elastic fibers in the dog, the deeper layer containing coarser fibers than the medial layer- In the cat, Niewenhuis noted, the long­ itudinal elastic fibers form a single layer much thicker and more compact than in the dog. Miller (1963) observed a few scattered circularly coursing elastic fibers in the ventral region of the sheep trachea. Trautmann and Fiebiger (1957) maintained that the submucosa in the pig contains many elastic fibers, fat cells, and mixed tubular glands. Paul (1913) reported numerous blood vessels and more submucosal fat ventrally. Bauersachs 8 (1910), according to Paul (1913), found fat cells, gland acini and a few scattered elastic fibers in the submucosa of r uminants. Near the musculature, he continued, the sub­ mucosal fat decreased with an increase in large glands* Parts of these glands are surrounded by thin elastic fibers and large lymph nodules* GLANDS According to Paul (1913), Frankenhauser (1879) noted the variation in the distribution of tracheal glands in domestic animals. swine, They are most abundant in cats; cows, sheep and horses follow in order of decreasing im­ portance* They are scanty in dogs, mice, and guinea pigs, and absent in chipmunks. The ventral and dorsal regions contain more glands, Frankenhauser continued. generally located in the submucosa, mucosa and outer fiber layer. These are seldom in the propria In some animals, glands are found in the trachealis muscle. Horse trachea contains fewer glands than swine tra­ chea. These convoluted, coiled or tubular type glands ap­ pear isolated or in groups (Paul, 1913). They are equally distributed but occasionally appear more ventrally and lat­ erally. Serous glands predominate, and there are no mixed glands. mucous glands are scanty Paul observed leucocytes around the glands in the horse but not in swine. According to Paul swine is irregular. (1913), gland distribution in the Most of the glands are in groups located 9 chiefly between the submucosal fat cells, some among the longutidinal elastic fibers in the deeper part of the pro­ pria mucosa, bundleso and some dorsally between the smooth muscle The few isolated glands present lie in the upper subepithelial propria mucosa. Paul (1913) observed mucous, serous and mixed sero-mucous glands in a ratio of 3:2:1 respectively. Mucous glands predominate dorsally. ally glands decrease in number Paul tubular, (1913) classified Gener­ toward the lung (Paul). the gland ducts as coiled simple alveolar or tubulo alveolar ducts. Waller and Bjorkmann (1882), according to Paul, reported tracheal glands in dogs, cats, and guinea pigs are predominately acinar. Niewenhuis (1961) confirmed this finding in dogs and cats. Mucous glands in cross section have a relatively large lumen. The gland cells are cuboidal or cylindrical containing flat, elongated or spherical nuclei lying toward the cell base (Paul, 1913). He explained that the shape and position of the nuclei change according to the function­ ing condition of the cell. are very small. The lumens of the serous glands Their cells are much darker; are spherical and lie more centrally. the nuclei Paul noted that the end pits of the glands converge to form large excretory ducts which pierce the epithelium and open in the tracheal lumen. Cells lining these excretory ducts range from simple cuboidal to cylindrical, to '•flat** epithelium. 10 CARTILAGE AND PERICHONDRIUM Prior to Paul*s (1913) investigation, many authors classified the cartilage of the trachea as hyaline. picric acid and acid fuschin, Paul Using (1913) observed a red network of elastic fibers in the cartilage of the horse trachea. He reported that the cartilage is impure hyaline and the elastic fibers come from the perichondrium. cartilage rings, ing dorsally. Paul Paul added, are widest ventrally, The decreas­ They are surrounded by elastic tissue. (1913) reported that the perichondrium of the pig and horse contains an irregular richness of elastic fibers and large c e l l s . that of the swine. It is thicker in the horse than The external perichondrium is thicker than the internal and the size of the cartilage cells in­ crease from the periphery toward the center. The cartilage rings are connected by "ligamenta annularia1* which consists of connective tissue, elastic fibers, small flat cells and occasional glands. MUSCLE According to Paul (1913), Lyeh observed the pres­ ence of longitudinal and circular muscle fibers connecting the dorsally incomplete cartilagenous rings of mammalian tracheas. Cuvier (Paul, 1913), lions, bears and ruminants, studying the trachea of noted that the muscle insertion in lions and bears is outside the cartilage rings; nants the insertion is inside the cartilage rings. in rumi­ 11 Ellenberger (1911) noted transverse muscle inside the car­ tilage rings in horse, cow, sheep, and swine* confirmed this finding in horse and swine* Paul (1913) Xn both of these species this muscle lies dorsally between the submucosa and the internal perichondrium* is thickest* Mid-dorsally, the muscle In the swine, Paul explained, the muscle in­ serts into the perichondrium by means of elastic fibers of irregular width* Xn the horse the muscle continued with the perichondrium by means of intermuscular connective tis­ sue containing elastic fibers* VESSELS AND NERVES According to Paul (1913), Frankenhauser (1879), and Kolliker and Ellenberger (1911), reported that vessels of the tracheal mucosa form a large plexus outside the mu­ cosa from which proprial and submucosal vessels developed and surrounded the gland acini* Most vessels are found in longitudinal section and they extend transversely to the submucosa where they form a capillary network and sur­ round the glands* Niewenhuis (1961) observed blood vessels of different sizes, nerve trunks, and ganglia in the adven­ titia of the dog and cat* Blood vessels increase in size from the lamina propria to the submucosa to the adventitia. Nerve trunks lie in the adventitia of the lateral and dor­ sal regions of the goat, cow, and sheep (Miller, 1963). MATERIALS AND METHODS The nine horses and fourteen Yorkshire pigs used in this investigation were obtained from Lang Feed Company, Romulus, Michigan, and the Michigan State University slaughter house respectively. These twenty-three healthy animals in­ cluded both males and females with ages ranging from six months to two years for the pigs and up to 20 years for the horseso Immediately after the animals were slaughtered the entire trachea was removed* Three consecutive cartilage rings from each of the following areas were selected: a) upper level— two segments below the larynx; b) middle level; and c) lower level— two segments above the carina cartilage. These sections were then fixed in a solution composed of commercial formalin, alcohol, 50 parts; 10 parts; ninety-five per cent ethyl two per cent glacial acetic acid, and distilled water, 40 parts 1949). 2 parts; (Lavdowsky’s Mixture, Guyer, After ninety-six hours, the tissues were removed from the fixative and stored in seventy per cent ethyl al­ cohol for four days * From each of the three levels, cross and longitud­ inal sections were obtained from the ventral, lateral, dorsal regions* and The dioxane method was used for dehydrating the tissues, and the paraffin method for infiltrating and embedding (Guyer, 1949). 12 13 Sections cut seven to nine microns thick were stained either with: a) hematoxylin and eosin for general histology; b) toluidine blue for serous and mucous glands, for the shape of the goblet cell, and for identification of mast cells; and c) Weigert-Van Gieson connective tissue stain for differentiation of elastic tissue and collagenous fibers (Gridley, 1960 )„ Using a calibrated micrometer the following measure­ ments were recorded in microns: Epithelium — height Cilia - length Elastic layer - thickness Elastic fiber - diameter Submucosa - thickness Cartilage - thickness Muscle — thickness An average of three measurements was taken from the dorsal, lateral, and ventral regions at the upper, middle, and lower levels* RESULTS AND DISCUSSION Microscopically the tracheal walls of the pig and horse show three main layers* periphery these are: From the lumen toward the a) a mucosa consisting of an epithel­ ium, a basement membrane, and a lamina propria; b) a sub­ mucosa consisting primarily of glands and blood vessels; and c) a fibroelastic cartilagenous layer (Plate 1)* MUCOSA Epithelium The tracheal mucosa of the pig and horse are lined by a pseudostratified, ciliated, columnar epithelium con­ taining goblet cells. Intermediate, non-ciliated cells and basal cells complete the epithelial cell types (Plate 2). The round to elongated basal cells located directly above the basement membrane extend not more than one-fourth the total epithelial length. Their conspicuous oval shaped nuclei lie horizontal to the basement membrane and occupy most of the cell space. in the horse, as Paul They are arranged in two layers (1913) had reported, but only one layer is present in swine (Plate 2). The narrow spindle-like intermediate cells are the least obvious of the four cell types. They are found on a lower level between the lateral boundaries of the ciliated cells. This finding supports that of Paul 14 (1913). Though 15 these cells span most of the epithelial thickness, not appear to reach the basement membrane. they do Toward the cell surface they taper and contain no cilia. Dispersed between the ciliated columnar cells, gob­ let cells attached to the basement membrane, open onto the free surface of the tracheal lumen. As Paul (1913) observed, these cells may appear single or in groups. The shape of the cells and their nuclei fluctuates with their activity. They appear long and narrow with an oval nucleus in the latent stage, and flask-like with a flat basal nucleus in the filled state. In the horse and pig, goblet cells de­ crease from the ventral to the lateral to the dorsal surface, similar to their distribution in ruminants reported by Miller (1963). In agreement with Paul (1913), they are more abun­ dant in the horse than in the pig and are least numerous at the middle level in both species. The very numerous ciliated cells also extend the entire thickness of the epithelium. Each ciliated epithel­ ial cell possesses a distinct cuticular border from which cilia protrude into the tracheal lumen. The nuclei of these cells are oval and lie midway or a little above in the cy­ toplasm. Cilia in the pig and horse increase in length from dorsal to ventral regions. than in the pig. They are longer in the horse In the horse the length of the cilia is greatest laterally and ventrally, whereas in the pig there 16 is a gradual increase from dorsal to lateral to ventral regionsc In both species cilia length increases from upper to lower level (Table 3). Similar to the findings of Paul (1913), patches of a transitional type of epithelium are present in the mucosal epithelium of the pig. However, also present in the horse this flat epithelium is (Plate 3)* This latter finding disagrees with the report by Paul (1913) who noted that the transitional type epithelium is not present in the horse* These patches appear mpre frequently in the pig but span longer distances in the horse* In both the pig and horse this transitional epithelium is found in the dorsal and lateral regions, though more frequently in the former area. Cilia and goblet cells are absent wherever this transitional epithelium is found. The epithelial height (Table 3) is greatest ventrally and least dorsally in both species. er in the horse than in the pig. Overall height is great­ The horse mucosa is uni­ formly wavy and the waves increase in height and width from the lateral to the dorsal region. In the pig the mucosal folds occur more Usually, the dorsal mu­ often dorsally. cosal folds in the pig axe larger and more uniform than those found ventrally and laterally. Short crypts appear only dorsally in the pig and occasionally in the horse. In sections where the cartilage was detached from the mu­ cosa, no mucosal folds were present. This observation sup­ ports the report by Miller (1913) in which he attributed 17 tracheal mucosal folds to diminution in diameter of the tracheal lumen caused by contraction of the tracheal is muscle and the inherent elasticity of the cartilage rings rather than resulting from the underlying elastic fibers* As Paul (1913) observed, lymphoid cells are pres­ ent in the tracheal epithelium of the pig and horse. They are more numerous among the basal cells but a few penetrate the epithelium to reach the lumen. lymphoid cells are diffuse; The majority of these others are found invading the epithelium from nodules present in the lamina propria. Basement Membrane The deepest layer of cells of the epithelial sheath rests on an acellular, clear, homogenous basement membrane (Plate 4). It is thinner and less obvious in the pig. In both species the basement membrane appears as a basal cuticular border formed by a condensation of the cellular cytoplasm in that area. This membrane does not appear con­ tinuous for any great distance, and in certain areas, it is obliterated by tiny fibers of the lamina propria. Lamina Propria The subepithelial layer (Paul, 1913), propria, is the deepest part of the mucosa. the lamina It lies beneath the basement membrane and above the submucosa. and horse, as Paul observed, In the pig this zone consists of elastic and collagenous fibers, connective tissue cells, gland acini and their ducts, diffuse lymphoid tissue and a few lymph 18 nodules, and capillaries* The collagenous fibers are dispersed throughout the lamina propria* In the pig and horse, the diameter and compactness of these irregularly coursing fibers in­ crease in the lower limits of the lamina propria; however, they are more abundant in the horse than in the pig* The elastic fibers are the most conspicuous feature of the lamina propria* In the pig and horse, the elastic fibers course longitudinally forming adjoining bundles giv­ ing the appearance of an elastic membrane* like arrangement was also observed by Paul This bundle(1913)* The diameter of the elastic fibers and the thickness of the elastic membrane are greater in the horse than in the pig (Table 3)* Here, too, the elastic membrane is thickest ventrally and thinnest laterally* In the pig the greatest diameter of the elastic membrane is on the lateral surface and least on the dorsal surface (Table 3)* Compared to the horse, elastic fibers are more compact in the pig. In a cross-sectional view of the pig trachea, cir­ cular elastic fibers are found in the superficial layer of the lamina propria (Plate 5)* These fibers are not pres­ ent in this area of the horse trachea* However, a few cross- sections of the trachea of the pig and horse show circular elastic fibers underlying the longitudinal elastic fibers of the deep lamina propria (Plate 6). These circular fibers extend for a short distance but do not form a complete layer* They are more abundant in the horse than in the pig. In 19 the pig some of these circular elastic fibers extend ob­ liquely blending with the fibroelastic layer of the carti­ lage ringso SUBMUCOSA The submucosa located between the elastic layer and the internal perichondrium consists predominately of glands, collagenous and elastic fibers, fat tissue and blood vesselso These components are also reported by Paul (1913)# The fibers of this layer course in random directions. In the lower limits of the submucosa they blend with the fibro­ elastic membrane of the cartilage rings. The submucosa of the horse trachea is thicker than that of the pig with the greatest thickness on the lateral and dorsal surfaces* In the pig, the submucosa is deepest on the ventral surface and shallowest on the lateral surface (Table 3). Scattered lymph nodules are present in the submucosa of the pig and horse. GLANDS Glands in the trachea of the pig and horse (also reported by Paul (1913)) contain mucous and serous cells and are branched tubuloacinar type (Plate 7). As Paul also observed, the glands are found isolated or grouped in clus­ ters chiefly in the submucosa. Some are located in the lamina propria, embedded in the muscle fibers of the dorsal region or extending as far as the area between the carti­ lage ends similar to the findings of Paul (1913). Either 20 isolated glands or clusters of glands may be all serous, all mucous, or a combination of sero-mucous glands. are numerous in the pig and, as Paul (1913) noted, Glands they generally decrease in numbers toward the tracheal bifurca­ tion . In agreement with Paul (1913), the glands of the horse are predominately serous (Plate 7); mucous glands are few and mixed sero-mucous glands are rarely present. Paul reported that there are no sero-mucous glands in the horse, but these are present occasionally (Plate 8). glands in the horse trachea are smaller, acini are more abundant than in the pig. Although the isolated gland These single gland acini are located in all levels of the lamina propria. are more frequently seen dorsally. As Paul They (1913) noted, larger gland clusters are found embedded between the longi­ tudinal elastic fibers of the deep lamina propria, and in the submucosa. The glands appear uniformly distributed from ventral to dorsal regions but may be more numerous ventrally and laterally. Often no submucosal glands are found in the dorsal region of the horse Paul (Plate 17). As (1913) observed, diffuse and aggregated lymphoid tis­ sue are found among and around the gland acini. In agreement with the report of Paul (1913), gland acini in the pig may be all serous, all muco u s , or of mixed sero-mucous type. dorsally. Mucous type glands are more abundant Isolated gland acini are few and are found with larger groups of glands in the deeper layer of the lamina 21 propria among the longitudinal elastic fibers* ing agrees with that of Paul (1913)* This find­ The submucosal glands are in larger clusters lodged between fat cells* In the pig glands are abundant ventrally and decrease in numbers laterally where the gland acini usually lie in the lower limits of the lamina propria (Plate 10a & 10b)* the glands increase in numbers (1913) observed, fibers Dorsally, (Plate 10c), and as Paul they extend between and outside the muscle (Plate 11) as far dorsal as between the open ends of the cartilage rings* In longitudinal section the submucosal glands in the pig may form a horizontal chain directly below the elas­ tic fibers of the lamina propria (Plate 12), or may occupy the superficial part of the triangular-like spaces between adjacent cartilages, with the apex directed between the segments of the cartilage rings* Lymphoid cells are closely associated with glands in the pig (Plate 10)* does not agree with the report of Paul This finding (1913) who reported that no lymphocytes are found in association with glands in the pig * Gland ducts of both the pig and horse may be lined with either stratified squamous cells or pseudostratified, ciliated, columnar cells* The cells lining the deepest portion of the ducts are lower in height, and as the ducts traverse the epithelial length to reach the surface, the height of the epithelium increases* In the upper third of the duct the epithelium becomes pseudostratified, ciliated, 22 columnar or stratified squamous depending on the lining of the lumen in that particular area* As Paul (1913) ob­ served, when glands are located between or beyond the muscle fibers, their ducts pierce the muscle layer, submucosa, and epithelium to reach the tracheal lumen. CARTILAGE The cartilage rings in the horse are larger in all aspects than those of the pig* The thoracic cartilages differ from those in the cervical region in that they do not overlap* All of the rings in the pig overlap with an increase of overlap toward the bifurcation. In the pig and horse the cartilage rings become thinner and narrower dorsally with the greatest thickness on the ventral surface. As Paul (1913) also observed, in the horse the cartilage width increases toward the lung (Table 3). The cartilage in the pig and horse is hyaline in nature* The tracheal rings of the nine horses used in this study revealed no content of an elastic character, as re­ ported by Paul as Paul (1913)* (1913) noted, The cartilage cells are oval and, these cells increase in size from the periphery toward the center of the ring* These cells are larger in the horse than in the pig* Longitudinal sections from the dorsal, lateral, and ventral regions were stained with methyl green and photographed grossly to show the cross-sectional appearance of the cartilage rings of the pig and horse tracheas (Plate 23 13)* The overlying arrangement allows for maximum tracheal movements« No calcification was observed in the pig trachea; however, some evidence of calcification was observed in the central area of the cartilage ring in the horse* A few of the chondrocytes were degenerating (Plate 14), and a few blood vessels were present in the deeper layer of the peri­ chondrium* These factors may indicate that a degree of calcification does occur in the trachea of horses* tunately, Unfor­ the exact ages of most of the horses used in this investigation were unknown* PERICHONDRIUM The perichondrium (Plate 15) is a layer of dense white fibrous connective tissue containing fibroblast— like cells* This membrane spans the entire circumference of the cartilage rings* Similar to the observation of Paul (1913), the perichondrium is thicker externally than internally* The cell nuclei lie parallel to the circularly oriented collagenous fibers* fibers,” Paul The "irregular richness of elastic (1913) observed in the perichondrium of the horse trachea was not present in any of the specimens stud­ ied* Therefore, his suggestion that the elastic fibers in the cartilage substance of the horse migrated from the peri­ chondrium is unlikely* ANNULAR LIGAMENT The annular ligament is a double sheath of collagenous 24 and elastic fibers that connects the cartilage rings one to another, and completely encloses them* This tube-like membrane is best seen in longitudinal section or in cross section where two or three cartilage rings lie parallel. The circularly disposed dense collagenous fibers are the chief components of the annular ligament. Elastic fibers are relatively few and show a random deposition. In a cross- sectional view circularly and longitudinally directed elas­ tic fibers are more concentrated between and covering the flat free ends of the cartilage rings (Plate 16). Here the circular elastic fibers extend for a short distance along the cartilage surface. This latter finding could have been the basis for Paul's (1913) report of the Melastic” character of the cartilage of the horse trachea. Elas­ tic fibers are more abundant in the annular ligament of the horse than in the pig. The following relationships between the width of the cartilage rings and the thickness of the annular ligament were observed: ventrally, the car­ tilage rings and annular ligament are thickest in both the pig and horse. This thickness decreases from ventral to lateral to dorsal surfaces. TRACHEALIS MUSCLE Circular smooth muscle bundles span the incomplete cartilage rings of the dorsal tracheal wall. with the finding of Paul (1913), In agreement this muscle in the pig and horse lies inside the cartilage rings external to the 25 submucosal glands. As Paul observed, in the horse the muscle inserts into the lateral internal perichondrium. Xn the pig the insertion is more dorsad. Paul also noted that in the pig the muscle narrows laterally becoming spindle­ like and attaches by means of a fibro—elastic tendon. muscle attachment in the horse may be indirect or direct. In the former the muscle fibers do not taper laterally but course obliquely and dip into the peripheral substance the cartilage ring (Plate 17). In the latter, fibers taper slightly and, as Paul The of the muscle (1913) observed, insert into the lateral internal perichondrium by means of a fibroelastic tendon (Plate 18). widest mid— dorsally. It is greater in the horse and de­ creases toward the lung. Paul In both species the muscle is In agreement with the report of (1913), in the pig the muscle decreases from the larynx toward the bifurcation. No longitudinal muscle fibers were observed in either species. Bundles of circular smooth muscle fibers were seen frequently in the ventral, lower level in the pig trachea but not in the horse. BLOOD AND LYMPHATIC VESSELS In the pig and horse blood vessels are located in the lamina propria, submucosa, between the open ends of the cartilage rings, and in the adventitia. Blood vessels in the lamina propria are of the smallest caliber. Here tiny networks are found directly beneath the basement membrane. In the elastic layer, the blood vessels have larger lumens 26 though their walls remain relatively thin* blood vessels have thicker walls. The submucosal The grouping of blood vessels in the submucosa as well as the networks found under the epithelium in the lamina propria suggest the plexus formation observed by Frankenhauser (1879) and Kolliker and Ellenberger (1911), as reported by Paul (1913). vessels are closely associated with gland acini. These Blood vessels of the largest caliber are found in the adventitia. A few thin walled blood vessels lie in the perichondrium in the horse. In agreement with the observations of Paul, the vessels in the horse are generally larger and more nu­ merous than in the pig. Tiny collapsed lymphatic vessels are found in the submucosa and adventitia in both the pig and horse (Plate 20). NERVES Larger nerve trunks are more frequently found dor­ sally in the space external to the muscle fibers and internal to the cartilage rings, and the dorsal adventitia. smaller nerve trunks are found laterally. and lie only in the dorsal adventitia. A few Ganglia are few No nerve trunks or ganglia were observed ventrally. ADVENTITIA This layer is found outside the dense fibro-elastic membrane that invests and connects the tracheal rings, is continuous with it. and It is a layer of loose collagenous and areolar connective tissue containing fat cells. This 27 layer transports and lodges blood vessels, nerves, end ganglia which supply the trachea* COMPARATIVE HISTOLOGY It is important that the veterinary histologist recognize normal organ histology, noting individual species specifications before physiological and pathological condi­ tions are differentiated* as those of Niewenhuis chart (page 29)* A summary of this study, as well (1961), and Miller (1963), are in a It is a concise report of those histolog­ ical characteristics regarded as the distinguishing features of the trachea of the dog, cat, cow, goat, and pig* 28 sheep, horse 29 Comparison of the Tracheas of the Dog, Cat, Cow, Goat, Sheep, Horse, Pig w g •r| p r> H P P jC P 01 0) -P H x: H 0 P u U 0 U p 01 E P p P 01 •H eh E — U p g p p ,Q X} H g P P P 44 g E P •rl H X) 0 2 2 /-N *r| U 44 t/) 44 •r| •r| P 01 03 Oi P p P g XI h 0 •rl H *4 —1m U —y 1 g g P rd e p 0) 43 PQ 6 01 0 0 2 S "O - ai 44 44 U P PH a 2 E O O -H U 03 P - l+H g 1 •H £3 43 O eh g >1 1 —1 44 P 2 01 X} 0 0 p 1—1xi p > 0 03 01 43 P g P P p XI p 03 •rl p < m P *> E 1 p 2 X) 03•H 2 —1 03 21 Mh P 43 4h p •r| 44 p T3 •r| p a g p p M O p 03 •H p g 43 p p Eh g xi 44 -p -p 01 •rl X1 g w e 1 p u p 01 p p cu i H P g 0 •H 44 X) •H 01 g P P H »P H p 44 44 id P 44 44 O 03 E 2 •rl i—1 P 43 44 •rl a p 01 01 p 2 P O 0 43 E O P P 44 M 2 p -rl O' a H 03 44 g H P P E g 43 P 0 p p 44 44 g 44 H P +) g rH p fd E g 43 p O p P -p -P X ■P w P -p g H p P E g 43 p o 01 P -p -P X -P w P 0 c U g •H -P 01 •H C H Q H 44 g p 01 p p 44 g P 03 43 3 a p H P g 0 •H 44 •rl 01 g p p Eh 44 g p 03 p p a 44 g p 03 P P a i P 01 0 u 2 E >1 H P > 44 •rl 0 44 p P a H E P 0 a 0 03 •r| Q H 44 g P p 2 03 oi n b p O 01 O -O g p 43 44 E 2 •rl rl P 43 44 •H a p •rl m •rl E 44 2 P *r| P H 44 01 01 43 44 > 1*r| h a H P p rH 44 h g p p 2 01 01 43 D P g p id -p -p oi P •rH 4h •H E -p 2 P -rl PH 44 p 01 43 44 > iH h a H P P g 01 0 2 •rl 0 03 g •P 03 Q >1 H H P 2 01 g o o X) O oi 44 g p 03 P P a 44 g p 03 p p a ■p g p 03 p p a 44 g p 03 p p a MQ3 q.'B oo p e i -H H 1 rH P g 0 .rl 44 *rl 01 g P 01*H g h Id P p .g +4 4J •H

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