50. Embryology - Development of the GI System

Development of the Digestive System: Dorsal and Ventral Mesentery

• The alimentary tract, from stomach to rectum, is suspended from the body wall within the peritoneal cavity.

• Dorsal Mesentery:

  • Dorsal mesogastrium: Suspends the stomach from the dorsal body wall. Contains the celiac artery and will give rise to the greater omentum, gastrosplenic, and splenorenal ligaments. Contains the developing spleen.
  • Dorsal mesoduodenum: Suspends the duodenum from the dorsal body wall. Contains the developing dorsal bud of the pancreas.
  • Mesentery proper: Contains the superior mesenteric artery and suspends the jejunum, ileum, cecum, ascending colon, and 2/3 of the transverse colon from the body wall.
  • Dorsal mesocolon: Suspends the distal 1/3 of the transverse colon, sigmoid colon, and rectum from the dorsal body wall. Contains the inferior mesenteric artery.

• Ventral Mesentery:

  • A double layered mesothelium derived from the septum transversum of the developing thoracic diaphragm. Shorter in length than the dorsal mesentery, and suspends the terminal esophagus, stomach, and proximal half of the duodenum.

Development of the Liver

• The liver develops within the septum transversum.
  • The superior surface of the liver (diaphragmatic surface) and the inferior surface of the diaphragm are not covered by peritoneum.
  • Mesoderm from the septum transversum forms the visceral peritoneal covering of the anterior, posterior, and inferior surfaces of the liver.
  • The anterior and posterior coronary ligaments suspend the liver from the inferior surface of the developing thoracic diaphragm.

Development of the Pancreas

• The pancreas develops from two endodermal outpockets: the ventral and dorsal pancreatic buds, arising from the mid-duodenum.

  • Ventral pancreatic bud: Associated with the proximal region of the hepatic diverticulum, develops within the ventral mesoduodenum.
    • Gives rise to the main pancreatic duct, uncinate process, and dorsal half of the head of the pancreas.
  • Dorsal pancreatic bud: Associated with the posterior longitudinal axis of the duodenum, develops within the dorsal mesoduodenum.
    • Gives rise to the ventral half of the head, neck, body, and tail of the pancreas, as well as the accessory pancreatic duct.
  • The visceral peritoneum covering the pancreas and duodenum fuses and is later reabsorbed, making the majority of the pancreas and duodenum retroperitoneal.

• Exocrine and Endocrine function: During the third month:

  • Pancreatic acini (glandular secretory cells) release enzymes into the duodenum that facilitate the breakdown of fats/lipids.
  • Islet cells differentiate into alpha and beta cells:
    • Alpha cells release glucagon, stimulating glycogen breakdown in hepatocytes and glucose release into the circulatory system.
    • Beta cells release insulin into the circulatory system, promoting glucose uptake by hepatocytes, and synthesis and storage of glycogen.

Development and Positioning of the Spleen

• The spleen is a vascular lymphatic organ not related to the digestive system.
• During the fifth week, the spleen develops from mesenchymal cells located between the layers of the dorsal mesogastrium. It receives its blood supply from a branch of the celiac artery.
• The spleen, along with the dorsal mesogastrium, are displaced to the left side of the body during the rotation and growth of the greater curvature the stomach.
• The spleen remains an intraperitoneal organ within the abdominal cavity.
  • The portion of the dorsal mesogastrium over the left kidney becomes the splenorenal ligament.
  • The portion of the dorsal mesogastrium between the spleen and stomach becomes the gastrosplenic ligament.

Development of the Midgut

• The midgut gives rise the distal half of the duodenum, jejunum, ileum, cecum, appendix, ascending colon, and 2/3 of the proximal transverse colon.

• All midgut structures receive blood supply from branches of the superior mesenteric artery, and are innervated by the vagal plexus (parasympathetic) and thoracic splanchnic nerves (sympathetic).

• Primary intestinal loop:

  • Formed by the rapid growth of the midgut between the 4th and 5th week, centered about the superior mesenteric artery, the dorsal mesentery, and the vitelline duct.
  • The primary intestinal loop rotates 180 degrees counterclockwise between the 5th and 8th week.
    • This rotation is driven by the rapid growth of the midgut portion between the distal duodenum and the vitelline duct, causing the loop to rotate about its axis.
    • During this rotation, the cecal bud and transverse colon pass ventral to the developing duodenum.
  • The rotation continues for an additional 90 degrees, driven by the growth of the jejunum, ileum, and distal gut tube that will become the large intestine.
    • This rotation is complete by the 10th week.

• Vitelline duct:

  • Connects the midgut to the yolk sac during early development.
  • The yolk sac is incorporated into a serous mesothelial lined sac within the amnion-covering of the umbilical cord.
  • This sac (extraembryonic cavity) remains continuous with the peritoneal cavity and accommodates the rapid growth of the small intestines and cecum between weeks 6-10.
    • This is known as physiological herniation.

• Retraction of the herniated intestinal loop:

  • Begins around the 10th week.
  • The proximal jejunum is the first part to reenter the abdominal cavity, with its coiled loops layering sequentially from the left side inferior to the stomach and spleen, progressing to the lower right side.
  • The cecal bud (proximal large intestine) is the last part of the herniated intestine to reenter the abdominal cavity.