Digestive Tract II: Esophagus through Intestines

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Esophagus Facts

• A tube within a tube, in that there is slippage between the muscularis mucosa & muscularis externa.

• Fairly straight tube from pharynx to stomach. Most is intrathoracic but terminal 2-4 cm is below diaphragm.

• Food passes rapidly and there is no absorption.

• Stratified squamous mucosal.

• Muscularis mucosae fairly prominent when compared to other segments of the GI tract.

• Submucosa has elastic fibers that contributes to lumen diameter change during swallowing.

• A few mucous esophageal glands in submucosa.

Esophagus Histology

• Upper 1/3 muscularis externa; all skeletal.

• Middle mixture of smooth & skeletal.

• Lower 1/3 all smooth.

Cardiac Glands

In lower esophagus, mucous glands in lamina propria are called cardiac glands due to their similarity to the cardiac region of the stomach.

GI Pathology

• Gastric reflux inflames columnar cells.

• Ulcers can lead to scarring & narrowing of the lumen so that swallowing is almost impossible.

Stomach

• Inner surface amplified by folds or rugae that flatten out when the stomach is full.

• Rugae generally only have a core of mucosa but large ones may have a submucosal component.

Functions of Stomach

1. Reservoir for food.

2. Site for continuation of digestion.

3. Kills bacteria & other live material.

4. Blender for food.

Stomach Cells

• Epithelial layer is highly invaginated to form gastric pits or foveloae which lead to gastric glands.

• Pylorus Foveolae are strictly mucous.

• Surface mucous cap cell (shaped like a chef's cap).

• Undifferentiated cells in neck, can go up or down.

• Mucous neck cells.

Parietal and Oxyntic cells

• Parietal, oxyntic cells produce HCl & intrinsic factor, a glycoprotein necessary for absorption of vitamin B12.

• Stain like large fried eggs.

• Eosinophilic cytoplasm.

Chief and Zymogen cells

• Chief, zymogen cells produce pepsinogen that is changed to pepsin due to presence of HCl.

• Pepsin then cleaves protein.

Chief and Zymogen cells

• Enteroendocrine cells modify adjacent cells by paracrine regulation.

• Basal secretions enter the lamina propria.

• Apical canalicular trough of parietal cells increases area for HCl production.

• Pump H+ ions into trough & combine with Cl-.

• Abundant mitochondria.

• Fundus More than one gastric gland can tap into one foveola.

• Mucous cap cell lives 4-6 days.

• Undifferentiated stomach cell types live > 2 days.

Gastric Glands

• Gastric glands are simple tubular but may branch.

• Surrounded by lamina propria.

• Longest ones crossed at base by muscularis mucosa.

• Name glands based on ratio of foveolae to gland length.

Cardiac Gastric Glands

• Cardiac is kind of weak.

• Fundus has funky glands.

• Pylorus has puny glands.

• Predominantly mucous.

• Protects esophageal- gastric junction.

• Produces most HCl & all pepsinogen.

• Short pits & long glands.

• Shift back to mostly mucous & enteroendocrine.

• Most parietals drop out.

• Deep pits & short glands.

Aspirin Effects

• Aspirin increases acid environ. & destroys mucous cap cells.

• Alcohol causes the same changes but slower.

Lesser Curvature

• Lesser curvature is main path for food.

• Always exposed to acid.

• Main site of ulcers.

• Can be damaged with ingestion of drain cleaner.

• Helicobacter pylori.

Intestinal Abrosption

Levels of amplification for absorption by the intestine:

A. Lengthen.

B. Plicae circulares.

C. Villi.

D. Microvilli.

E. Glycocalyx.

Large & Small Intestines

• Small intestine is ~6-7 meters.

• Can remove up to 1/3.

• Large intestine forms a picture window that "frames" the small intestine.

• Plicae circulares (valves of Kerckring) have core of submucosa.

• About 800 plicae.

• These are permanent and are present when the intestine is full.

• Spiral around 1/3 to 3 turns.

• Villi 1/2 to 1.5 mm long.

• Core of lamina propria, nerve, smooth muscle, single arteriole, capillary plexus, single venule & lymphatic (central lacteal).

• Each absorptive cell of a villus has ~3,000 microvilli.

• Small intestine completes digestion and absorbs products of digestion to blood or lymph.

• Produces hormones.

• Glands of intestine may occur as goblet cells (unicellular gland) or in lamina propria, mucosa, submucosa or external & connected by ducts.

• Evagination = villus.

• Invagination = crypt of Lieberkuhn.

Intestine Cell Types:

• Columnar absorptive cells (enterocytes) with microvilli and glycocalyx (PAS positive).

• Goblet cells, PAS positive, some consider them unicellular glands.

• Secretes mucin which hydrates with water to form mucous.

• Enteroendocrine cells. Two important factors are cholecystokinin and secretin.

• Paneth cells are highly differentiated and never leave the crypt.

• Contain eosinophilic zymogen granules that produce lysozyme that digests cell wall of bacteria and regulates the natural bacterial flora of the intestine.

• M-cells (microfold cells) process antigen and associated region where basement membrane has "holes" for interaction with lymphocytes.

• Undifferentiated cells.

Small Intestine Cell life span

• Esophagus 2-3 days.

• Gastric 4-6 days.

• Intestine 3-6, generally 6 days.

• Paneth cells 30 days.

Duodenum

• 180 million crypts of Lieberkuhn.

• Mucous glands of Brunner in submucosa that produce urogastrone, a gastric acid inhibitor.

• Secretions are alkaline (pH 8.1-9.3).

• Brings intestinal contents to optimal pH for pancreatic enzymes.

• Brunner's tap into base of crypt.

Ileum

• Ileum with Peyer's patches, M cells, etc.

• There are about 30 patches, visible with the naked eye.

• Mostly in submucosa but can project into mucosa.

• Antigen sampled by M cell & presented to lymphocytes in LP.

• Migrate to lymph nodes & circulate/ Exit at gut lamina propria.

• Develop into plasma cells.

• Secrete IgA into gut lumen.

• In lactating women contributes to milk.

• Vessel plexus in submucosa supplies villi.

• Prominent mucous cells produces a "mucous water slide" for fecal matter to surf out.

• Passively absorbs water.

• Elimination of residual foodstuffs.

• No important enzymatic activities, just left over from small intestine.

Large Intestine

• Absorbs water.

• Prominent mucous cells produces a "mucous lubricant" for the forming fecal masses.

• Elimination of residual foodstuffs.

• No important enzymatic activities, just left over from small intestine.

• Taenia coli thickened longitudinal layer of muscularis externa.

• Haustra, out- pocketings caused by taenia.

Intestine Histology

• Serosa covers caecum, appendix, transverse & sigmoid colon.

• Keratinized SS Mucosal SS = ectoderm (proctodeum)

• Columnar = endoderm (hindgut).

• Muscularis mucosae terminates at junction of rectum and anus and lamina propria blends with submucosa.

• Skin around anus forms the zona cutanea and has no sweat glands but apocrine glands that secrete oily fluid related to sexual activity in lower animals.

• Circular smooth muscle of anal canal is thickened as the internal anal sphincter.

• External anal sphincter is skeletal muscle.

Hemorrhoids

Hemorrhoids are dilations of the internal (above pectinate line) &/or external (below pectinate line) hemorrhoidal plexuses.

Appendix

The appendix has no taenia coli, circumferrential lymphatic nodules with star shaped to obliterated lumen.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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