DIGESTIVE SYSTEM = responsible for processing, digestion, and absorption of food material; also elimination of wastes or undigestible residue.

The digestive system may be divided into 3 general sections: I. ORAL CAVITY/TONGUE

1. Lined by stratified squamous epithelium (nonkeratinizing); except keratinized on gums, dorsal surface of tongue, and hard palate.

2. Epithelium (mucosa) underlain by submucosa of dense irregular CT with collagen and elastic fibers. Contains small mucous and mucoserouc glands. These glands secrete mucus and digestive enzymes (salivary amylase ® breaks down carbohydrates). Underlying CT is immunologically active.

3. Tongue = skeletal muscle sheathed in a mucous membrane continuous with that of the oral cavity.

- Anterior 2/3 of tongue covered by lingual papillae = projections of mucosal surface
with submucosal core. 4 Types:

1) Filiform = located over entire surface, slender and conical
2) Fungiform = located singly among rows of filiform, more numerous toward
tip, mushroom- shaped with secondary papillae of CT
3) Circumvallate = only 10-14 in humans, located along central sulcus of
tongue, surrounded by circular furrow, taste buds present in lateral walls
4) Foliate = leaf-like folds on posterolateral margins, taste buds occur in
grooves between folds; rudimentary in humans, well-developed in rabbits

Dorsoventral intrinsic = move tip of tongue
Lateral intrinsic = move tongue sideways, cup tongue
Extrinsic = protrude tongue

Taste Buds = barrel-shaped neuroepithelium (taste receptor cells, sustentacular
cells, basal cells); opens via taste pore. Taste Receptor cells = chemoreceptors,
respond to dissolved molecules

4. SALIVARY GLANDS = compound exocrine glands, developed from epithelial
- Divided into 4 regions (organs) by sphincters (muscular valves)
1. Esophagus
2. Stomach
3. Small Intestine
4. Large Intestine (colon)
- All four regions show similar structural arrangement of walls - consists of 4 layers:

1. Mucosa Lumen
2. Submucosa

3. Muscularis Externa
4. Serosa (adventitia) Outside

1) Mucosa = consists of:
2) Submucosa = loose CT between muscularis mucosae and muscularis externa
3) Muscularis Externa = consists of at least 2 layers of smooth muscle, more
substantial the m. mucosae
4) Serosa = outermost layer composed of somewhat denser areolar CT with
elastic fibers

Although all regions of the digestive tract show common structural arrangement,
each particular region is modified according to its function.

1. Esophagus - relatively straight tube extending from oral cavity to stomach,
functions mainly in rapid transport of food
2. Stomach = muscular-walled reservoir capable of retaining food and
digestion/absorption of food

- Gastric Glands contain:
1. Mucous Neck Cells = found in "neck" region of gland, mucus-secreting (acid
mucopolysaccharides, proteoglycans, GAGs); basal cytoplasm basophilic, apical
granules stain with PAS (carbo.-containing)
2. Parietal Cells = scattered singly or in small groups throughout length of gland;
acidophilic or pale cytoplasm, secretes HCl
3. Chief Cells = found at base of gland; basophilic cytoplasm due to high
numbers of ribosomes, secretes pepsinogen (converted to pepsin by HCl) protein
4. Enteroendocrine Cells = found at base of gland; usually with clear unstained
cytoplasm (stain with Ag2+, Cr2+; secrete serotonin, peptide hormones involved
in digestion (secretin stimulates pancreas secretion, gastrin stimulates HCl
secretion, cholecystokinin stimulates pancreatic enzyme secretion), endorphins;
release secretion to lamina propria
Germinative Zone occurs at isthmus/neck junction, continuously replaces dead
mucous columnar cells of pit and surface

To Lecture 15b