GENERAL STRUCTURE OF TUBULAR ORGANS

A general structural pattern exists for all tubular organs of the digestive, respiratory, urinary, and reproductive systems (Fig. 10-1). Familiarity with this general pattern is helpful in understanding the specific characteristics of each organ. The wall of a typical tubular organ is composed of four coats. Each coat is called either a tunic or a tela. A tela has a delicate weblike structure while a tunic is comprised of denser tissue.

FIGURE 10-1 Schematic drawing of cross sections through various portions of the digestive tract. Esophagus without submucosal glands (A). Esophagus with submucosal glands (B). Small intestine with and without submucosal glands and with aggregated lymphatic nodules (C). Large intestine (D). Tunica mucosa: epithelium (E), lamina propria (F), lamina muscularis (G). Tela submucosa (H). Tunica muscularis: circular layer (I), longitudinal layer (J). Tunica serosa (K). Tunica adventitia (L).

Tunica Mucosa

The tunic next to the lumen is the tunica mucosa. The tunica mucosa is also referred to as a mucous membrane or simply, the mucosa. A mucosa lines all organs that communicate to the outside of the body and is protected by a layer of mucus, a viscous material containing cast-off epithelial cells and leukocytes, in addition to mucin, a product of specialized glands. Structures associated with, or located within, the oral cavity, such as the lips, cheeks, and tongue, have a mucosa, even though they are not typical tubular organs. The mucosa is composed of three layers or laminae: an epithelium, a lamina propria, and a lamina muscularis.

The mucosal epithelium is constantly present and may consist of any of the types of surface epithelia, depending on the function of the specific organ. The epithelium rests on a basement membrane.

The lamina propria is a layer of connective tissue immediately beneath the epithelium. In most organs, this is a loose connective tissue containing fine collagen, elastic, and reticular fibers as well as all of the cells typical of loose connective tissue (see Chapter 3). The lamina propria is also classified as diffuse lymphatic tissue because it contains immunocompetent T and B lymphocytes. The lymphocytes initiate the immune response to injurious agents that have penetrated the epithelium. Blood vessels essential for the nourishment of the epithelium, along with lymph capillaries and nerves, are also present in the lamina propria. In some organs, the lamina propria contains glands that are referred to as mucosal glands because they are confined to the mucosa.

The lamina muscularis is inconstantly present. It consists of one to three layers of smooth muscle. The lamina muscularis allows independent movement of the mucosa, possibly to facilitate the movement of luminal contents or to assist in the expression of secretions from mucosal glands.

Tela Submucosa

The tela submucosa, or simply submucosa, is a layer of connective tissue that may contain glands (submucosal glands). In most organs, the connective tissue of the submucosa is more dense than that of the lamina propria. Also present are blood vessels, lymph vessels, and the submucosal (Meissner’s) plexus, a ganglionic nerve plexus of the autonomic nervous system. In organs without a lamina muscularis, the lamina propria and submucosa blend without a clear line of demarcation, forming a propria-submucosa.

Tunica Muscularis

The tunica muscularis is the coat of smooth muscle or skeletal muscle responsible for moving the ingesta through the tract and for mixing the ingesta with glandular secretions. Usually, two layers of muscle are present in the tunica muscularis of the tubular organs of the digestive system. The muscle fibers of the inner layer are oriented circularly or in a tightly coiled pattern, whereas those of the outer layer are arranged longitudinally or in a loosely coiled pattern. Between these two layers is a ganglionic nerve plexus of the autonomic nervous system, the myenteric (Auerbach’s) plexus.

Tunica Serosa/Adventitia

The outermost tunic may be either a tunica serosa or tunica adventitia. The tunica serosa (serosa or serous membrane) is composed of a layer of connective tissue with a covering of mesothelium. Organs that border the pleural, pericardial, and peritoneal cavities are covered by a serosa. In each of these locations, the serosa is given a special name: pleura, epicardium, and peritoneum, respectively. All organs not bordering these cavities, such as the cervical part of the esophagus, lack a mesothelium. They have a layer of connective tissue, called a tunica adventitia, or simply adventitia, which blends with the surrounding fascia (Fig. 10-1).

ORAL CAVITY

The junction between the integument and the digestive system occurs on the lips, which are covered on the outside by skin and on the inside by a mucosa. Near the mucocutaneous junction, the skin is devoid of hair follicles and the epidermis is thicker, with a more elaborate interdigitation with the underlying connective tissue (Fig. 10-2). The mucosa of the lips is covered by stratified squamous epithelium that is keratinized in ruminants and horses, but nonkeratinized in carnivores and pigs. The lamina propria and submucosa blend without a clear line of demarcation. Aggregates of serous or seromucous minor salivary glands, referred to as labial glands, are distributed throughout the propria-submucosa. The tunica muscularis consists of skeletal muscle fibers of the orbicularis oris muscle.

Cheeks

The cheeks, like the lips, are composed of an external covering of skin, a middle muscular layer (the buccinator muscle), and an internal mucosa lined by stratified squamous epithelium that may or may not be keratinized, depending on the particular area or species. In ruminants, the mucosa is studded with macroscopic, caudally directed, conical buccal papillae that facilitate the prehension and mastication of food (Fig. 10-3). The buccal glands are minor salivary glands located in the propria-submucosa and among the skeletal muscle bundles of the cheek, with some secretory units extending into the dermis. The glands are compound tubuloacinar glands and may be serous, mucous, or seromucous, depending on the location and the species.

FIGURE 10-2 Lip (cat). Junction of keratinized stratified squamous epithelium of skin and mucosa (A); junction of dermis and lamina propria of lip (B); orbicularis oris muscle (skeletal) of lip (C). Hematoxylin and eosin (×45).

Hard Palate

The bones of the hard palate are covered by a mucosa, which exhibits a series of transverse ridges called rugae. The mucosa is covered by a keratinized stratified squamous epithelium, which is particularly thick in ruminants (Fig. 10-4). The lamina propria has a well-developed papillary layer that blends with the submucosa without an intervening lamina muscularis, forming a propria-submucosa. The propria-submucosa is composed of a dense network of collagen and reticular fibers and blends with the adjacent periosteum. A dense network of capillaries and veins, especially well developed in horses, permeates the propria-submucosa. Branched tubuloacinar mucous and seromucous minor salivary glands (palatine glands) are located in the caudal part of the hard palate in all domestic mammals, except pigs. The rostral portion of the mucosa of the hard palate is especially thick in ruminants and forms the dental pad (pulvinus dentalis). The dental pad consists of a heavily keratinized stratified squamous epithelium overlying a thick layer of dense irregular connective tissue (Fig. 10-5). The lower (inferior) incisor teeth press against the pad, forming a tight grip on forage during grazing.

FIGURE 10-3 Cheek (large ruminant). Conical buccal papilla covered with keratinized stratified squamous epithelium (A); lamina propria (B); skeletal muscle (C); buccal glands (D). Hematoxylin and eosin (×12).

FIGURE 10-4 Hard-palate mucosa (large ruminant). Propria-submucosa (A); keratinized stratified squamous epithelium (B); caudal surface of ruga (arrow). Hematoxylin and eosin (×22).

FIGURE 10-5 Dental pad (sheep). Propria-submucosa (A) with papillae interdigitating with stratified squamous epithelium (B); stratum corneum (C). Trichrome (×48).

Soft Palate

The soft palate consists of a core of skeletal muscle fibers with a mucosa covering both surfaces. The oropharyngeal (ventral) surface is covered by a stratified squamous epithelium. The nasopharyngeal (dorsal) surface is covered by a stratified squamous epithelium caudally and a ciliated, pseudostratified columnar epithelium rostrally. Between the two types of epithelium, a narrow transition zone consisting of transitional epithelium is present. The propria-submucosa contains branched tubuloacinar mucous and seromucous palatine glands. Lymphatic tissue occurs in the mucosa of both the oropharyngeal and the nasopharyngeal surfaces; in pigs and horses, a macroscopically visible tonsil is present on the oropharyngeal surface. Longitudinally oriented skeletal muscle fibers (the palatinus muscle) and connective tissue are located between the two mucous membranes.

Tongue

The tongue is a muscular organ covered by a mucosa. It is important in the prehension, mastication, and deglutition of food.

The epithelium covering the tongue is stratified squamous. It is keratinized and thick on the dorsum, and nonkeratinized and thin on the ventral surface. The dorsum bears numerous macroscopic lingual papillae. These papillae differ somewhat in shape, are named according to their morphologic characteristics, and serve either a mechanical or a gustatory function. The filiform, conical, and lenticular papillae are purely mechanical; they facilitate the movement of ingesta within the oral cavity. The fungiform, vallate, and foliate papillae are gustatory; that is, they contain the taste buds, which are responsible for perception of the sense of taste.

The filiform papillae are the most numerous type. They are slender, threadlike structures that project above the surface of the tongue and are covered by a keratinized stratified squamous epithelium with a thick stratum corneum. They are supported by a highly vascularized connective tissue core. Equine filiform papillae consist of very fine keratinized threads projecting above the surface (Fig. 10-6). The connective tissue core ends at the base of the thread. In ruminants, a keratinized cone projects above the surface, and the connective tissue core has several secondary papillae. Cats have large papillae with two prominences of unequal size (Fig. 10-7). The caudal prominence is especially large and gives rise to a caudally directed keratinized spine, supported by a more rounded rostral papilla with a thinner stratum corneum. The filiform papillae of dogs may have two or more apices; the caudal apex is largest and has a stratum corneum thicker than that of the other(s) (Fig. 10-8).

Conical papillae occur on the root of the tongue in dogs, cats, and pigs, and on the torus linguae of ruminants (see Special Lingual Structures section below). They are larger than the filiform papillae and usually are not highly keratinized. They contain both primary and secondary connective tissue papillae. In pigs, the conical papillae are more correctly referred to as tonsillar papillae, because they contain a core of lymphatic tissue and, therefore, collectively constitute the lingual tonsil.

FIGURE 10-6 Tongue (horse). Filiform papillae are keratinized threads extending from the surface of the stratified squamous epithelium on the dorsum of the tongue (A); submucosa propria (B); skeletal muscle (C). Hematoxylin and eosin (×26).

Lenticular papillae are flattened, lens-shaped projections that are found on the torus linguae of ruminants. They are covered by keratinized stratified squamous epithelium and have a core of dense irregular connective tissue.

The fungiform papillae are scattered among the filiform papillae and have a dome-shaped upper surface in horses and pigs (Fig. 10-9). The shape is suggestive of a mushroom, and thus the name fungiform. The papillae are covered by a nonkeratinized stratified squamous epithelium containing one or more taste buds on the upper surface. The taste buds are sparse in these papillae in the tongues of horses and cattle, more numerous in those of sheep and pigs, and abundant in those of carnivores and goats. The connective tissue core is rich in blood vessels and nerves.

FIGURE 10-7 Tongue (cat). Filiform papilla with a caudally directed keratinized spine arising from the caudal prominence (A); lamina propria (B); supporting rostral papilla (C). Hematoxylin and eosin (×35).

FIGURE 10-8 Tongue (dog). Filiform papillae with caudally directed apices (A); lamina propria (B). Hematoxylin and eosin (×75).

The vallate papillae are located on the dorsum of the tongue, just rostral to the root. They are large, flattened structures completely surrounded by an epithelium-lined sulcus (Fig. 10-10). They extend only slightly, if at all, above the lingual surface and are covered by a stratified squamous epithelium. The epithelium on the papillary side of the sulcus contains many taste buds. Groups of serous gustatory glands are located deep to the sulcus and have ducts that open into the sulcus at various levels (Fig. 10-10). Mucous glands may also be found beneath the papillae, but their secretory products are emptied directly onto the lingual surface. The connective tissue core is rich in blood vessels and nerves. The number of vallate papillae varies with the species; horses and pigs typically have one pair, carnivores have four to six pairs, and ruminants have eight to 24 pairs.

The foliate papillae are parallel folds of the lingual mucosa located on the margin of the tongue just rostral to the palatoglossal arch. Taste buds are located in the epithelium on the sides of the folds. The folds are separated by gustatory sulci (Figs. 10-11 and 10-12). Deep to the sulci lie serous gustatory glands, the ducts of which empty into the sulci. Foliate papillae are absent in ruminants; they are rudimentary and lack taste buds in cats.

FIGURE 10-9 Tongue (pig). Fungiform papilla (A) with taste buds (arrows). Hematoxylin and eosin (×75).

FIGURE 10-10 Tongue (large ruminant). Vertical section of a vallate papilla with a surrounding sulcus (A); taste buds in the epithelium (B); a gustatory gland duct opening into the sulcus (C); gustatory gland (D). Hematoxylin and eosin (×30). (From Stinson AW, Brown EM. Veterinary histology slide sets. East Lansing, MI: Michigan State University, Instructional Media Center, 1970.)

The taste buds are ellipsoid clusters of specialized epithelial cells embedded in the stratified squamous epithelium of the fungiform, vallate, and foliate papillae of the tongue (Fig. 10-12). They also occur widely dispersed in the soft palate, epiglottis, or other areas of the oral cavity and pharynx. The taste bud consists of a cluster of spindle-shaped epithelial cells that extend from the basement membrane to a small opening, the taste pore, at the epithelial surface (Fig. 10-12). In most mammalian species, three cell types have been identified. They are referred to as type I cells, type II cells, and type III cells. Type I and type II cells have apical microvilli that project into the taste pore; type III cells have a club-shaped apex that also projects into the taste pore. The type III cell is characterized by clusters of cytoplasmic vesicles, resembling synaptic vesicles, adjacent to intraepithelial nonmyelinated afferent nerve fibers. Therefore, the type III cell is considered to be the chemoreceptor (taste) cell, whereas the type I and type II cells are believed to serve a sustentacular (supportive) role. The average life span of the cells is approximately 10 days. New cells are recruited from mitotically dividing cells in the perigemmal region (Latin gemma, meaning “bud”).

FIGURE 10-11 Tongue (rabbit). Foliate papillae with prominent taste buds (A). Hematoxylin and eosin (×110).

FIGURE 10-12 Taste buds (rabbit). Gustatory sulcus (A); taste pore (B); nonmyelinated nerve fibers (C). Hematoxylin and eosin (×615).

The proper (intrinsic) lingual muscles consist of longitudinally, transversely, and perpendicularly arranged bundles of skeletal muscle (Fig. 10-6). Because of the diverse arrangement of these muscle fibers, the tongue has extensive mobility to facilitate movement of food into and within the oral cavity.

The ventral surface of the tongue is covered by nonkeratinized stratified squamous epithelium. The mucosa contains an abundance of capillaries, arteriovenous anastomoses, and branches of the lingual artery and vein. They participate in thermoregulation.

Scattered among the muscle fibers and in the propria-submucosa of the tongue are clusters of seromucous minor salivary glands, which are collectively referred to as the lingual glands.

Special Lingual Structures

The lyssa of the tongue of carnivores is a cordlike structure enclosed in a dense irregular connective tissue capsule and extends longitudinally, in the midline, near the ventral surface of the apex of the tongue. The lyssa of dogs is filled with white adipose tissue, skeletal muscle, blood vessels, and nerves, but that of cats contains mainly white adipose tissue (Fig. 10-13). The tongue of pigs contains a similar structure. A middorsal fibroelastic cord with hyaline cartilage, skeletal muscle, and white adipose tissue is present in the tongue of horses. It is termed the dorsal lingual cartilage.

The ruminant tongue has a large prominence, the torus linguae, covering the caudal portion of the dorsum and characterized by a thickened mucosa. Connective tissue papillae extend almost to the surface of the epithelium, which is thicker than that on other regions of the tongue. Lenticular papillae and conical papillae are scattered over the surface of this area.

FIGURE 10-13 Lyssa (dog). Skeletal muscle (A); white adipose tissue (B); dense irregular connective tissue capsule (C); intrinsic lingual muscles (D); ventral surface of tongue (E). Hematoxylin and eosin (×28).

Teeth

Teeth are highly mineralized structures in the oral cavity that serve domestic mammals during the procuring, cutting, and crushing of food and as weapons of offense and defense. The tooth consists of a highly mineralized outer part surrounding the pulp cavity, which contains the dental pulp, a core of connective tissue, blood vessels, lymph vessels, and nerves (Fig. 10-14).

Brachydont and Hypsodont Teeth

Two types of teeth occur in the domestic mammals: brachydont and hypsodont. These teeth differ in their rates of growth and in the arrangement of the layers of mineralized tissue.

Brachydont teeth are short and cease to grow after eruption is completed (Fig. 10-14). They have a crown (the portion above the gingiva), a neck (the constricted region just below the gingival line), and one or more roots embedded in a bony socket called the alveolus. Brachydont teeth include all those of carnivores (and human beings), the incisor teeth of ruminants, and the teeth of pigs, except for the canine teeth.

FIGURE 10-14 Schematic drawing of a longitudinal section through a brachydont tooth in situ. The dental pulp fills the pulp cavity of the tooth. The location of the odontoblasts, at the periphery of the dental pulp, is indicated by arrows. (From Dellmann HD. Veterinary Histology: An Outline Text—Atlas. Philadelphia: Lea & Febiger, 1971.)

Hypsodont teeth are much longer than brachydont teeth and continue their growth throughout a portion, if not all, of the adult life of the animal (Fig. 10-15). They do not have a crown and neck but, instead, have an elongated body; in some species, the roots and neck form only after a delayed period. The tusks of the boar continue to grow throughout life and never develop roots. Hypsodont teeth include all those of horses, the cheek teeth of ruminants, and the canine teeth of pigs.

Structure

The mineralized tissues of the teeth are enamel, dentin, and cementum. Each of these has a separate origin and differs morphologically and in degree of mineralization.

Enamel covers the external surface of the crown of brachydont teeth and lies beneath a layer of cementum in hypsodont teeth. It is the hardest substance in the body, composed of 99% mineral (hydroxyapatite) and 1% organic matrix by weight. Histologically, enamel is composed of long, slender rods, enamel prisms, held together by interrod enamel. Parallel bundles of rods pursue a wavy or oblique course from the inner to the outer surface of the enamel layer (Fig. 10-16). Curved lines (incremental lines) appear where these bundles change directions. Enamel is produced by ameloblasts that differentiate from the inner enamel epithelium of the enamel organ (see Development section below). Ameloblasts disappear from the fully developed brachydont tooth, but a small population of columnar cells remains at the base of the hypsodont tooth to continue enamel production.

FIGURE 10-15 Schematic drawing of a longitudinal section through a hypsodont tooth in situ.

FIGURE 10-16 Ground tooth (human). Junction of enamel (A) and dentin (B) with the odontoblastic processes penetrating the dentinal tubules; interglobular dentin (C). Unstained (×235).

Cementum resembles bone in all its structural features. Acellular cementum is composed of lamellae oriented parallel to the surface of the tooth (Fig. 10-17). Cellular cementum has cementocytes, which occupy lacunae and canaliculi similar to those of bone. Bundles of collagen fibers, called cementoalveolar (Sharpey’s) fibers, extend from the alveolar bone into the cementum of the tooth (Fig. 10-17). Collectively, these fibers form the periodontal ligament, which anchors the tooth in the alveolus. Cementoblasts at the junction of the cementum and the periodontal ligament produce the fibrous matrix of the cementum, and then later mineralize the cementum by depositing hydroxyapatite crystals within the matrix. Once the cementoblasts are surrounded by matrix, they are known as cementocytes. The roots of brachydont teeth are covered by a layer of cementum that may slightly overlap the enamel on the neck. Cementum covers the outside surface of equine and ruminant hypsodont teeth, both above and below the gingiva. It begins just above the area at the base of the tooth where the ameloblasts produce enamel. Equine cementum has cementocytes throughout and lacks the brachydont equivalent of acellular cementum. Equine cementum is also unique in that it is vascular and innervated.

Dentin is a highly mineralized tissue that constitutes the major part of the tooth. It underlies the enamel of the crown and the cementum of the root in brachydont teeth, and also underlies the enamel of the body in hypsodont teeth. Dentin also forms the wall of the pulp cavity. It consists of a matrix of organic material, mainly randomly oriented collagen fibrils and glycoproteins, upon which is deposited minerals including primarily hydroxyapatite with some carbonate, magnesium, and fluoride. The composition is approximately 70% mineral and 30% organic matter. Dentin is produced by a columnar layer of cells, called odontoblasts, which are located adjacent to the interior surface of the dentin in the outer layer of the dental pulp. Odontoblast processes lie in roughly parallel anastomotic channels, the dentinal tubules, that extend from the inner to the outer surface of the dentin. Peritubular dentin immediately surrounds the odontoblast processes and is more highly mineralized than intertubular dentin, which constitutes the remainder of the dentin. Unmineralized organic material, termed predentin, lies between the apex of the cell body of the odontoblasts and the mineralized dentin. Interglobular dentin is composed of small, unmineralized or incompletely mineralized areas within the dentin at its periphery, immediately adjacent to the enamel or cementum. These areas are more numerous in the root of the tooth and form the stratum granulosum of the dental root at the dentinocementum junction (Fig. 10-18). The odontoblasts continue to produce dentin throughout the life of the tooth, although at a slower rate after the tooth erupts.

FIGURE 10-17 Ground tooth (human). Lamellae of acellular cementum (A) oriented parallel to the surface of the root. Cellular cementum with cementocytes (B) (×185). Inset: cementoalveolar (Sharpey’s) fibers (arrow) embedded in the cementum. Unstained (×185).

Unlike brachydont teeth, the hypsodont cementum and enamel layers invaginate into the dentin. The invaginations that extend from the occlusal surface down into the tooth are known as infundibula, whereas similar invaginations along the sides of the tooth form enamel plicae. These invaginations are common in the cheek teeth (premolars and molars) of horses and ruminants. Because enamel is the hardest of the mineralized tissues, it is most resistant to wear and projects above the occlusal surface as sharp enamel crests; dentin and cementum are less resistant and wear away more readily. The uneven wearing of the mineralized tissues creates a corrugated surface, which is highly effective for grinding food.

Dental pulp occupies the pulp cavity of the tooth. It is composed of connective-tissue cells and fibers, amorphous ground substance, numerous blood and lymph vessels, and nerves. It resembles embryonic connective tissue in texture, with delicate collagen fibers coursing through the amorphous ground substance. The most peripheral part of the pulp is the layer of odontoblasts, from which the odontoblast processes extend into the dentinal tubules. Basal processes from the odontoblasts extend into the amorphous ground substance or unite with similar processes from neighboring cells. Because dentin is continuously deposited on the inside of the tooth, the size of the pulp cavity is gradually reduced as the animal ages.

FIGURE 10-18 Ground tooth (human). Dentinocemental junction. Dentin (A) containing dentinal tubules (the dark lines); stratum granulosum of the root (B); cementum (C) with lacunae. Unstained (×185).

Development

In the embryo, an invagination of the oral ectoderm into the underlying mesenchyme forms the dental lamina (Fig. 10-19), a continuous, arch-shaped sheet of epithelial cells extending along the future site of the gingiva in both the upper and lower jaws. Isolated thickenings arise on the labial side of the dental lamina, where each deciduous and permanent tooth develops. These thickenings are the primordia of the enamel organ, which eventually gives rise to the enamel. As the enamel organ develops, it takes on the appearance of an inverted cup, attached to the dental lamina by a thin stem (Figs. 10-19 and 10-20). The epithelial cells lining the inside of the cup form the inner enamel epithelium, and those covering the outside form the outer enamel epithelium. The epithelial cells between these two layers become stellate and take on the appearance of connective tissue, thus forming the stellate reticulum of the enamel organ. The mesenchyme (derived from neural crest ectoderm) enclosed by the cup of the enamel organ condenses to form the dental papilla, the future dental pulp. The internal contour of the cup is a replica of the shape of the tooth crown to be produced (Fig. 10-21).

FIGURE 10-19 Stages in the development of the tooth. The dental lamina degenerates (holes) as the teeth mature.

As the enamel organ enlarges, the cells of the inner enamel epithelium take on a distinct columnar shape and differentiate into ameloblasts, which later produce the enamel. The neural crest-derived mesenchymal cells of the dental papilla immediately adjacent to the ameloblasts differentiate into odontoblasts, which produce dentin. Dentin is deposited as sheaths of mineralized material around the odontoblast processes that are anchored to the basement membrane of the inner enamel epithelium. As more dentin is produced, the cell body of the odontoblast recedes toward the developing pulp cavity. Shortly after the first dentin is deposited, the ameloblasts begin to produce the enamel matrix (Figs. 10-21 and 10-22). The deposition of the dentin and enamel begins at the apex of the crown and continues down the sides of the crown to the neck of the tooth.

FIGURE 10-20 Developing tooth (dog). Oral ectoderm (A); dental lamina (B); outer enamel epithelium of enamel organ (C); stellate reticulum of enamel organ (D); inner enamel epithelium of enamel organ (E); dental papilla (F) with peripheral odontoblasts (arrowheads); developing dental sac (G). Hematoxylin and eosin (×85).

FIGURE 10-21 Developing permanent tooth (dog). Ameloblasts (A); enamel (B); dentin (C); odontoblasts (D); dental pulp (E); outer enamel epithelium (F) (×25). (Courtesy of A. Hansen.)

The formation of the root of the tooth begins shortly before eruption. The root is formed by a downward growth of a sheet of cells originating from the enamel organ at the junction of the inner and outer enamel epithelia. This downward-growing sheet of cells, the epithelial sheath of Hertwig, surrounds the connective tissue of the dental papilla and induces the formation of odontoblasts. The dentin of the root is produced by these odontoblasts.

FIGURE 10-22 Developing tooth (dog). 1. Odontoblasts (A); predentin (B); dentin (C); enamel (D); ameloblasts (E); stellate reticulum (F) (×200). 2. Area marked in 1. Trichrome (×300).

The entire enamel organ and developing tooth are enclosed by the dental sac, a thickened connective tissue layer that completely surrounds the developing tooth (Fig. 10-20). In brachydont teeth, the crown erupts through the dental sac, which then collapses against the dentin of the root. The cells of the collapsed dental sac then differentiate into cementoblasts, which deposit a covering of cementum over the roots. In hypsodont teeth, the dental sac collapses before the tooth erupts, and therefore, cementum covers the entire tooth.

SALIVARY GLANDS

The parotid salivary gland in domestic mammals is predominantly serous, although occasional isolated mucous secretory units may occur in dogs and cats. Structurally, it is a compound acinar gland composed of numerous lobules separated by thin connective tissue septa. The lobule consists of acini formed by pyramid-shaped cells with basal nuclei surrounded by basophilic cytoplasm (Fig. 10-23). The apex of each cell is filled with secretory granules, referred to as zymogen granules, containing precursors of digestive enzymes. Myoepithelial cells are located between the secretory cells and the basement membrane.

The narrow lumen of the acinus opens into a short intercalated duct lined by low cuboidal epithelium (see Fig. 2-18). The intercalated duct joins a large striated or salivary duct lined by simple columnar epithelium that is characterized by striations in the basal portion of the cells (see Figs. 2-18 and 10-24). This appearance results from perpendicularly oriented mitochondria within numerous cytoplasmic compartments formed by deep infoldings of the basal cell membrane. This arrangement creates a large basal membrane surface area containing energy-requiring ion pumps located near energy-producing mitochondria, thus facilitating active transport of substances between the cells and the underlying tissue. The striated ducts are easily recognized as the largest structures within the lobule and participate in the secretory process. The striated ducts extend to the edge of the lobule, where they join interlobular ducts located in the connective tissue septa between lobules (see Fig. 2-18).

Interlobular ducts are lined by simple columnar epithelium, which changes to stratified columnar epithelium as the ducts become larger and fuse with similar ducts draining other lobules. The interlobular ducts converge to form the parotid duct. The epithelium changes from stratified columnar to stratified squamous where the parotid duct opens into the vestibule of the oral cavity.

Mandibular Salivary Gland

The mandibular salivary gland is a seromucous (mixed) compound tubuloacinar gland. The morphologic structure of the secretory unit is somewhat variable from one species to another but generally consists of a tubular unit with an enlarged terminal acinus. Mucus-secreting cells border the lumen of the tubule and acinus, and serous demilunes occur at the periphery (Fig. 10-25). The serous secretory product reaches the lumen through intercellular canaliculi between the mucous cells. Variations in this basic pattern may include separate serous and mucous acini or mucous tubular units with enlarged, serous, acinar end pieces. In dogs and cats, the mucous elements predominate. Myoepithelial cells are present around the secretory units (see Fig. 2-19). The duct system is like that of the parotid salivary gland. In the epithelium of the main duct, goblet cells may occur.

FIGURE 10-23 Parotid salivary gland (horse). Serous acini (A) opening into intercalated ducts (arrows); serous acinus (B). Hematoxylin and eosin (×440). Inset: serous acinus. Hematoxylin and eosin (×1382).

FIGURE 10-24 Parotid salivary gland (horse). Intercalated ducts (A) joining striated duct (B). Hematoxylin and eosin (×425).

Sublingual Salivary Gland

Like the mandibular salivary gland, the sublingual salivary gland is also a seromucous (mixed) compound tubuloacinar gland (Fig. 10-26). The number of mucous acini and serous demilunes and the seromucous nature of their secretory product vary among species. Sublingual glands of cattle, sheep, and pigs are almost entirely mucous, with relatively few serous demilunes. In addition to the typical mucous acini and demilunes, the glands of dogs and cats contain clusters of serous acini with periodic acid-Schiff (PAS)–positive granules in their basal portion (Fig. 10-27). The mucous cells form tubular secretory units that connect the serous acini with intercalated ducts. Striated and intercalated ducts are present, but not prominent, in cats and dogs. In horses, ruminants, and pigs, however, they are well developed. The interlobular ducts have, at their origin, a low simple columnar epithelium that increases in height and becomes two-layered in larger ducts. The main duct is lined with stratified cuboidal epithelium, and goblet cells occur in cattle and pigs.

FIGURE 10-25 Mandibular salivary gland (horse). Capsule (A); mucous acinus (B) and serous demilunes (C); intercalated duct (D); striated duct (E). Hematoxylin and eosin (×384). Inset: serous acinus (F); mixed acinus (G). Hematoxylin and eosin (×530).

FIGURE 10-26 Sublingual salivary gland (dog). Mucous acini with lumina (arrows) emptying into intercalated duct (A); serous demilunes (B). Hematoxylin and eosin (×280).

Minor Salivary Glands

Clusters of serous, seromucous, or mucous minor salivary glands, occurring throughout the oral cavity, are generally named according to their location. The lingual glands are located in the propria-submucosa and between the intrinsic muscle bundles of the tongue. The gustatory glands, associated with the vallate and foliate papillae (Fig. 10-10), are entirely serous and their ducts open into the sulcus at the base of the papillae. The labial, buccal, palatine, and pharyngeal glands also contribute mucous and serous secretory products to the saliva. Histologically, the secretory units resemble those of the major salivary glands and occur in a variety of forms (i.e., acinar, tubuloacinar, or tubular). Mucous tubules and acini frequently have serous demilunes associated with them; however, striated ducts are not characteristic of the minor salivary glands. The duct system is lined with simple cuboidal epithelium within the lobules and two-layered cuboidal epithelium in the larger interlobular ducts. The stratification of the duct epithelium increases as it reaches the oral cavity, where it changes to a stratified squamous type.

FIGURE 10-27 Sublingual salivary gland (dog). Serous acini (A); serous demilunes (C) on tubular mucous secretory unit. Hematoxylin and eosin (×275).

Among the domestic species, the zygomatic salivary gland is present only in carnivores. The parenchyma is composed of long, branched tubuloacinar secretory units that are predominantly mucus-secreting (Fig. 10-28). Intercalated and striated ducts are almost nonexistent. The interlobular and main ducts are similar to those of the other glands.

The molar salivary gland of cats is histologically similar to the zygomatic salivary gland. It is a compound tubuloacinar gland that is predominantly mucus-secreting. Intercalated and striated ducts are not present, and the interlobular ducts have a two-layered cuboidal epithelium. Several main ducts empty into the vestibule of the oral cavity opposite the molar teeth.

FIGURE 10-28 Zygomatic salivary gland (dog). Mucous tubules (A); mucous acini (B). Hematoxylin and eosin (×300).

PHARYNX

ESOPHAGUS

The esophagus (Table 10-1) joins the laryngopharynx with the stomach and contains all the layers of a typical tubular organ of the digestive system (Fig. 10-1). An internal annular fold, the pharyngoesophageal limen, marks the junction of the laryngopharynx and esophagus in carnivores.

The mucosa is composed of three layers: a stratified squamous epithelium, a lamina propria, and a lamina muscularis. The degree of keratinization of the stratified squamous epithelium varies with the species. It is usually nonkeratinized in carnivores, slightly keratinized in pigs, more so in horses, and keratinized to a high degree in ruminants. The lamina propria consists largely of a dense feltwork of fine collagen fibers with an abundance of evenly distributed elastic fibers; the esophagus is atypical in that the connective tissue of the lamina propria is more dense than the connective tissue of the submucosa (Fig. 10-29). The lamina muscularis contains only longitudinally oriented smooth muscle bundles. It is absent in the cranial end of the esophagus of pigs and dogs, but cats, horses, and ruminants have isolated smooth muscle bundles near the pharynx that increase in number and become confluent toward the stomach. In pigs, the lamina muscularis is especially well developed in the caudal end, where it is as thick as the outer layer of the tunica muscularis.

TABLE 10-1 Characteristics of the Esophagus

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