Gastrointestinal Tract Histology

Overview of Gastrointestinal Tract Histology

• Radial organization of gastrointestinal tract
  • wall of tubular gastrointestinal tract consists of 4 concentric layers:
    • mucosa
      • epithelium
        • typically a simple cuboidal or a simple columnar epithelium
        • secretory function in stomach
        • secretory and absorptive functions in small intestine and large intestine
      • lamina propria
        • supports avascular mucosal epithelium
          • contains blood and lymphatic vessels
      • muscularis mucosae
        • typically a double layer of smooth muscle
          • inner layer of circularly oriented smooth muscle
          • outer layer of longitudinally oriented smooth muscle
        • contraction causes local movement in mucosa
    • submucosa 
      • a layer of connective tissue 
      • contains large blood vessels and large lymphatic vessels
      • contains submucosal (Meisnner’s) nerve plexus
      • anchors the mucosa to the muscularis externa
    • muscularis externa
      • a double layer of smooth muscle
        • inner layer of circularly oriented smooth muscle
        • outer layer of longitudinally oriented smooth muscle
    • contains myenteric (Auerbach’s) nerve plexus in between double layer of smooth muscle
    • contraction causes peristalsis
    • adventitia / serosa
• a layer of connective tissue

Distinctive Features of Gastrointestinal Tract Histology

• Esophagus
  • esophageal mucosa
    • non-keratinizing, stratified squamous epithelium
    • muscularis mucosae is a single layer of longitudinally oriented smooth muscle
  • esophageal muscularis externa
    • upper one third of esophagus
      • striated muscle
    • middle one third of esophagus
      • striated muscle and smooth muscle
    • lower one third of esophagus
      • smooth muscle
• Stomach
  • gastric mucosa
    • gastric glands occupy gastric mucosa
      • simple, branched, tubular glands that extend from muscularis externa to bottom of gastric pits
      • consist of mucus neck cells, parietal cells, chief cells, and G cells
      • elaborate gastric secretions into lumen of stomach via gastric pits
• Small Intestine
  • overview
    • small intestinal mucosa
      • exhibits numerous projections, or villi, that protrude from epithelial layer of mucosal surface
        • villi increase surface area over which digestion and absorption occurs
        • epithelial layer of small intestinal mucosa is heterogeneous, composed of:
          • mucus-secreting cells (goblet cells)
          • absorptive cells (enterocytes)
            • exhibit numerous projections, or microvilli, that protrude from apical border
              • microvilli increase surface area over which digestion and absorption occurs
              • microvilli are responsible for characteristic striated border, or brush border, of enterocytes
      • frequency of villi and of microvilli in small intestine
        • jejunum > duodenum and ileum
      • frequency of goblet cells in small intestine increases as you progress down the small intestine
        • duodenum < jejunum < ileum
  • duodenum
    • duodenal mucosa
      • crypts of Lieberkühn, or intestinal glands, occupy duodenal mucosa
        • simple tubular glands that extend from muscularis externa to base of villi
        • elaborate small intestinal secretions into lumen of duodenum
    • duodenal submucosa
      • Brunner’s glands, or submucosal glands, occupy duodenal submucosa 
        • elaborate alkaline (basic pH) secretions
          • likely function to neutralize acidic chyme propelled from stomach to duodenum of small intestine
        • peptic ulcer disease presents with hypertrophy of Brunner’s glands  
  • jejunum 
    • jejunal mucosa
      • crypts of Lieberkühn, or intestinal glands, occupy jejunal mucosa
    • jejunal submucosa
      • plicae circulares are circularly arranged transverse folds containing a core of submucosa that extend partially around jejunal lumen
  • ileum
    • ileal mucosa 
      • Peyer’s patches, or aggregations of nodules of unencapsulated lymphatic tissue, occupy ileal lamina propria (and ileal submucosa)
        • M cells, overlying Peyer’s patches, function as antigen-transporting cells
          • take up microorganisms and macromolecules
          • deliver antigens to antigen-processing macrophages
            • macrophages present processed antigen to lymphocytes
              • triggers secretory immunity
                • stimulates B cells in germinal centers of Peyer’s patches to differentiate into IgA-secreting plasma cells that reside in ileal lamina propria
      • crypts of Lieberkühn, or intestinal glands, occupy ileal mucosa
    • ileal submucosa
      • plicae circulares are circularly arranged transverse folds containing a core of submucosa that extend partially around ileal lumen (proximal ileum)
        • increase surface area over which absorption occurs
• Large intestine
  • colon
    • colonic mucosa
      • “smooth” surface devoid of villi
      • crypts of Lieberkühn, or intestinal glands, occupy colonic mucosa
    • colonic muscularis externa
      • outer layer of longitudinally oriented smooth muscle is organized into 3 bundles, or teniae coli 
  • anal canal
    • anal canal mucosa
      • keratinizing, stratified squamous epithelium