Al-Tikriti: Digestive tract histology

1. Esophagus
2. Stomach-cardiac, fundus, body and pylorus regions
3. Small intestine-duodenum, jejunum, Ileum
4. Large intestine-Cecum and vermiform appendix, Colon (Ascending, transverse, descending and sigmoid); wall has Taenia coli, Sacculation and Haustra
5. Terminal portion-rectum, anal canal

1. Esophageo-cardiac junction
2. Pyloro-duodenal junction
3. Duodeno-jejunal junction
4. Ileo-cecal junction
5. Recto-anal junction

A. Epithelial lining-stratified squamous (nonkeratinized) and simple columnar
B. Lamina propria-loose CT rich in blood and lymph vessels and smooth muscle cells, nerve plexuses, glands and lymphoid tissues
C. Muscularis mucosae-thin inner circular and outer longitudinal layer of smooth muscle cells separating mucosa from the submucosa
D. Function-a selectively permeable barrier b/t lumen and tissues of the body
i. facilitates transport and digestion of food
ii. promotes absorption of products of digestion
iii. produces hormones that affect activity of digestive system
iv. secretes mucus for lubrication and protection

1. SUBMUCOSA-dense CT w/ many blood and lymph vessels, submucosal (Meissner’s) nerve plexus, glands and lymphoid tissues
Lymphoid tissues protect from bacterial invasion due to macrophages and lymphoid cells producing IgA (resistant to proteolytic enzymes)
2. MUSCULARIS-composed of smooth muscles that runs into inner circular and outer longitudinal layers; myenteric (Auerbach’s) nerve plexus is located b/t the layers, as well as blood and lymph vessels
In the stomach it becomes thick due to the presence of oblique innermost layers
It promotes movement (motility) that causes propulsion of chyme
3. SEROSA-composed of thin layer of loose CT, rich in blood, lymph vessels and adipose tissue, covered with simple squamous epithelium (mesothelium)
Serosa may be replaced by tunica adventitia if the digestive organ is bound to other organs or structures

muscular tube; transports foodstuffs from mouth to stomach; lined by nonkeratinized stratified squamous epithelium; longitudinal folds of lumen expand during passing of food
Esophageal glands are compound tubuloalveolar glands found in submucosa
a. upper proper part-produces acidic mucus for lubrication and protection
b. lower part (esophageal cardiac glands)-located in lamina propria; produce neutral secretion to protect the esophagus from regurgitated gastric content
The muscular coat is divided into 3 parts
a. proximal third-striated muscle
b. distal third-smooth muscle
c. middle third-mixed
The abdominal part is covered by serosa, while the rest is covered by adventitia

1. Continue digestion of carbohydrate (CHO) initiated in the mouth
2. Add acidic fluid to ingested food, transform it by muscular activity into a viscous mass (chyme)
3. Promote initial digestion of protein with enzyme (pepsin)
4. Produces gastric lipase to digest triglycerides
5. Mucosa and submucosa lie in longitudinal folds (RUGAE)

The surface epithelium of the stomach is thrown into invaginations deep into the lamina propria, forming GASTRIC PITS; emptying into gastric pits are cardiac, gastric (fundic) and pyloric glands
Gastric pit is covered by simple columnar epithelium, and all the cells secrete alkaline mucus; mucus forms a thick gel layer that protects epithelium from the effects of strong acid secreted by the stomach; tight junctions around surface and pit cells form barrier against acid (HCl), pepsin, lipases (lingual & gastric)

A. CARDIA-narrow, circular band forming a transition b/t esophagus and stomach; mucosa contains simple or branched tubular cardiac glands; secretory cells produce mucus, has few parietal cells
B. FUNDUS & BODY-filled with branched, tubular “gastric (fundic) glands” in bottom of each gastric pit; fundic gland has long neck and a broad base or body connecting to the gastric pits by a short segment called isthmus, which is a site for cell replication
PYLORUS (gatekeeper)-has pyloric glands with longer pits and shorter coiled secretory portions; mainly contains cells that secrete mucus with few parietal cells among them
Enteroendocrine cells are also found scattered among glandular epithelium which is mainly of Gastrin (G-cell) producing cells

A. MUCOUS NECK CELLS-located b/t parietal cells in neck of gastric glands; secrete mucus; irregular in shape, with nucleus at base of cell and secretory granules near apex
B. OXYNTIC (PARIETAL) CELLS-rounded/pyramidal cells, central nucleus and eosinophilic cytoplasm which contains mitochondria and intracellular canaliculus
Parietal cell releases H+ and Cl- ions into the lumen of the glands; secretory activity of parietal cells is initiated by cholinergic nerve endings (parasympathetic), histamine and gastrin; oxyntic cell also produces intrinsic factor, a glycoprotein that binds avidly to Vitamin B12
C. CHIEF (ZYMOGENIC) CELLS: predominate in lower region of tubular glands; basophilia is due to abundant rER; granules in cytoplasm contain pepsinogen converted by HCL into active pepsin; produce lipase
D. ENTEROENDOCRINE CELLS-found near the bases of gastric glands in the fundus; secretes 5-hydroxytryptamine (serotonin)
E. STEM CELLS-low columnar cells with oval nuclei; have a high rate of mitotic division, replaces pit and surface mucous cells; others differentiate into mucous neck cells, parietal, chief and enteroendocrine cells

A. Submucosa-composed of dense CT, contains blood and lymph vessels; infiltrated by lymphoid cells, macrophages and mast cells, the submucosa (Meissner’s) plexus
B. Muscularis externa-smooth muscle fibers oriented in 3 main directions: ext. is longitudinal, mid is circular, internal is oblique; at the pylorus, middle is thickened to form pyloric sphincter
Myenteric or (Auerbach’s) plexus is present b/t muscle layers
C. Serosa-loose CT covered with mesothelium

1. Glucagon (A cell)-stomach; hepatic glycogenolysis
2. Gastrin (G cell)-pylorus; stimulates gastric acid secretion
3. Secretin (S cell)-small intestine; pancreatic and biliary HCO3 & H2O secretion
4. Gastric inhibitory polypeptide (GIP)-K cell; small intestine; inhibition of gastric acid secretion
5. Glicentin (glucagon-like)-A&L cell; small intestine; hepatic glycogenolysis
6. Ghrelin (Gr cell)-stomach; GH secretion
7. Cholecystokinin (I cell)-small intestine; pancreatic enzyme secretion, gallbladder contraction
8. Somatostatin (D cell)-pylorus, duodenum; inhibit others
9. Motilin (Mo cell)-small intestine; increased gut motility
10. Serotonin (substance P)-outside the digestive tract; increase guts motility
11. Vasoactive intestinal polypeptide (VIP)-D1cell; digestive tract; ion and water secretion, increased gut motility

The small intestine is the site of terminal food digestion, nutrient absorption and endocrine secretion
MUCOUS MEMBRANE-series of permanent folds, plicae circulares (Kerckring’s valves); most developed in jejunum
Intestinal villi project into the lumen; in duodenum, they are leaf-shaped, assuming fingerlike shapes as they reach the ileum; b/t villi are small openings of simple tubular glands called intestinal glands (crypts) or glands of Lieberkühn

A. Absorptive cells-tall columnar cells w/ oval nucleus in basal half of the cell; projecting from apical surface are straight microvilli visible in LM and called striated (brush) border
Microvilli increase the surface of intestinal lining for absorption; it is a site for hydrolyzes of disaccharides and protein into monosaccharide and amino acids for easily absorption; lipid digestion by pancreatic lipase and bile occurs in duodenum and upper jejunum
B. Goblet cells-interspersed b/t absorptive cells; increased number in ileum; produce acid glycoproteins of mucin type that protect and lubricate lining of small intestine
C. Paneth cells-exocrine cells w/ secretory granules in their apical cytoplasm found in basal portions of intestinal glands; secretory granules are very large; contain lysozyme that digests the cell wall of some bacteria
D. M (microfold) cells-specialized epithelial cells overlying the lymphoid follicles of Peyer’s patches; characterized by presence of apical membrane invaginations rather than microvilli, forming pits containing many intraepithelial lymphocytes and antigen-presenting cells (macrophages)

Diffuse neuroendocrine system (secrete by exocytosis); hormone may be local (paracrine) or blood borne (endocrine); 2 classes:
1. Open type-apex presents microvilli and contacts lumen of organ
2. Closed type-cellular apex is covered by other epithelial cells

A. Muscularis mucosa-no peculiarities
B. Submucosa-in duodenum, has the duodenal (Brunner’s glands) seromucous type, produces alkaline secretions; protects mucosa against effects of acid gastric juice and to bring pH of intestine into optimum for pancreatic enzyme action
C. Muscularis layer: well-developed, made up of internal circular and outer longitudinal layer
Intestinal immunological system-GALT (gut-associated lymphatic tissue); Ab secreting plasma cells, macrophages and lymphocytes found in mucosa and submucosa
Peyer’s patches-part of GALT; oval area on antimesenteric side of intestine; epithelium is made up of M cells

nourish the intestine and remove absorbed products of digestion; penetrate the muscularis and form a large plexus in the submucosa; from submucosa, extend through muscularis mucosa and lamina propria and into the villi; each villus receives one or more vessels that form a capillary network
Lymph vessels begin as closed tubes in core of villi called Lacteals; form a plexus to submucosa where they anastomose repeatedly and surround lymph nodules; important for lipid transportation

i. Intrinsic component: Myenteric (Auerbach’s) plexus b/t outer longitudinal and inner circular layers of muscularis and Meissner’s (submucosal) plexus in submucosa contains chemoreceptor and mechanoreceptors; other nerve cells are effectors and innervate muscle and hormone secreting cells which cause intestinal contraction
ii. Extrinsic component: parasympathetic cholinergic nerve fibers stimulate and sympathetic adrenergic nerve fibers depress intestinal smooth muscle activity

divided into ascending, descending, transverse and sigmoid colon; this division reflects its anatomical location and relationship with the other organs in the abdominal cavity
The mucosal membrane has no folds except in rectal portion; the mucosa has no villi; intestinal glands are long, abundant goblet cells and absorptive cells and enteroendocrine cells; the muscularis mucosa is thin and regular; absorptive cells are columnar and have short, irregular microvilli
Functions-absorbs water, forms fecal mass and produces mucus which lubricates and covers bacteria and particulate matters
Lamina propria is rich in lymphoid cells and nodules (GALT) due to abundant bacterial population of the large intestine; teniae coli are formed by the outer longitudinal layer bands of smooth muscle

It is an evagination of cecum, small, narrow and irregular lumen due to presence of abundant lymphoid follicles in its wall; has fewer and shorter intestinal glands and has no teniae coli
Because it is close-ended, its contents are not renewed rapidly, and it frequently becomes a site of inflammation (appendicitis), which can progress to the point of destruction of this structure, with consequent infection of the peritoneal cavity
Appendices epiploicae-small, pendulous “tear-dropped” outpocketings fats found in serosa of the colon
Anal region-mucous membrane forms a series of longitudinal folds (rectal columns of Morgagni)
About 2 cm above the anal opening, intestinal mucosa is replaced by stratified squamous epithelium
Lamina propria contains a plexus of large veins that produce hemorrhoids

The epithelial cells of the entire GIT are constantly being cast off and replaced w/ new ones formed thru mitosis of stem cells located in:
a. basal layer of esophageal epithelium
b. Isthmus of gastric glands
c. Lower half of intestinal glands
d. bottom third of the crypts of the large intestine
The high rate of cell renewal explains why intestine is affected by cancer chemotherapy which inhibits cell proliferation-->atrophy of epithelium-->defective absorption, fluid loss and diarrhea