Gastrointestinal System Lecture Notes PDF

Gastrointestinal System Kristine Krafts, M.D. Gastrointestinal System Lecture Objectives • Describe the general layered organization of the GI system, including the location and appearance of: • Mucosa with epithelium, lamina propria, glands, and muscularis mucosae • Submucosa with glands and Meis...

Gastrointestinal System Kristine Krafts, M.D. Gastrointestinal System Lecture Objectives • Describe the general layered organization of the GI system, including the location and appearance of: • Mucosa with epithelium, lamina propria, glands, and muscularis mucosae • Submucosa with glands and Meissner’s submucosal nerve plexus • Muscularis (externa) with Auerbach’s myenteric nerve plexus • Serosa or adventitia • Describe how each layer changes as one moves through the entire digestive tract. More GI System Lecture Objectives • Describe the structure and function of the cells that are present in: • Esophageal glands and esophageal cardiac glands • Gastric pits and gastric glands • Intestinal glands or crypts (glands of Lieberkuhn) of the small intestine and the large intestine • Duodenal glands (mucous glands of Brunner) Gastrointestinal System Lecture Outline • Introduction • Esophagus • Stomach • Small intestine • Large intestine Gastrointestinal System Lecture Outline • Introduction Components of the GI System The digestive tract consists of: • Oral cavity • Esophagus • Stomach • Small and large intestine Organs associated with the digestive tract include: • Salivary glands • Pancreas • Liver • Gallbladder Layers in the GI System • Mucosa with epithelium, loose connective tissue of lamina propria, and muscularis mucosae • Submucosa with glands, denser connective tissue, and Meissner’s submucosal nerve plexus • Muscularis with Auerbach’s myenteric nerve plexus • Adventitia or serosa General Structure of the GI Tract General Structure of the GI Tract General Structure of the GI Tract Mucosa Epithelium Lamina propria Muscularis propria Mucosa: Epithelium • Stratified squamous in oral cavity, esophagus and anal canal • Simple columnar in stomach, small intestine, colon, appendix and rectum • Basement membrane underlies the epithelium • Epithelium can invaginate into underlying connective tissue, forming glands for secretion • Epithelium can evaginate into lumen, forming villi for absorption and secretion Mucosa: Lamina Propria • Composed of loose connective tissue • Contains blood and lymphatic vessels, and sometimes glands. • May contain lymphoid tissue which is part of the mucosa-associated lymphoid tissue (MALT) system. Mucosa: Muscularis Mucosae • Thin layer of smooth muscle • Moves independently of muscularis externa • Not present in oral cavity and anal canal • Composed of dense, irregular connective tissue. • Contains blood vessels, lymphatic vessels, submucosal plexus of autonomic nerves, and sometimes glands and MALT. Submucosa • From esophagus through large intestine: inner circular layer of smooth muscle, outer longitudinal layer of smooth muscle. • Myenteric plexus of autonomic nerves lies between smooth muscle layers and innervates them. Muscularis externa • Stomach has third layer of smooth muscle. • Upper esophagus and external anal sphincter contain skeletal muscle. • Oral cavity has skeletal muscle in muscularis. Muscularis externa • Adventitia consists of loose connective tissue without mesothelium. • Serosa consists of adventitia covered by mesothelium (simple squamous epithelium). In the abdominal cavity, it covers most of the digestive tract and accessory glands. Serosa or adventitia Gastrointestinal System Lecture Outline • Introduction • Esophagus Esophagus: Mucosa • Epithelium: non-keratinized stratified squamous • Lamina propria: • May have lymphoid tissue (MALT) • Esophageal mucous (cardiac) glands located close to stomach • Muscularis mucosae: typical Differences between upper, middle lower esophagus Esophagus: very low power view Epithelium Lamina propria Muscularis mucosae Mucosa Submucosa Esophagus: low power Esophagus: mucosa Esophagus: Other Layers • Submucosa: glands and MALT • Muscularis externa: • Upper third: all skeletal muscle • Middle third: mixed skeletal and smooth muscle • Lower third: all smooth muscle • Adventitia surrounds almost all of the esophagus Esophagus: submucosa with glands Esophagus: muscularis externa Esophagus: inner layer of muscularis externa Esophagus: outer layer of muscularis externa Esophagus: Myenteric plexus Esophagus: adventitia Gastrointestinal System Lecture Outline • Introduction • Esophagus • Stomach Functions of the Stomach • Mixes food to produce chyme • Begins digestion process • Produces intrinsic factor for absorption of vitamin B12 from the ileum • Absorbs a few nutrients Stomach regions Stomach: Mucosa • The epithelium of the stomach consists of simple columnar mucous epithelial cells which invaginate into lamina propria to form gastric pits or foveolae. • Rest of lamina propria contains gastric glands. • Muscularis mucosae is typical. Stomach histology Mucosa of Body and Fundus • Surface epithelium forms short pits (1/4 of the mucosal thickness). • 2-4 straight-ish, long glands empty into each pit. • Cells present include mucous neck cells, stem cells, parietal cells, chief cells, enteroendocrine cells. Mucosa of body and fundus short pits parietal cells long straight glands Mucosa of body and fundus Stomach: fundic mucosa Fundic mucosa: gastric pits Fundic mucosa: gastric glands Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Chief cells • Stem cells • Enteroendocrine cells Cells of Gastric Glands in Body/Fundus • Mucous neck cells: make protective mucus Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Make intrinsic factor (necessary for B12 absorption) and HCl • Have intracellular canaliculi and microvilli • Located mostly in neck and body of gland • Intensely eosinophilic (lots of mitochondria) Parietal cell, resting and active Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Chief (or zymogenic) cells • Secrete pepsinogen, which is converted into pepsin by HCl • Pepsin hydrolyzes proteins • Located in base of glands • Very basophilic due to rough ER Parietal and chief cells Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Chief cells • Stem cells • Replenish epithelium • Located mostly in isthmus Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Chief cells • Stem cells • Enteroendocrine cells • G cells: release gastrin when there is protein in the stomach. Gastrin stimulates parietal and chief cell secretion, and increases contraction of the muscularis externa. Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Chief cells • Stem cells • Enteroendocrine cells • G cells: release gastrin • D cells: release somatostatin when acid levels in the stomach go up. Somatostatin inhibits gastrin secretion. Cells of Gastric Glands in Body/Fundus • Mucous neck cells • Parietal cells • Chief cells • Stem cells • Enteroendocrine cells • G cells: release gastrin • D cells: release somatostatin • Ghrelin cells: release ghrelin during fasting conditions. Ghrelin increases appetite. Mucosa of Pyloric Region • Surface epithelium forms long pits (2/3 of the mucosal thickness). • 2-4 branched, coiled glands empty into each pit. • Cells present include mucous neck cells and stem cells. Mucosa of pyloric region long pits short, branched glands Mucosa of pyloric region Stomach: pyloric mucosa Pyloric mucosa: gastric pits Pyloric mucosa: gastric glands Mucosa of Cardiac Region • Similar to mucosa of pyloric region, except pits are short (1/4 of the mucosal thickness). • Glands (coiled) and cells (mucous neck and stem cells) are same as pyloric region. Summary: Pits and Cells Pit Cells Cardia Fundus/Body Pylorus short short long Mucous, stem Mucous, stem, parietal, chief, enteroendocrine Mucous, stem Stomach: Remaining Layers • Submucosa • Connective tissue with blood vessels and lymphatics • Meissner’s autonomic plexus • Muscularis externa • 3 layers of smooth muscle (inner oblique, middle circular, outer longitudinal) • Myenteric nerve plexus between middle and outer layers • Serosa Meissner’s submucosal Auerbach’s myenteric Autonomic plexuses (plexi?) Inner oblique(ish) Middle circular Nerve plexus Outer longitudina l Muscularis externa Gastrointestinal System Lecture Outline • Introduction • Esophagus • Stomach • Small intestine Functions of Small Intestine • Complete digestion of food • Absorb final products of digestion (amino acids, monosaccharides, fatty acids, etc.) • Secrete protective mucus • Secrete hormones Proximal Plicae circulares or valves of Kerckring Distal Villus Plicae circulares Mucosa Submucosa Muscularis Serosa Mesothelium Small Intestine: Anatomic Regions • Duodenum: immediately distal to pylorus of stomach (12 inches) • Jejunum: distal to duodenum (8 feet) • Ileum: most distal region (12 feet) Your small intestine is basically a 22-foot-long pipe that has a diameter of an inch or so. What’s the total surface area? This is the actual surface area (2700 square feet)! How can the surface area be so large? Folds, folds and more folds. • Plicae circulares: circular infoldings of mucosa and submucosa in duodenum and upper jejunum • Villi: mucosal folds • Microvilli: tiny folds on cell surface Small intestine: plicae circulares Epithelium Lamina propria Muscularis mucosa Submucosa Muscularis externa Auerbach’s plexus Serosa Small intestine: layers Small intestine: plicum and villi Small Intestine: Villi and Glands • Villi are fingerlike processes of epithelium (with a core of lamina propria) projecting into the lumen. There is smooth muscle in the lamina propria which helps the villi move. • Intestinal glands (Crypts of Lieberkuhn) are invaginations of epithelium into the lamina propria. • Epithelium covering the villi and lining the crypts is continuous (but there are different cell types in different parts). Villus Simple columnar epithelium Lacteal or central lymphatic Crypt of Lieberkuhn (intestinal gland) Goblet cells Lamina propria Capillary network with an artery and vein Smooth muscle Lamina propria Muscularis mucosae Undifferentiated columnar cells Paneth cells Intestinal villi Crypts of .. Lieberkuhn Small intestine: villi and glands (crypts) Cells of Epithelium Covering Villi • Simple columnar absorptive cells • Most numerous • Have nice microvilli (“brush border”) • Function: absorb nutrients • Mucous goblet cells • Produce mucus for protection • Increase in number as you move down the intestinal tract Small intestine: epithelial cells and goblet cells Cells in Epithelium of Glands (Crypts) • Simple columnar absorptive cells • Mucous goblet cells Cells in Epithelium of Glands (Crypts) • Simple columnar absorptive cells • Mucous goblet cells • Enteroendocrine cells • Secrete a bunch of hormones related to digestion (like cholecystokinin) Cells in Epithelium of Glands (Crypts) • Simple columnar absorptive cells • Mucous goblet cells • Enteroendocrine cells • Paneth cells • Deep in crypts • Produce lysozyme and defensins (antibacterial substances) Cells in Epithelium of Glands (Crypts) • Simple columnar absorptive cells • Mucous goblet cells • Enteroendocrine cells • Paneth cells • M (microfold) cells • Eat bugs/foreign stuff and present antigens to immune cells Cells in Epithelium of Glands (Crypts) • Simple columnar absorptive cells • Mucous goblet cells • Enteroendocrine cells • Paneth cells • M (microfold) cells • Stem cells • Replenish cells of epithelium Small intestine: glands Small intestine: Paneth cells How to Tell Apart Small Intestine from Stomach • Villi. Small intestine has them; stomach does not. • Epithelium. Small intestine has alternating absorptive cells (with microvilli) and goblet cells. Stomach has simple columnar mucous cells (no microvilli) and no goblet cells. • Paneth cells. Small intestine has them; stomach does not. Duodenum, Jejunum and Ileum • All have similar epithelium and glands (crypts). • All have villi. • All have similar muscularis mucosae and externa. • Differences lie in lamina propria and submucosa! Duodenum • Coolest feature of duodenum: Brunner’s glands. • Present in submucosa; extend into lamina propria. • Secrete alkaline mucous. Duodenum: low power Duodenum: Brunner’s glands Ileum • Coolest feature of ileum: Peyer’s patches. • Lymphoid tissue present in submucosa, usually in nodules (follicles). • Good example of mucosa-associated lymphoid tissue (MALT). • Good idea to have additional, special lymphoid tissue. The gut is basically a tube open to the outside – think of all the bugs and foreign material it is exposed to! Ileum: Peyer’s patch Jejunum • Coolest feature of jejunum: well, there isn’t a particularly cool feature unique to the jejunum. Sorry jejunum. • To distinguish it from the duodenum and ileum, though, note that it does NOT have either Brunner’s glands (like the duodenum) or Peyer’s patches (like the ileum). Jejunum: Submucosa without Brunner’s glands or Peyer’s patches Summary: Features of Small Intestine Duodenum Jejunum Ileum Villi yes yes yes Microvilli on absorptive cells yes yes yes Goblet cells some more most Paneth cells yes yes yes Brunner’s glands yes no no Peyer’s patches no no yes Gastrointestinal System Lecture Outline • Introduction • Esophagus • Stomach • Small intestine • Large intestine Functions of the Large Intestine • Absorption of water • Secretion of mucus to lubricate dehydrated feces Anatomical Regions of Large Intestine Wall of large intestine Crypts of Lieberkühn Lamina propria Muscularis mucosae Lymphatic nodule Mucosa of large intestine Wall of large intestine Submucosa Muscularis externa Muscularis mucosa Epithelium and lamina propria Serosa or adventitia Large intestine: low-power view Epithelium and lamina propria Muscularis mucosa Submucosa Muscularis externa Large intestine: layers Mucosa of Large Intestine • Epithelium is simple columnar and contains: • Absorptive cells with microvilli • Mucous goblet cells (abundant!) • Enteroendocrine cells • Stem cells • Epithelium invaginates into lamina propria forming Crypts of Lieberkuhn (intestinal glands). • Differences from small intestine: Villi and plicae circulares are not present, and Paneth cells are rare. Epithelium Lamina propria Muscularis mucosa Colon: mucosa Large Intestine: Remaining Layers • Submucosa is similar to small intestine; may have lymphoid nodules extending from lamina propria • Muscularis externa • Inner circular layer • Outer longitudinal layer (in 3 bundles: taenia coli) • Serosa or adventitia • Transverse colon has serosa • Ascending and descending colon have adventitia Inner circular layer Auerbach’s plexus Outer longitudinal layer (taenia coli) Colon: muscularis externa Appendix • Attached to cecum • Histology similar to colon except: • No taenia coli • Lots of lymphatic tissue (both diffuse and in nodules) present in lamina propria and submucosa Diffuse lymphoid tissue Lymphoid nodules Appendix: super low-power view Crypts Lamina propria Appendix: mucosa Rectum • Last 6-8 inches of large intestine • Epithelium has tons of goblet cells • Mucosa has longitudinal folds called the rectal columns of Morgagni Sigmoid colon Middle rectal valve Superior rectal valve Inferior rectal valve Levator ani muscle Rectal column Rectal sinus Internal hemorrhoidal plexus External sphincter Anorectal line Anatomic anal canal External hemorrhoidal plexus Internal sphincter Rectal column or columns of Morgagni Simple columnar epithelium Anal valves Internal anal sphincter (smooth muscle) Anus Stratified squamous mucosal epithelium External anal sphincter (skeletal muscle) Anal orifice Anus • Last 1-1 ½ inches of large intestine • Stratified squamous epithelium; no muscularis mucosae or lamina propria • Submucosa has large veins (susceptible to dilation) • Internal anal sphincter is composed of circular layer of smooth muscle • External anal sphincter is composed of skeletal muscle (under voluntary control) Pectinate line Colorectal zone Anal canal Recto-anal junction: super low-power view Simple columnar epithelium of colorectal zone Pectinate line Stratified squamous epithelium of the anal canal Recto-anal junction: super low-power view Here it is! Pectinate line Crypts with tons of goblet cells Stratified squamous epithelium Recto-anal junction Crypts with tons of goblet cells Stratified squamous epithelium Recto-anal junction Anal glands Internal anal sphincter (smooth muscle) External anal sphincter (skeletal muscle) Recto-anal junction Gastrointestinal System Lecture Outline • Introduction • Esophagus • Stomach • Small intestine • Large intestine