Digestive system and Histology

• The lecture is about the Digestive system (Part 1) and Histology

Digestive System

• Consists of the digestive canal/tube (alimentary tract) from the mouth to the anus, and digestive glands

General Structure of the Alimentary Tract

• The alimentary tract has four tunics:
• Tunica mucosa
• Tunica submucosa
• Tunica muscularis
• Tunica adventitia/serosa

Tunica Mucosa

• The innermost layer
• Lamina epithelialis contains epithelium
• Lamina propria contains connective tissue
• Lamina muscularis mucosa: smooth muscle fibers which are absent in the pharynx, rumen, reticulum, and some esophageal parts
• In parts where the lamina muscularis mucosa are absent the lamina propria continues with the submucosa in one layer called “propria submucosa”

Tunica Submucosa

• Contains loose connective tissue
• Contains submucosal glands; these are present in the duodenum and esophagus of dogs plus some parts of the large intestine

Tunica Muscularis

• Consists of inner circular and outer longitudinal smooth muscle fibers, except in the pharynx and esophagus of dogs and ruminants.
  • In these the muscle fibers are of striated type
• This layer helps the passage of ingesta and the mixing of the contents of the digestive tube.

Tunica Adventitia/Serosa

• The outermost layer
• Lamina subserosa: Loose C.T.
• Lamina epithelialis serosa: Simple squamous epithelium (mesothelium)

Oral Cavity

• Includes lips, cheeks, hard palate, soft palate, tongue

Lips

• Have a central mass of skeletal muscle fibers that run in different directions
• Covered externally by skin and internally by a cutaneous mucous membrane
• Skin forms the epidermis (stratified squamous keratinized epithelium) and dermis (dense irregular fibrous conective tissue) including hair follicles, sweat glands, and sebaceous glands)
• The inner mucous membrane has a Lamina epithelialis
  • Stratified squamous non-keratinized epithelium "carnivores and pigs" and keratinized epithelium "horse and ruminants"
• Lamina propria submucosa contains dense irregular C.T. and labial glands

Cheeks

• Contain a central mass of skeletal muscle fibers of various directions
• Covered externally by skin and internally by a cutaneous mucous membrane
• Contain seromucous buccal glands in the propria submucosa and the intermuscular C.T.
• In ruminants, the inner mucous membrane has conical macroscopic papillae for prehension and mastication of food.

Hard Palate

• Lamina epithelialis: Stratified squamous keratinized epithelium
  • In ruminants, the rostral part is highly keratinized, forming the dental pad
• Propria-submucosa: Dense irregular fibrous C.T.
  • Includes well developed papillary bodies and cavernous blood spaces and blends with the periosteum of the bony roof of the oral cavity
• Mucous and mixed palatine glands are present caudally in all domestic animals except pigs.

Soft Palate

• The double-surfaced mucosal fold extends behind the hard palate and divides the pharynx into the oropharynx and nasopharynx
• Oral surface: Stratified squamous non-keratinized epithelium
• Aboral surface: Pseudo-stratified columnar ciliated epithelium
• Propria submucosa of both surfaces: Includes dense irregular fibrous C.T, seromucous palatine glands, and lymphoreticular tissue
• Central core: Longitudinally arranged skeletal muscle fibers

The Tongue

• Contains masses of skeletal muscles arranged in different directions covered dorsally and ventrally by a mucous membrane.
• Has intermuscular fibroelastic C.T. (fat cells, nerves, blood and lymph vessels and lingual glands)
• The dorsal surface is covered by thick, stratified squamous keratinized epithelium, and is rough due to lingual papillae
• The ventral surface is covered by thin, stratified squamous non-keratinized epithelium, and is smooth (lacks the lingual papillae).
• Propria submucosa: Fibroelastic C.T.
• Lingual glands are located in the propria submucosa and intermuscular C.T.
• Von Ebner's glands are clusters of seromucous glands, mainly serous, and associate with vallate and foliate papillae
• Weber's gland: mainly mucous, at the root of the tongue

Tongue of Carnivora (Lyssa)

• Lyssa is a cord-like structure surrounded by a dense collagen sheath in the midventral tongue part
  • In dogs: Consists of fat cells, skeletal muscle fibers, blood vessels, nerves, and islands of cartilage at the caudal part
  • In cats: mainly fat cells

Lingual Papillae

• Mucosal projections on the dorsal and lateral surfaces of the tongue.
• Each papilla has an epithelial cap and C.T. core (dense irregular C.T. derived from lamina propria)

Mechanical Papillae

• They perform a mechanical function providing a rough surface which facilitates food movement during chewing
• Have a keratinized epithelial cap
• Types include filiform, coniform, and lentiform papillae

Gustatory Papillae

• Have taste buds in a non-keratinized epithelial cap
• Types include fungiform, circumvallate, and foliate papillae

Filiform Papillae (Mechanical)

• The most numerous type
• Project above the tongue surface covered by a keratinized stratified squamous epithelium
• Supported by a highly vascularized connective tissue core
• In cats, form a hard brush to clean their fur

Lentiform Papilla (Mechanical)

• Flattened lentil-like shape
• Epithelial cap: Stratified squamous highly keratinized epithelium
• Primary papilla branched into secondary papillae
• Occurrence: Caudal part of the dorsum of the ruminant tongue

Coniform Papilla (Mechanical)

• Conical in shape
• Epithelial cap: Stratified squamous highly keratinized epithelium
• Primary papilla branched into secondary papillae
• Occurrence: Root of the tongue and sides of lips and cheeks of ruminants

Fungiform Papilla (Gustatory)

• Mushroom shaped and protrude above the surface of the tongue.
• Possess a dome shaped top and slightly constricted neck
• Stratified squamous non-keratinized epithelium
• Presence of taste buds on the upper (free) surface of this cap
• Has a highly innervated and vascularized primary papilla that branches into secondary papillae
• Between filiform papillae

Circumvallate Papilla (Gustatory)

• The largest papillae that doesn't project above the surface
• Completely surrounded by a circular moat or trench
• Stratified squamous non-keratinized epithelium
• Primary papilla branches into secondary papillae, which is highly innervated and vascularized
• Ducts open into the trench, secreting to clean food particles and help taste reception
• Broad above and narrow below

Foliate Papilla (Gustatory)

• Each papilla is formed of parallel, leaf-like projections that are separated from each other by gustatory furrows
• Stratified squamous non-keratinized epithelium
• Includes a highly innervated and vascularized C.T. core
• Seromucous Von Ebner's glands located at the bottom of the furrows
• Ducts open into the bottom of the gustatory furrows
• Well-developed in rabbits, rudimentary in cats

Taste Buds

• Intraepithelial, ellipsoid or oval bodies with pale staining
• Arranged perpendicularly to the surface epithelium where they are embedded
• Contain three types of cells:
  • Gustatory (sensory or taste) cells
  • Supporting or sustentacular cells
  • Basal cells
• Found in gustatory lingual papillae and the soft palate
• Bipolar neuroepithelial cells with tapering ends and nucleus enlargement
• Apical part: microvilli (taste hairs) that project of the taste pore (taste pit)
• Basal end: afferent sensory ending connection
• Long curved and pointed cells
• Support gustatory cells
• Small polyhedral cells near to the basement membrane
• They divide and give rise to other two types