GI - Anatomy/Histology

GI - Anatomy/Histology

1. Oral Cavity

   • Teeth (32)
     • Enamel (outermost layer; HARDEST STRUCTURE IN BODY)
     • Dentin (inside enamel)
     • Pulp (inside dentin)
     • Cementum (secures teeth to jaw)
     • Periodontal ligaments (hold teeth in place)
   • Tonsils
     • Palatine
     • Pharyngeal
     • Lingual
   • Tongue
     • Filiform Papillae (anterior)
     • Circumvallate Papillae (on innervation border b/w CN VII and IX; contain taste buds)
     • Stratified squamous, nonkeratinized epithelium
   • Salivary Glands
     • Parotid (30%; ALL SEROUS; good for digestion)
     • Submandibular (60%; MIXED)
     • Sublingual (10%; ALMOST ALL MUCOUS; good for lubrication)

2. Digestive Tube

   • Esophagus
     • Nonkeratinized, stratified squamous epitheiium
     • Lamina propria, submucosa - mucous lubrication
     • Muscularis externa
       • Upper 1/3 - skeletal Middle - skeletal and smooth Lower 1/3 - smooth
     • Adventitia (above diaphragm); Serosa (below diaphragm)
     • Hiatal Hernia: stomach traverses diaphragm where esophagus usually penetrates
     • Barrett's Esophagus: dysplasia of epithelium due to stomach acid exposure; CARCINOGENIC
   • Stomach
     • Physical regions: cardia, fundus, body, pylorus; rugae are inner folds
     • Gastric Pits: surface mucous cells secrete mucous, protect lining
       • Peptic ulcers: lose lining, acid erodes stomach wall (can be due to H. pylori bacteria)
     • Gastric Glands (from left to right, these cells are organized superficial to deep)
       • Mucous neck cells
         • Produce thinner mucous, facilitating movement of things in lumen
       • Parietal cells
         • -Pump H+ and Cl- ions into lumen, which combine to form HCl (i.e. stomach acid) -Produce GIF, which is necessary to absorb Vitamin B12 -Appear as eosinophilic "fried egg cells" under microscope (red b/c lots of mitochondria for pumping)
         • Pernicious anemia: no B12 to properly make RBC's b/c no GIF from parietal cells
       • Chief cells
         • Produce proenzymes pepsinogen and lipase, activated upon cleavage in acidic environment; digest protein and fats, respectively
       • Enteroendocrine cells
         • Secrete hormones (motilin, secretin, CCK) into lamina propria, allowing shuttling of signals to other parts of GI and enabling crosstalk b/w the different parts
       • Stem cells
         • Stem cells that replace surface mucous cells every 4-7 days to ensure safety of stomach lining
   • Small Intestine
     • Includes: Duodenum (4 sections; SEE NOTE!), jejunum (plicae circularis), ileum
       • Duodenum -INTRAPERITONEAL AT BEGINNING OF FIRST SEGMENT AND DUODENOJEJUNAL JUNCTION!!-Intestinal glands to protect lining from stomach acid Jejunum -Few arcades, long vasa recta Ileum -Many arcades, short vasa recta -Lymph nodules in the form of Peyer's Patches
     • Simple, columnar epithelium
       • Surface absorptive cells
         • MICROVILLI HERE
       • Goblet cells
         • PRODUCE MUCOUS
       • Enteroendocrine cells
         • Secrete hormones (motilin, secretin, CCK) into lamina propria, allowing shuttling of signals to other parts of GI and enabling crosstalk b/w the different parts
       • Paneth cells
         • -Produce bacterial-killing lysozyme -Red granules in a circle under microscope
       • M cells
         • APC's
       • Stem cells
     • Lamina propria
       • Contains intestinal glands (crypts of Lieberkuhn)
     • Muscularis mucosa
     • Submucosa
       • -Submucosal (Meissner's) plexus -Duodenal (Brunner's) glands secrete mucous, alkaline fluid
     • Muscularis externa
       • -Myenteric (Auerbach's) plexus
     • Serosa/Adventitia
     • Innervation: Submucosal and Myenteric plexuses
     • Malabsorption diseases (topical sprue, celiac disease)
       • These are damaging through inflammation of the epithelial lining and destruction of villi, as well as compromise of the gut-vascular barrier, making infection more likely if gut flora get into bloodstream
     • Enteritis (inflammation --> bowel stenosis)
   • Large Intestine
     • Includes: Colon, Cecum, Appendix, Rectum
     • Structures: Taenia coli, haustra, epiploic appendages, lymph nodules
     • Simple, columnar epithelium
       • Contains: -surface absorptive cells -mucosal goblet cells -enteroendocrine cells -Stem cells (replenish every 6-7 days)
     • HAS SAME LAYERS AS SI
     • Appendicitis
   • Rectum/Anus
     • Transition from simple columnar to stratified squamous epithelium
     • Structures: Anal columns, anal valves/sinuses, pectinate line
       • Pectinate line is the transitional zone from where blood supply is going portally to systemically (SO PORTAL HYPERTENSION CAN MANIFEST HERE)
     • Layers: Lamina propria, muscularis mucosa, submucosa, muscularis externa
       • Lamina propria -anal and circumanal glands Muscularis mucosa -ends at pectinate line Submucosa -hemorrhoidal plexuses Muscularis externa -internal anal sphincter (smooth) -external anal sphincter (skeletal)
     • Hemorrhoids: internal (not painful) and external (painful)
   • 4 MAIN LAYERS
     • Mucosa (innermost)
       • Contains: -Epithelium (innermost) -Lamina propria -Muscularis mucosa (outermost)
     • Submucosa
       • Contains: -Submucosal (Meissner's) Plexus-Submucosal glands (ONLY in the esophagus and duodenum; secrete buffer to neutralize stomach acid, as these locations are on either flank of the stomach)
     • Muscularis externa
       • Contains: -Inner layer (circular; SQUEEZES) -Outer layer (longitudinal; MOVES FOOD ALONG) -Myenteric (Auerbach's) Plexus
     • Adventitia (or Serosa; outermost)
       • Connective supportive layer; referred to as adventitia if organ is retroperitoneal, or serosa is organ is intraperitoneal
   • Blood Supply: Celiac artery, SMA, IMA
     • Branches of celiac trunk: -left gastric artery -common hepatic artery -splenic artery Branches of SMA: -ileocolic artery -right colic artery -middle colic artery -jejunal and ileal branches Branches of IMA: -left colic artery -sigmoid arteries -superior rectal artery
   • Innervation
     • Parasympathetic: CN X and pelvic splanchnics
     • Sympathetic: Greater, lesser, least, and lumbar splanchnics

3. Liver

   • Portal Triad
     • Blood supply: hepatic artery
     • Portal vein: filters systemic circulation through sinusoids
       • Perisinusoidal space b/w hepatocytes and sinusoids allows for greater filtration due to slower fluid velocity through here
     • Hepatic duct: carries out bile
       • Pathway of bile: Bile canaliculi (in lateral plasmalemma) --> choliangoles --> canals of Hering --> interlobular bile ducts --> hepatic duct
   • Hepatic vein: blood leaving liver to systemic circulation
   • Cell types
     • Hepatocytes
     • Kuppfer cells: macrophages in perisinusoidal space
   • Lobules
     • Classic Lobule
     • Portal Acinus
       • -Two neighboring portal triads drain into two neighboring central veins (diamond-shaped) -3 processing zones: zone 1 receives unfiltered nutrients and this heavy processing occurs here, zone 3 receives the most processed blood (already went through zones 1 and 2)
     • Portal Lobule
       • Portal triad drains here (triangle of central veins b/w three neighboring hepatocytes)
   • Hepatitis: can produce referred pain in shoulder
   • Portal HTN: manifests at portocaval anastomoses
     • Caput medusae
     • Internal hemorrhoids
     • Esophageal varices
     • Retroperitoneal varices
   • Jaundice: can indicate cirrhosis or hepatitis

4. Pancreas

   • Pancreatic acinus: produces zymogen
   • Centroacinar cells; secrete bicarb-rich, enzyme poor solution
     • This keeps proenzymes from becoming activated until they reach the duodenum
   • Intercalated ducts
   • Intra- and interlobular ducts
   • Pancreatic cancer
   • Pancreatitis

5. Gallbladder

   • Stores and releases bile to emulsify dietary fat
   • Layers: epithelium (in and out), lamina propria, muscularis externa (oblique)
   • Gallstones

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