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

• Epithelial lining of digestive tube and associated glands is derived from endoderm.
  
• Gut lining secretes mucus which functions to protect against corrosive HCl, aid passageof contents down the digestive tract, and protect against enzymatic degradation of epithelial lining.

The digestive system may be divided into 3 general sections:

• Oral Cavity
• Digestive Tube
• Digestive Glands

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.

• Muscle fibers are both intrinsic (= confined to tongue) and extrinsic (= originate outside of tongue).
• Between muscle fibers are mucous glands (at base of tongue), serous glands (in the body of the tongue), and mixed acinar glands (at tip of tongue).
• Serous glands of the tongue are called von Ebner’s Glands = act to wash taste buds free to allow new tasting.

- 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
lining

a) Parotid = located below and anterior to ear in humans

- ducts open to cheek opposite 2nd upper molar, ducts prominent
- serous secretion (enzymes [amylase] and polysaccharides)
- compound tubuloalveolar
- enclosed by dense irregular CT capsule
- secretory unit empties to intercalary duct -- secretory ducts (modify ionic
composition) -- excretory ducts -- main duct
- divided into lobes and lobules by septa (from capsule)

b) Submandibular = lies in floor of mouth beneath the body of the mandible
(lower jaw)

- duct opens below tip of tongue, ducts prominent
- mixed secretion (serous predominates)
- mucous secretory units often capped with crescent-shaped serous demilunes
- compound alveolar or tubuloalveolar, encapsulated

c) Sublingual = lies underneath tongue

- multiple ducts open near submandibular ducts, fewer ducts within glands
- mixed secretion (mucous predominates)
- compound tubuloalveolar
- CT capsule less well-developed

II. DIGESTIVE TUBE
- 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:

a) epithelial membrane - lubricated by mucus
b) lamina propria = underlying loose CT layer (rich in blood vessels & lymphatics)
c) muscularis mucosae = thin layer of smooth muscle forming boundary
- Mucosa generally shows finger-like projections or folds into lumen termed villi - act
to greatly increase surface area for absorption/digestion
- Invaginations between villi form intestinal glands of "crypts" and gastric glands
(stomach)
- Lamina propria tends to become lymphoid (lymph nodules)
- Contractions of M. mucosae cause folding of mucous membrane

2) Submucosa = loose CT between muscularis mucosae and muscularis externa

- Supplied with blood vessels and lymphatics
- Often contains accumulation of lymphoid tissue
- Contains nerve plexus of autonomic fibers and ganglion cells = Meissner's Plexus - innervates m. mucosae
- Contains mucus secreting glands in esophagus and duodenum of small intestine

3) Muscularis Externa = consists of at least 2 layers of smooth muscle, more
substantial the m. mucosae

- Striated muscle replaces smooth in upper esophagus (assoc. with swallowing)
- Layers: generally inner circular (constricts lumen)
outer longitudinal (shortens gut, widens lumen)
- Innermost oblique layer occurs in stomach
- M. externa responsible for peristalsis -- moving food along tract
- Between inner circular and outer longitudinal layers lies Aeurbach's Plexus =
Autonomic nerve fibers and ganglion cells - innervates m. externa, mostly
parasympathetic

4) Serosa = outermost layer composed of somewhat denser areolar CT with
elastic fibers

- When lined by mesothelium = serosa, when outside of body cavity (retroperitoneal,
portions of small intestine) = adventitia
- Contains many blood vessels, lymphatics, nerves

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

a) Mucosal epithelium is nonkeratinizing str. squamous in humans; in herbivorous
mammals (swallow rough material) keratinized str. squamous is usually present
b) At upper and lower ends, mucosal glands (cardiac glands) present in lamina propria
(secrete mucus)
c) Submucosal mucous glands present throughout
d) Muscularis externa of upper esophagus composed of skeletal muscle, middle
esophagus = smooth muscle replaces skeletal muscle, lower esophagus = smooth
muscle in inner circular-outer longitudinal arrangement
e) Submucosal/Mucosal folds extending into lumen in empty esophagus, when full
these become distended; this response is allowed by prominent elastic fibers in
submucosa (similar to remainder of digestive tract)

2. Stomach = muscular-walled reservoir capable of retaining food and
digestion/absorption of food

- Consists of 3 regions histologically:


a) Cardiac	anterior
b) Fundic
c) Pyloric	posterior

- Food enters stomach as bolus of semisolid material, exits as chyme = semifluid
pulplike mass
- Absorption limited to salts, water, glucose, alcohol and some drugs
- Gastric Juice secreted by stomach contains HCl, mucus, pepsin (a proteolytic enzyme),
lipase (fat catabolism), rennin (curdles milk)
- Submucosal/Mucosal folds into lumen (rugae), flatten out when stomach is full
- Mucosal epithelium = simple columnar, contains:

a) Gastric Glands = simple tubular or branched tubular glands
b) Gastric Pits = open to surface, 2 or 3 glands empty into one pit
c) Cardiac Glands = only for short initial portion of cardium, mucus-secreting, gastric
glands short, glands coiled and dilated

d) Surface lined with Mucous Columnar Cells

- 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
hydrolysis
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