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Embryology of GIT Part 1 BDS II 2024 (1)

Embryology of the GIT

Overview

  • Focus: Development of the oesophagus, stomach, duodenum, liver, biliary tract, and pancreas.

  • Main divisions of the GIT: Foregut, Midgut, Hindgut.

1. Subdivisions of GIT

  • Foregut:

    • Oral cavity, tongue, tonsils, salivary glands, pharynx, oesophagus, stomach, duodenum (including entrance of common bile duct).

    • Blood supply: Coeliac artery.

  • Midgut:

    • Duodenum, jejunum, ileum, cecum, appendix, ascending colon, transverse colon (proximal 2/3).

    • Blood supply: Superior mesenteric artery.

  • Hindgut:

    • Transverse colon (distal 1/3), descending colon, sigmoid colon, rectum, anal canal (above pectinate line).

    • Blood supply: Inferior mesenteric artery.

2. Developmental Milestones

  • Established by the 4th week of embryonic development.

  • Stomach develops from the stalk of the yolk sac/vitelline duct.

3. Germ Layer Contributions

  • Endoderm:

    • Forms epithelium and mucosal/submucosal glands.

  • Mesoderm:

    • Forms lamina propria, muscularis mucosae, blood vessels, muscularis externa, adventitia/serosa.

  • Ectoderm:

    • Neural crest contributes neurons and nerves (Meissner’s and myenteric plexuses).

4. Foregut Development

  • Extends from mouth to the duodenum (opening of sphincter of Oddi).

  • Includes lower respiratory tract, liver, biliary tract, pancreas as endodermal outgrowths.

5. Derivatives of Foregut

  • Forms tracheoesophageal folds which fuse into tracheoesophageal septum, leading to the formation of trachea and lungs ventrally, and the oesophagus dorsally.

6. Rotation of the Foregut

  • Abdominal foregut rotates 90° clockwise around its longitudinal axis:

    • Left side of the stomach moves to the anterior surface; right side to the posterior surface.

    • Results in the positioning of the lesser and greater curvatures of the stomach.

7. Lesser Sac/Omental Bursa Formation

  • Rotation forms a small compartment between the stomach and posterior abdominal wall, opening into the greater sac of the peritoneal cavity.

8. Structural Changes Post-Rotation

  • Liver, lesser omentum, pylorus of stomach, and duodenum shift to the right.

  • Spleen, pancreas, greater omentum shift to the left.

  • Contributions from ventral and dorsal mesenteries to form ligaments and omenta.

9. Development of Duodenum

  • Develops from the terminal part of the foregut and proximal midgut.

  • Forms a C-shaped loop, rotates right, becoming retroperitoneal.

10. Development of the Liver and Biliary Tract

  • Liver bud arises from the distal foregut; divides into parts to form the liver and gallbladder.

  • Hepatic ducts develop, and with stomach rotation, the common bile duct opens into duodenum.

11. Development of the Pancreas

  • Arises from dual pancreatic buds: larger dorsal and smaller ventral.

  • Dorsal bud grows first, followed by ventral bud near bile duct and rotates to fuse with dorsal bud.

  • Main pancreatic duct forms from the union of both bud ducts, entering the ampulla of Vater.

  • Ventral bud contributes to the uncinate process and part of the head of the pancreas.