Digestive System Lecture Notes

Overview

• Discussion on the histology of the esophagus, stomach, small intestine, and large intestine.
• Key features to distinguish between these parts.

Esophagus

• Histology: Stratified squamous epithelium visible at low magnification due to a darker border at the bottom.
• Layers:
  • Mucosa with three sublayers: stratified squamous epithelium, lamina propria, muscularis mucosa.
  • Submucosa.
  • Muscularis externa with inner circular and outer longitudinal layers.
  • Adventitia as the outer layer (no serosa).
• Muscle Types:
  • Superior part: skeletal muscle.
  • Middle part: mix of skeletal and smooth muscle.
  • Inferior part: smooth muscle.

Stomach

• Histology: Ridges at the top but can appear as "cheerios" in cross-section.
• Epithelium: Simple columnar.
• Layers:
  • Mucosa with gastric pits (surface epithelium) and glands (hormone, pepsinogen release).
  • Muscularis mucosa marks the bottom of the mucosa.
  • Submucosa.
  • Muscularis externa with an innermost oblique layer, middle circular, and outer longitudinal layer.
  • Serosa as the outer layer (due to high mobility within the peritoneum).

Small Intestine

• Histology: Characterized by finger-like villi.
  • Villi lined by simple columnar epithelium and contain lamina propria.
  • Microvilli on simple columnar cells to increase surface area.
• Structures:
  • Intestinal crypts/glands at the base of villi.
  • Goblet cells (secrete mucus) visible.
• Layers:
  • Mucosa with prominent villi.
  • Submucosa.
  • Muscularis externa (longitudinal and circular layers).
  • Serosa (except duodenum which has adventitia due to its fixed position).
• Sections:
  • Duodenum: Retroperitoneal with adventitia and Brunner's glands in the submucosa.
  • Jejunum and ileum: Have serosa, ileum distinguished by the presence of Peyer's patches (though hard to identify).

Large Intestine

• Histology: Lacks villi but may have microvilli.
  • Goblet cells are more numerous.
  • Mucosa appears more organized and is proportionally smaller than in the stomach.
• Layers:
  • Mucosa, muscularis mucosa at the base.
  • Submucosa.
  • Muscularis externa.
• Sections:
  • Ascending and descending colon: Retroperitoneal with adventitia.
  • Transverse colon: Has serosa.
  • Sigmoid colon: Verification needed, likely has adventitia.

Practical Identification Tips

• Esophagus: Look for stratified squamous epithelium with a dark border, large muscularis mucosa, and adventitia.
• Stomach: Identify by knuckle-like ridges or "cheerios", prominent mucosa.
• Small Intestine: Look for finger-like villi, microvilli, goblet cells, and submucosal glands in the duodenum.
• Large Intestine: Characterized by a high number of goblet cells, absence of villi, and organized mucosa.

Conclusion

• Understanding these distinguishing features and layers is essential for identifying different parts of the digestive tract histologically.