Digestive System Embryology

As a result of cephalocaudal and lateral folding, a portion of the yolk sac lined by endoderm is incorporated into the embryo to form the primitive gut. The primitive gut is divided into three parts: foregut, midgut, and hindgut. The midgut communicates with the yolk sac through the temporary omphalomesenteric duct.

Foregut Development

It extends from the oropharyngeal membrane to the second portion of the duodenum (ampulla of Vater).

Esophagus Formation

Its primordium arises from the tracheoesophageal septum around the 4th week. The endoderm proliferates to temporarily occlude the lumen. At the end of the embryonic period, the lumen recanalizes, and the endoderm differentiates into stratified squamous epithelium. The muscle of the upper 1/3 is striated and derives from the 4th and 6th pharyngeal arches, while the distal 2/3 is smooth muscle derived from splanchnic mesoderm.

Stomach Development

It develops as a dilation whose dorsal wall grows faster than the ventral wall. It then rotates 90 degrees clockwise, bringing the dorsal side to the left and the ventral side to the right. This rotation pulls the dorsal mesentery to the left, attaching it to the posterior abdominal wall and forming the omental bursa (lesser sac). In its anterior mesentery, the liver develops, leading to the formation of the gastrohepatic ligament.

Duodenum Formation

It develops from the caudal foregut and the cephalic portion of the midgut. The bile duct enters at this level. The lumen of the duodenum is temporarily occluded by proliferating endoderm, which forms the epithelial lining and later recanalizes. The anterior and posterior mesenteries contribute to the formation of the ligament of Treitz.

Midgut Development

It extends from the entrance of the bile duct to the junction of the proximal two-thirds and distal one-third of the transverse colon. Its rapid growth results in physiological herniation outside the abdominal cavity between the 6th and 10th weeks.

The midgut loop is located in the sagittal plane, and its apex is connected to the yolk sac by the omphalomesenteric duct. The midgut loop rotates 90 degrees counterclockwise around the superior mesenteric artery. A dilation, the cecal diverticulum, appears on the caudal limb.

As the physiological hernia reduces, the intestine returns to the abdominal cavity. This reduction is accompanied by a further 180-degree counterclockwise rotation, resulting in a total rotation of 270 degrees. The cecum is then located in the right lower quadrant.

The cranial limb forms the jejunum and upper ileum, while the caudal limb forms the lower ileum, cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon.

Hindgut Development

It extends from the junction of the proximal two-thirds and distal one-third of the transverse colon to the cloacal membrane.

In the 6th week, the caudal portion (cloaca) communicates ventrally with the allantois and laterally with the Wolffian (mesonephric) ducts. The urorectal septum grows caudally, dividing the cloaca into a dorsal part (rectum and upper anal canal) and a ventral part (urogenital sinus).

The cloacal membrane is formed by the cloacal endoderm and surface ectoderm. The anal membrane ruptures at the end of the 7th week.