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GI Histology I—Alimentary

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BIOL 1840U
Peter Cheung

GI Histology I—Alimentary Canal I. Overview a. Function—to break down food into macro & micronutrients for body’s use b. How it does it? By maximizing surface area! c. Layers: i. Mucosa 1. Specialized Epithelium—faces lumen 2. Lamina Propria—underlying connective tissue; contains all sorts of stuff 3. Muscularis Mucosa—us. 2 layers (inner circular & outer longitudinal); contributes to local movement ii. Submucosa—fibrous connective tissue w/ blood vessels lymphatics & nerves iii. Muscularis—relatively thick; inner circular (diameter controlling) & outer longitudinal (length controlling) layers; 1. responsible for propulsion & mechanical breakdown of food (peristalsis) 2. Pacemaker cells—set electrical rhythm & speed but DO NOT initiate contractions (requires neuronal & hormonal input) iv. Adventitia/ Serosa—outermost connective tissue layer; (known as serosa when it’s covered by mesothelium) d. Glands: i. Mucosal—epithelial invaginations into lamina propria ii. Submucosal—epithelial invaginations into submucosa iii. Ducts from accessory glands (salivary, liver, pancreas) e. Blood Vessels i. Enter thru adventitia or mesentery ii. Largest arteries run longitudinally in submucosa iii. Capillaries are fenestrated in mucosa & continuous in muscle f. Lymphatics i. Arise as blind tubes ii. Called Lacteals in SI—chylomicra preferentially enter lacteals (rather than veins) in SI → key for lipid absorption! iii. MALT—mucosal associated lymphoid tissue iv. IgA—antibody secreted from lymphocytes in lamina propria; transcytosed to lumen where it plays important protective role g. Innervation i. Extrinsic 1. Somatic—motor/sensory to skeletal mm in mouth, pharynx, 1/3 esophagus, & EAS 2. Autonomic—Parasympathetic: stimulates Sympathetic: inhibits ii. Intrinsic (Enteric Nervous system) 1. Auerbach’s Plexus—myenteric plexus (found between layers in muscularis) 2. Meissner’s Plexus—submucosal plexus (found in submucosa) 3. neural connections in plexuses permit intratract neural reflexes that er independent of CNS 4. BUT vagus & sacral nerves (CNS) still influence enteric nervous system iii. Hirschprung’s Disease—(congenital megacolon) caused by absence of meissner’s & aurbach’s plexuses, usually in colon II. Esophagus a. Structure—approx 10 in. long; mucosa thrown into many folds (flatten as food passes) 1 b. Key Distinguishing Features i. Longitudinal folds ii. Thick stratified squamous epithelium iii. Relatively few cells in lamina propria iv. Thick fibrous submucosa v. Muscularis: Upper 1/3 = striated; Middle 1/3 = mixed; Lower 1/3 = smooth c. Gastroesophageal Sphincter—(Lower Esophageal sphincter: LES) last 4 cm of esophagus before entering stomach III. Stomach a. Structure—dilated part of GI tract where ingested food undergoes mechanical and chemical digestion => chyme i. Divided into 4 anatomical regions: cardia, fundus, pylorus, & body ii. Divided into 3 histological regions: cardiac, fundic, & pyloric glands 1. cardiac glands—mainly mucus-secreting; short pit & short gland
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