Microanatomy of the Gastrointestinal System II

General Concepts and Definitions

  • Stomach Nomenclature:
    • Latin: Ventriculus
    • Greek: Gastēr
  • Stomatitis: Explicitly defined as inflammation of the oral cavity (not to be confused with stomach inflammation).
  • Stomach Basic Function: A distention of the digestive tube equipped with sphincters at the entry (CARDIA) and at the exit (PYLORUS).
  • Histological Layers of the Glandular Mucosa:
    • Vessels (V): Present within the tissue.
    • Submucosa (SM): Layer beneath the mucosa.
    • Mucosa (M): The innermost layer containing the glands.
    • Muscularis externa (ME): Composed of three layers of smooth muscle (oblique, circular, and longitudinal).
    • Serosa (S): The outer covering of the stomach.

Stomach Microanatomy and Epithelium

  • Epithelium Types:
    • Non-glandular: Stratified squamous epithelium.
    • Glandular: Simple columnar epithelium.
  • Esophagus-Stomach Junction:
    • In species like the dog, the stratified squamous epithelium of the esophagus ends abruptly at the point where the columnar epithelium of the stomach begins.
  • Structural Classifications (Species-Specific):
    • Simple Stomach: Found in carnivores and humans; characterized by glandular mucosa only.
    • Compound Multichambered Stomach: Found in ruminants. The forestomachs (rumen, reticulum, omasum) are lined with stratified squamous epithelium, while the abomasum (the "true stomach") is glandular.
    • Composite Stomach: Found in pigs and horses; contains both glandular and stratified squamous mucosa.
  • Margo Plicatus (mp): In the horse, this is the distinct junction between the glandular (g) and non-glandular (ng) regions of the stomach.

Glandular Regions of the Stomach

  • General Features:
    • All domestic species possess three glandular regions: Cardia, Fundus (or body), and Pylorus.
    • Surface Lining: Simple columnar mucous cells line the surface in all three regions.
    • Gastric Pits: Functional invaginations of the lining epithelium that lead into the glands.
    • Rugae: Large folds that allow for stomach distention.
  • Specific Glandular Regions:
    • Cardiac Gland Region: Composed mainly of mucous glands.
    • Fundic Region (Proper Gastric Glands): Composed of several specialized cell types:
      • Parietal Cells: Secrete hydrochloric acid (HClHCl) and intrinsic factor.
      • Chief Cells: Produce pepsinogen and gastric lipase.
      • Endocrine G Cells: Produce the gastrointestinal hormone gastrin, which enters the bloodstream.
      • Mucous Neck Cells: Secrete acidic fluid containing mucin.
    • Pyloric Region: Contains mucous glands and endocrine G cells. (Note: G cells are generally not visible without the use of special stains).

Microanatomy of the Small Intestine

  • Nomenclature:
    • Latin: Intestinum
    • Greek: Enteron
  • Segments:
    • Duodenum
    • Jejunum
    • Ileum
  • Key Structural Modifications for Absorption:
    • Circular Folds: Large-scale tissue folding.
    • Villi: Papillary projections into the lumen (confined to the small intestine); the primary site of absorption.
    • Microvilli: Detailed foldings of the cell membrane containing actin microfilaments. Their function is to increase surface area. Approximate size is 1 μm1\,\mu\text{m}. They form the "striated border."
    • Intestinal Crypts (Crypts of Lieberk$k$hn): Invaginations of the lining epithelium at the base of the villi; the primary site of cell production and division.
  • Cellular Composition:
    • Enterocytes: Absorptive cells with microvilli.
    • Goblet Cells: Produce mucus.
    • Stem Cells: Located in the crypts for regeneration.
    • Enteroendocrine Cells: Secrete various hormones.
    • Paneth Cells: Provide innate immunity.

Regional Specializations of the Small Intestine

  • Duodenum:
    • Contains Brunner’s glands (duodenal glands) in the submucosa.
    • Function: Produce alkaline mucus to protect the duodenum from acidic stomach content.
    • Presence: Found only in mammalian species.
  • Jejunum:
    • Characterized by a lack of specific specialized structures in the connective tissue of the submucosa compared to the duodenum or ileum.
  • Ileum:
    • Features Peyer’s patches, which are vast aggregations of lymphoid nodules and Mucosal Associated Lymphoid Tissue (MALT).
    • Found in the distal small intestine (jejunum and ileum).
  • Central Lacteals:
    • Blind-ended lymphatic vessels located in intestinal villi.
    • They carry chyle, which is filtered through intestinal lymph nodes before entering the blood at the jugular veins.
  • M Cells: Specialized epithelial cells that sample antigens from the luminal environment and transport them to underlying lymphoid tissue.

Microvilli vs. Cilia

  • Microvilli:
    • Movement: Non-motile.
    • Structure: Folded membrane with actin microfilaments.
    • Size: approximately 1 μm1\,\mu\text{m}.
    • Function: To increase surface area.
  • Cilia:
    • Movement: Motile.
    • Structure: 9+29+2 microtubule arrangement.
    • Size: 1−10 μm1-10\,\mu\text{m}.
    • Function: Movement (of substances over the cell surface).

Enteric Nervous System

  • A division of the autonomic nervous system consisting of nets of nerves connecting ganglia.
  • Submucosal Plexus (Meissner’s Plexus): Located in the submucosa.
  • Myenteric Plexus (Auerbach’s Plexus): Located between the layers of the muscularis externa.

Microanatomy of the Large Intestine

  • Segments:
    • Caecum
    • Colon
    • Rectum
  • Functions:
    • Absorption of water (H2OH_2O).
    • Secretion of mucus.
    • Fermentation and absorption of volatile fatty acids (VFAVFA) in hindgut fermenters.
  • Histological Features:
    • No Villi: The surface is relatively smooth.
    • Intestinal Crypts: Present and relatively long compared to the small intestine.
    • Epithelium: Simple columnar epithelium, dominated by goblet cells.
  • Taeniae Coli: Longitudinal flat bands of smooth muscle and elastic fibers found in pigs and horses.
  • Haustra: Sacculations formed by the contraction of the taeniae coli.

Rectum and Anus

  • Rectum:
    • Lined by simple columnar epithelium.
    • Function: Stores feces and secretes mucus.
  • Anus:
    • Lined by stratified squamous epithelium.
    • Rectoanal Junction: Also known as the mucocutaneous junction, where simple columnar mucosa transitions to stratified squamous mucosa.
  • Glandular Regions of the Anus:
    • Anal Glands: Tubuloacinar modified apocrine sweat glands emptying into the anal lumen (found in carnivores and pigs).
    • Anal Sac Glands: Located in carnivores; glands emptying into the paired anal sacs lateral and below the anus. Lined by keratinized stratified squamous epithelium. These discharge foul-smelling secretions for territorial scent marking.
      • Dogs: Glands are apocrine tubular sweat glands.
      • Cats: Glands are both sebaceous and tubular apocrine sweat glands.
    • Circumanal Glands (Perianal/Hepatoid Glands):
      • Present in the subcutis around the anus in dogs.
      • Features: Nonpatent ducts (unknown function). Proper cells resemble hepatocytes, hence the name "hepatoid."
      • Clinically significant: They rank third in frequency as a site of canine tumors.
      • Morphology: Upper portion is sebaceous; lower portion is nonsebaceous.

Questions & Discussion

  • Q: In the stomach, where are the glands located?
    • A: Mucosa (not the submucosa).
  • Q: Where are Brunner’s glands located?
    • A: Submucosa.
  • Distinction Summary:
    • Small Intestine: Has both villi and intestinal crypts.
    • Large Intestine: Has no villi; only intestinal glands (crypts) are present.
  • Slide Identification Practice:
    • Identified structures in the visuals include: Duodenum (with Brunner’s glands), Ileum (with Peyer's patches), Esophagus (stratified squamous), and Colon (no villi, many goblet cells).