intro to digestive system

Small Intestine

• Core of the villus is formed by connective tissue, blood vessels, and lymph vessels of the lamina propria.
• Villi are covered by simple columnar epithelium (enterocytes) with some goblet cells in between.
• Small intestine lamina propria contains connective tissue, blood vessels, lymph vessels, and intestinal glands (crypts of Lieberkuhn) which open into the lumen between the villi.
• Crypts are mainly lined by simple columnar cells with microvilli (enterocytes) which are absorptive in function.

Small Intestine Lamina Propria

• Contains connective tissue, blood vessels, and lymph vessels, and intestinal glands (crypts of Lieberkuhn) which open into the lumen between the villi.
• Crypts are mainly lined by simple columnar cells with microvilli (enterocytes) which are absorptive in function.
• Other cells lining the crypts include:
  • Goblet cells (produce mucin)
  • Paneth cells (defensive in function)
  • Neuroendocrine cells (produce locally acting hormones that regulate motility and secretion of GIT)
  • Stem cells (replace other cells)
• Small intestine lamina propria of ileum contains solitary lymphoid follicles called Peyer’s patches.

Muscularis Mucosa

• Made up of inner circular and outer longitudinal layer of smooth muscle fibers.
• Some of these fibers extend into the core of the villus.

Submucosa

• Contains connective tissue, blood vessels, lymph vessels, and nerve fibers.
• Characterized by the presence of mucous glands (Brunner’s glands) in the duodenum.
• Contains Peyer’s patches (aggregated lymphoid follicles) in the ileum.

Muscularis Externa

• Has outer longitudinal and inner circular layer of smooth muscle fibers.

Esophagus

Histology of Esophagus

• Wall of the esophagus presents 4 layers:
  • Mucosa (mucous membrane)
  • Submucosa
  • Muscularis externa
  • Adventitia
• Mucous membrane (mucosa) is the innermost layer and contains 3 parts:
  • Epithelium
  • Lamina propria
  • Muscularis mucosa
• Epithelium is of stratified squamous non-keratinized type, except in the lowest part, which is lined by simple columnar type epithelium.

Lamina Propria

• Made up of loose irregular connective tissue with blood vessels.
• Muscularis mucosa is a thin layer made up of muscle fibers, arranged in 2 layers:
  • Inner layer of circularly arranged muscle fibers
  • Outer layer of longitudinally arranged muscle fibers

Submucosa

• Contains dense irregular connective tissue, blood vessels, and mucous glands (esophageal glands).
• Larger ducts of the glands extend to the lumen and are lined by stratified columnar epithelium.
• Glands secrete mucus, which aids in the passage of food.

Muscularis Externa

• Made up of thick layer of muscle fibers, arranged in 2 layers:
  • Inner layer of circularly arranged muscle fibers
  • Outer layer of longitudinally arranged muscle fibers
• Muscularis externa in the upper part of the esophagus is made of skeletal muscle fibers.
• In the lower part, it is made of smooth muscle fibers.
• In the middle part, it is made of both skeletal and smooth muscle fibers.

Adventitia

• Is the outermost layer.
• Made up of loosely arranged connective tissue with blood vessels.
• Adventitia is replaced by serosa in the abdominal part of the esophagus.

Stomach

Histology of Stomach

• Stomach is the most dilated part of the GI tract.
• Stomach has 4 parts – cardia, fundus, body, pyloric part.
• Interior of the empty stomach presents longitudinal folds of mucosa – gastric rugae.
• HISTOLOGY OF CARDIA OF STOMACH:
  • Has 4 main layers: the inner mucosa, submucosa, muscularis externa, and outer serosa.
  • Mucosa made up of 3 layers (epithelium, lamina propria and muscularis mucosa) of the cardia has a simple columnar epithelium with many invaginations that form millions of gastric pits.
  • These layers are similar throughout the length of the digestive tract but there is regional modification and specialization.

Colon

Histology of Colon

• Epithelium (simple columnar with goblet cells)
• Intestinal glands in lamina propria
• Muscularis mucosa
• Submucosa
• Muscularis externa
• Serosa

Appendix

Histology of Appendix

• Mucosa:
  • Epithelium: Simple columnar epithelium with lots of goblet cells.
  • Lamina propria: Contains aggregation of lymphocytes, which extend into the submucosa breaking muscularis mucosa.
  • Also contains few intestinal glands
• Muscularis mucosa: Inner circular and outer longitudinal layer of smooth muscle fibers.
• Submucosa: It is thin with connective tissue, blood vessels, lymph vessels, and nerve fibers.
• Muscularis externa: Has outer longitudinal and inner circular layer of smooth muscles.

Test your knowledge of the layers of the gastrointestinal tract, including the epithelium, lamina propria, muscularis mucosa, and more. Learn about the structure and function of each layer and how they work together to facilitate digestion. This quiz covers the regional modifications and specializations of the GIT layers.