Embryo

Development of the Digestive System

• The digestive system develops from the endoderm-lined yolk sac cavity, which is incorporated into the embryo as a result of cephalocaudal and lateral folding.
• The primitive gut forms a blind-ending tube, consisting of the foregut, midgut, and hindgut.

Foregut

• The foregut forms the pharyngeal gut or pharynx, extending from the oropharyngeal membrane to the respiratory diverticulum.
• The remainder of the foregut lies caudal to the pharyngeal tube and extends as far caudally as the liver outgrowth.
• The pharyngeal gut is particularly important for the development of the head and neck.

Midgut

• The midgut begins caudal to the liver bud and extends to the junction of the right two-thirds and left third of the transverse colon in the adult.
• The midgut is suspended from the dorsal abdominal wall by a short mesentery and communicates with the yolk sac by way of the vitelline duct or yolk stalk.
• During development, the midgut undergoes rapid elongation, resulting in the formation of the primary intestinal loop.

Hindgut

• The hindgut gives rise to the distal third of the transverse colon, the descending colon, the sigmoid, the rectum, and the upper part of the anal canal.
• The endoderm of the hindgut also forms the internal lining of the bladder and urethra.

Development of the Diaphragm

• The diaphragm develops ventral to the foregut and the pleuroperitoneal membrane, growing dorsally from the ventrolateral body wall.
• During the sixth week, the septum transversum, pleuroperitoneal membranes, and mesentery of the esophagus fuse to form a complete partition between the thoracic cavity and the abdominal cavity.

Development of the Pancreas

• The pancreas develops from the dorsal mesoduodenum, initially growing into the dorsal mesoduodenum.
• The tail of the pancreas extends into the dorsal mesogastrium, which fuses with the dorsal body wall, resulting in a retroperitoneal position.
• The pancreas becomes fixed in a retroperitoneal position, with the main pancreatic duct forming from the distal part of the dorsal pancreatic duct and the entire ventral pancreatic duct.

Development of the Omentum and Ligaments

• The greater omentum forms from the dorsal mesogastrium, which bulges down and grows over the transverse colon and small intestinal loops, eventually fusing to form a single sheet hanging from the greater curvature of the stomach.
• The lesser omentum and falciform ligament form from the ventral mesogastrium, which itself is derived from mesoderm of the septum transversum.

Abnormalities

• Pyloric stenosis occurs when the circular and longitudinal musculature of the stomach in the region of the pylorus hypertrophies.
• Reversed rotation of the intestinal loop occurs when the primary loop rotates 90° clockwise, resulting in the transverse colon passing behind the duodenum and lying behind the superior mesenteric artery.