Histology of Gastrointestinal Tract

Basic Histological Layers of the Digestive System

• The digestive system consists of four main layers: Mucosa, Submucosa, Muscularis, and Serosa
• Mucosa: composed of Epithelium, Lamina Propria, and Muscularis Mucosae
• Submucosa: contains Submucosal plexus (“Plexus of Meissner”)
• Muscularis: contains Myenteric plexus
• Serosa: outermost layer, composed of a thin layer of connective tissue and epithelium

Esophagus

• Mucosa: Stratified squamous non-keratinized epithelium
• Submucosa: contains Meissner's plexus and oesophageal glands
• Muscularis externa: composed of skeletal fibers (upper one-third), mixed fibers (middle one-third), and smooth fibers (lower one-third)
• Adventitia: loose connective tissue

Stomach

• Divided into three histological regions: Cardiac, Fundic, and Pyloric
• Mucosa: simple columnar epithelium with gastric pits
• Gastric glands:
  • Found in fundus and body of stomach
  • Secrete gastric juice, mucus, and gastrin
  • Each gastric pit communicates with several gastric glands
  • Two types of secretory cells: parietal cells and chief cells
• Cells of fundic region:
  • Mucous neck cells
  • Parietal (oxyntic) cells
  • Chief (peptic/zymogen) cells
  • Enteroendocrine cells
  • Undifferentiated cells
• Parietal cells:
  • Secrete intrinsic factor and hydrochloric acid (HCl)
  • Mostly found in proximal portions of glands
• Chief cells:
  • Secrete pepsinogen (inactive proenzyme)
  • Most abundant near base of gastric gland
  • Pepsinogen is converted by HCl in the gastric lumen to pepsin (active proteolytic enzyme)

Small Intestine

• Divided into three segments: Duodenum, Jejunum, and Ileum
• Mucosa:
  • Characterized by Plicae circularis (valves of Kerkring)
  • Villi and Microvilli
  • Goblet cells (few)
  • Crypts of Lieberkuhn (intestinal glands)
  • Glands are lined by columnar cells, goblet cells, Paneth cells, and enteroendocrine cells
• Plays key role in digestion and absorption of nutrients
• 90% of nutrient absorption occurs in the small intestine
• Segments of the S.I.:
  • Duodenum: 25 cm long, receives chyme from stomach, digestive secretions from pancreas and liver
  • Jejunum: 2.5 meter long, middle segment, location of most chemical digestion and nutrient absorption
  • Ileum: 3.5 meter long, final segment, joins large intestine at ileocecal valve

Intestinal Folds and Projections

• Structural modifications of the small intestine wall increase surface area
• Plicae: largest, deep transverse (circular) folds in intestinal lining, permanent features
• Intestinal Villi: series of fingerlike projections of mucosa, covered with simple columnar epithelium, which have many plasma membrane projections called microvilli
• All serve to increase surface area for absorption (altogether by 600x)

Large Intestine

• Also called large bowel
• Horseshoe-shaped, about 1.5 meters long and 7.5 cm wide
• Extends from end of ileum to anus
• Lies inferior to stomach and liver
• Functions:
  • Reabsorption of water (the last 15-20%)
  • Compaction of intestinal contents into feces
  • Absorption of important vitamins produced by bacteria
  • Storage of fecal material prior to defecation
• Parts of the Large Intestine:
  • Cecum: pouchlike first portion, has wormlike appendix projecting from it
  • Colon: largest portion
  • Rectum: last 15 cm of digestive tract
  • Anal canal
• Mucosa:
  • Absence of Plicae circulares and villi
  • Presence of Microvilli
  • Presence of Crypts of Lieberkuhn
  • Presence of Goblet cells in large number

This quiz covers the basic histological layers of the digestive system, including the esophagus, mucosa, submucosa, muscularis, and serosa.