Foregut

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The foregut is the anterior part of the alimentary canal, from the mouth to the intestine, or to the entrance of the bile duct (1).

Development

During development, the caudal foregut gives rise to the Stomach, the proximal half of the duodenum, the liver, gall bladder, bile duct, pancreas (all members of the [[gastrointestinal system[[) and the spleen. Also during development, the fate of the mesenteries is decided.

Stomach

  • appears as a spindle-shaped dilation of the foregut, just distal to the esophagus
  • Growth is faster along its dorsal surface, causing a pronounced curve along both the dorsal surface (greater curvature) and ventral surface (lesser curvature)
  • Rotates around its longitudinal axis, so that the original left side becomes the definitive ventral (anterior) surface and the right side becomes the dorsal (posterior) surface
  • There is also rotation around the antero-posterior axis, so that the proximal end of the stomach (cardia) moves to the left and the distal end (pylorus) to the right

Duodenum

  • the first half is derived from the foregut (hence supplied by the coeliac artery) and the second half from the midgut (hence supplied by the superior mesenteric artery)
  • Rotation of the developing stomach swings the pyloric end to the right, carrying the first part of the duodenum with it
  • Consequently, the duodenum becomes a C-shaped loop

Liver, Gall Bladder and Bile Duct

  • Liver, Gall bladder, and bile duct all develop from an outgrowth of the most caudal part of the foregut
  • The outgrowth (hepatic diverticulum/liver bud) grows into the mesoderm of the ventral mesentery
  • The endoderm forms the lining of the bile duct system, as well as the epithelial parenchymal cells of the liver
  • The mesoderm of the ventral mesentery and septum transversum will form the other components of the liver
  • The gall bladder and cystic duct arise as outgrowths from the main hepatic diverticulum, as does part of the pancreas

Pancreas

  • The pancreas arises as a ventral pancreatic bud from the hepatic diverticulum, and a dorsal pancreatic bud which grows into the dorsal mesentery from the foregut
  • As the stomach and duodenum rotate, the dorsal pancreatic bud moves to the left and the ventral bud to the right
  • The ventral bud, together with the bile duct, continues to migrate, passing posterior to the duodenum
  • Eventually, the two pancreatic buds meet and fuse
    • The dorsal bud forms the body and tail of the pancreas
    • The ventral bud forms the head and the uncinate process
  • The ducts of the two buds also join, and it is the terminal part of the ventral duct that usually forms the main duct, thus retaining its connection with the bile duct

Spleen

  • Not part of the gastrointestinal system
  • Develops from mesoderm in the part of the dorsal mesentery that suspends the stomach and receives its blood supply from the coeliac artery

Fate of the Mesenteries of the Foregut

Dorsal Mesentery

  • Rotation of the stomach around its longitudinal axis causes the dorsal mesentery attached to it (dorsal mesogastrium) to be stretched and pulled to the left
  • As a result, a recess, the lesser sac (omental bursa) is formed posterior to the stomach
  • The part of the dorsal mesogastrium containing the spleen partially fuses with the parietal peritoneum anterior to the developing left kidney
  • The spleen comes to lies on the left side of the body and its mesentery is known as the lienorenal ligament (lien = spleen)
  • The part of the dorsal mesogastrium between the spleen and stomach is the gastrosplenic ligament
  • Growth of the dorsal mesogastrium in an inferior direction results in an extensive fold of mesentery (greater omentum) hanging down from the greater curvature of the stomach, and containing an extension (inferior recess) of the lesser sac
  • Fusion of the two layers of the greater omentum later obliterates most of the inferior recess.

Ventral Mesentery

  • As the liver grows into the ventral mesentery, the latter is converted into a thin envelope of visceral peritoneum
  • When the growing liver reaches the septum tranversum (part of the diaphragm), the visceral peritoneum is reflected directly from the liver (bare area) on to the diaphragm, as parietal peritoneum
  • A persisting, inferior part of the ventral mesentery between the liver and the anterior body wall becomes the falciform ligament, and the umbilical vein runs in its lower free border
  • The part of the ventral mesentery extending between the liver and stomach is the lesser omentum (gastrohepatic ligament)
  • The lesser omentum also has a free border (lower border of ventral mesentery) which contains the bile duct, hepatic artery and portal vein
  • It is here that the lesser sac communicates with the rest of the peritoneal cavity (greater sac), through a narrow opening, the epiploic foramen.

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