Digestive 2

• Layers of the GIT

  • Mucosa
  • Submucosa
  • Tunica muscularis/ muscularis externa
  • Serosa
• Mucosa


  Epithelial lining of the organ
• Functions of the mucosa

  • Protection
  • Absorption
  • Secretion
• Mucosa


  • Lamina propia- loose connective tissue with blood vessels, lymphatics and small glands
  • Muscularis mucosa – thin layer of smooth muscles; allowing minimal movement of the mucosa
• Submucosa


  Denser irregular connective tissue with larger blood vessels and submucosal plexus of nerves (Meissner plexus)
• Muscularis layer


  • Inner- Circular muscles contract to compress and mix the content of the lumen
  • Outer – Longitudinal muscles shorten the tube and propel the food bolus
  • Contains Myenteric nerve plexus (Auerbach)
• Sphincters in the GIT

  • Pharyngoesophageal
  • Inferior esophageal sphincter
  • Pyloric sphincter
  • Ileocecal valve
  • Internal anal sphincter
• Serosa


  Large blood vessels and lymphatics travel through the serosa to reach the wall or lumen of the organ
• Adventitia


  Covers organs outside the peritoneal cavity or attached to the wall of abdominal cavity
• Esophagus


  • 25 cm long tube which delivers food from pharynx to stomach
  • Upper, middle, lower third
  • Located in the thorax, passing through the diaphragm to join the stomach, its terminal 1-2 cm is in the abdominal cavity
• Esophageal layers


  • Mucosa
  • Submucosa
  • Tunica muscularis/muscularis externa
  • Adventitia/Serosa
• Esophageal mucosa


  • Stratified squamous, non-keratinizing
  • Lamina propia has diffuse type of lymphatic tissue and nodular types
  • Muscularis mucosae is composed of smooth muscles
• Esophageal submucosa


  • Dense irregular connective layer
  • Contains esophageal glands (for lubrication) in the lower segment
  • Contains large blood vessels
  • Contains ganglion cells of the Meissner's plexus
• Esophageal muscularis


  • Has ganglion cells of the Myenteric/Auerbach plexus
  • Two layers: inner circular, outer longitudinal
  • Proximal ⅓ is skeletal muscle, middle ⅓ is combination of skeletal and smooth, distal ⅓ is smooth muscle
• The first 23cm of the esophagus is covered by adventitia, the last 2cm is covered by serosa
• On cross section, the esophageal lumen appears collapsed and has numerous folds
• Stomach


  • The most dilated/expanded portion of the GIT
  • Mainly for digestion and storage of food
  • 4 regions: Cardia, Fundus, Corpus/body, Pylorus
• Cardia and Pylorus have short glands with mucus cells, Fundus has taller glands
• Stomach mucosa and submucosa

  • Form longitudinal folds called rugae which allows the stomach to distend when filled
  • Rugae are more prominent in the lower regions
• Stomach mucosa


  • Simple columnar epithelium which invaginates into lamina propria forming gastric pits (foveolae) each with opening into the stomach lumen
  • Each gastric pit leads to a gastric gland
• Stomach surface mucous cells


  • Simple columnar cells
  • Contains mucinogen granules
  • Secretes mucus to protect cells from HCl
  • Secretes bicarbonate to neutralize acid
  • Secretes prostaglandin E2 which enhance bicarbonate secretion
• Stomach mucosa - Isthmus area


  • Site of stem cells and can give rise to mucous cells or gland cells

  
• Stomach gastric pit

  • Long, branched tubular glands through the full thickness of the lamina propia (NECK and BASE)
• Cells in the stomach glands

  • Parietal cells
  • Chief cells
  • Enteroendocrine cells
• Food in the stomach dilates pylorus and presence of proteins


  G cells produce Gastrin
• Gastrin acts on parietal cells

  Parietal cells produce HCl
• Gastrin acts on Enterochromaffin like cells - ECL


  ECL produce Histamine
• Histamine acts on Parietal cells


  Parietal cells produce HCl
• Chief cells produce Pepsinogen


  Pepsinogen is converted by HCl to Pepsin
• Pepsin


  Digests proteins in the diet
• Mucous neck cells


  • Smaller than parietal cells
  • Irregular in shape
  • Found between parietal cells
• Parietal cells


  • Found at the neck
  • Larger than mucous neck cells
  • Triangular in shape
  • Acidophilic/
• Chief cells


  • Found at the base of the glands
  • Basophilic cytoplasm because of numerous Rough ER
  • Acidophilic apex because of inactive pepsinogen (zymogen granules)
• Muscularis externa of the stomach

  • Inner oblique
  • Middle circular
  • Outer longitudinal
• The cardia and pylorus have short glands with mucus cells, while the fundus has taller glands
• Gastroesophageal junction


  • Deep gastric pits leading to short, coiled pyloric branched glands
  • Most of the cells are mucosa secreting (pale staining appearance)
  • Fewer parietal cells
• Small intestine


  • 4-7 meters in length
  • 3 segments: Duodenum, Jejunum, Ileum
• Small intestine wall


  • Mucosa – lining epithelium
  • Absorptive cells with microvilli
  • Plicae circularis or Valves of Kerckring

  
• Absorptive cells


  Tall columnar, with an oval nucleus in the basal half of the cell, and a striated border at the apex
• The function of the columnar absorptive cell is the absorption of water, minerals, amino acids, simple sugars, and lipid