Embryology of the Human Digestive System

Embryology of the Human Digestive System

• The digestive system develops from germ tissues during gastrulation.
• By week 2, the endoderm, mesoderm, and ectoderm are developed.
• The primitive gut tube is formed by week 3-4 by incorporating the yolk sac.
• Foregut: develops into the esophagus, stomach, liver, gallbladder, pancreas, and proximal duodenum.
• Midgut: develops into the distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal 2/3 of the transverse colon.
• Hindgut: develops into the distal 1/3 of the transverse colon, descending colon, sigmoid colon, and upper anal canal.

Molecular Regulation of Gut Tube Development

• Retinoic acid (RA) gradients initiate regional specification of the gut tube.
• Higher RA concentrations in the colon influence gene expression compared to other areas.
• SOX2 expression determines esophagus and stomach development.
• PDX1 expression determines duodenum development.
• CDX2 expression determines small intestine development, and
• CDX1 expression determines large intestine and rectum development.
• Sonic Hedgehog (SHH) expression initiates epithelial-mesenchymal interactions for initial patterning.

Mesentery Development

• Mesentery is a fold of peritoneum that suspends digestive organs, blood vessels, and nerves from the posterior abdominal wall.
• It provides support and allows mobility to the organs.
• The foregut, midgut, and hindgut are initially attached to the posterior mesenchymal wall until week 5.
• The connecting tissue then narrows, attaching the gut tube to the abdominal wall via the dorsal mesentery.

Organogenesis

• The Foregut: Develops into the esophagus, stomach, liver, gallbladder, pancreas, and the caudal portion of the duodenum. (Week 4.)
• Esophagus: The respiratory diverticulum partitions the foregut into ventral (respiratory primordium) and dorsal (esophagus) portions.
• Stomach: Formation of fusiform dilation is seen in the foregut with a noticeable change in positional and growth differences in an anterior and posterior aspect.

Liver Development

• The liver forms from endoderm within the foregut.
• Surrounding tissues (ectoderm, mesoderm, and notochord) release inhibitors to locally establish the expression of liver-specific genes.
• Fibroblast Growth Factor 2 (FGF2) secreted by cardiac mesoderm and blood vessels initiates liver gene expression.
• Bone Morphogenetic Proteins (BMPs) produced by the septum transversum prepare the endoderm to respond to FGF2.

Pancreas Development

• The pancreas develops from two separate endodermal buds: dorsal and ventral.
• The dorsal bud forms first within the dorsal mesentery.
• The ventral bud is close to the bile duct and shifts during gut rotation to merge with the dorsal bud.
• The main pancreatic duct of Wirsung forms from the fusion of both buds.
• Proximal parts of the dorsal bud may persist as accessory pancreatic duct of Santorini.

Midgut Development

• The midgut extends from the bile duct to the junction of the transverse and descending colon.
• It forms a significant portion of the small intestine and proximal large intestine.
• During development, the superior mesenteric artery is the axis of a 90-degree counter-clockwise rotation of the midgut.

Hindgut Development

• The hindgut extends and forms the distal transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal.
• It plays a role in the internal lining formation of the bladder and urethra.
• The urorectal septum divides the hindgut and the urogenital sinus.

Cloacal Development

• The cloaca is a terminal portion of the hindgut.
• The cloaca is divided into:
  • Anorectal canal (rectum and upper anal canal),
  • Urogenital sinus(bladder and urethra).
• The development of the cloaca includes the rupturing of the cloacal membrane between the ectoderm and endoderm and the creation of separate openings for the anal canal and the urogenital sinus with the help of the urorectal septum.

This quiz covers the embryological development of the human digestive system, focusing on the formation and differentiation of the gut tube from germ tissues during gastrulation. It includes insights into the roles of specific molecular regulators like retinoic acid and gene expressions in developing various parts of the digestive system.