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Histology/Pathology – Lecture 2: Histology of the GI Tract 8/3/12 Alimentary Canal o Mouth esophagus stomach duodenum jejunum ileum cecum large intestine anus General Structure of Digestive Tract o Lumen w/ a 4-layered wall Mucosa Epithelial lining underlying the lamina propria Muscularis mucosae o Thin layer of smooth muscle that separates mucosa from submucosa Submucosa Submucosal gland (only found in esophagus and duodenum) Denser CT w/ blood and lymph vessels Meissner submucosal plexus of autonomic nerves Muscularis Inner circular layer of smooth muscle Outer longitudinal layer of muscle Myenteric nerve plexus in b/t layers; also CT w/ blood vessels Serosa (Adventitia) Loose CT, blood vessels, lymphatics, fat, etc. Covers vast majority of GI tract Simple squamous covering epithelium (mesothelium) Components of the Wall of GI Tract o Meissner submucosal plexus Found in the submucosa Coordinates peristalsis o Auerbach’s myenteric plexus Coordinates peristalsis Found b/t the circular and longitudinal muscle layers Main Fxns of the Epithelial Lining o Provide selectively permeable barrier b/t contents of tract and body tissues o Facilitate transport and digestion of food o Promote absorption of products of digestion o Produce hormones that affect GI system o Produce mucus for lubrication and protection Anatomy and Physiology of Esophagus o Adult esophagus: 25 cm long o Fixed superiorly at cricopharyngeus muscle = upper esophageal sphincter o Exits thorax through hiatus of diaphragm o Lower esophageal sphincter is not a true sphincter; rather a functional one Tonic muscular contraction at lower end of esophagus creates a flutter-like wave motion o Anatomic Relationships Begins at back of pharynx Posterior to trachea and arch of the aorta Histology of Esophagus o See Lab for slides and explanations** Normal Esophageal Mucosa o Stratified squamous epithelium Tough – can handle coarse foods Pearly white color Can see a sharp line of demarcation on transition to columnar epithelium o The gastroesophageal (GE) junction Stomach o 4 parts Cardia Mucous cells o Protect the stomach against acid Fundus Parietal cells o Hydrochloric acid & intrinsic factor Chief cells o Pepsin Much thicker glandular mucosa than in cardia Body Antrum Mucous cells G cells (endocrine cells) o Gastrin (driver of HCl production) o Anatomy Rugae Folds that increase the surface area; flatten out upon distention Most prominent in the proximal stomach Gland structure in body of stomach . See slides for images from various parts of the stomach o Gastroduodenal Junction Marked by the pyloris – area that gets tighter/thicker Duodenum contains Brunner glands Small Bowel o Has several segments w/ plica circularis o Mucosa Many villi covered by epithelial cells of 3 types: Columnar absorptive cells Mucin-secreting cells (goblet cells) Few endocrine cells Villi terminate in the lamina propria as glandular lumina called crypts of Lieberkuhn Each villi contains a small lymph channel – lacteals Paneth cells – secrete IgA; found in the jejunum o The foldings of the small intestine – increase the surface area for absorption Plica circularis o Peyer’s Patches Part of the MALT system found in the ileum Appendix o Found at the base of the cecum, near the connection w/ the ileum o Contains Serosa, mesentery, follicle, lamina propria Colon o Storage organ o 9 L of water pass through/day o Contains many crypts of Lieberkuhn o Mainly a flat surface o Contains many goblet cells lubrication for peristalsis and defecation o Layers Circular layer of muscles Longitudinal layer of muscles o Also has lymphatic nodules Recto-anal Junction o Line of Hilton (aka pectinate line) Jxn b/t rectum and anus Rectum has columnar mucosa Anus has stratified squamous mucosa