DEVELOPMENTAL ANOMALIES OF THE SMALL INTESTINE
The duodenum derives from the distal portion of the foregut. During the fourth to eighth weeks of gestation, rapid growth of the epithelial lining of the foregut results in obliteration of the lumen. This is the solid core stage. Recanalization occurs by the process of vacuolization, beginning during the 8th to 10th weeks. Failure of vacuoliza tion can lead to congenital obstruction, in the form of either atresia or stenosis. Congenital atresias in the jejunum, ileum, and colon do not apparently occur by this mechanism, but rather result from intrauterine bowel ischemic events.
Because the pancreas and the extrahepatic biliary ductal system develop at the same time as the duodenum, combined anomalies sometimes occur. The dorsal portion of the embryonic pancreas arises from the wall of the duodenum and the ventral portion arises between the duodenum and the hepatic bud. During the process of bowel rotation, the ventral pancreas migrates along the right side of the duodenum to merge with the dorsal pancreatic bud. Incomplete rotation of the ventral pancreas can partially or completely surround the duodenum; this annular pancreas anomaly is often associated with atresia or stenosis of the duodenum. Other biliary and pancreatic anomalies that occasionally occur in association with duodenal atresia and stenosis include biliary atresia, choledochal cyst, pancreatic lipomatosis, dual pancreatic duct, and anomalous bile duct insertion.
Normal fetal development of the gut includes rotation of the intra-abdominal components. Early in embryogenesis, the primitive gut develops in an extracoelomic location. As the gut returns to the coelomic cavity, the bowel undergoes rotation. Before 6 weeks gestational age, the duodenum and cecum rotate 90° counterclockwise, as viewed from the front. Between 6 and 8 weeks gestation, the midgut (i.e., the portion of bowel extending from the duodenal–jejunal junction to the midportion of the transverse colon) develops in an extracoelomic location. During this period, the duodenum undergoes an additional 90° counterclockwise rotation, and the cecum remains relatively stationary. At approximately 10 weeks of gestation, the midgut begins to return to the coelomic cavity, and the duodenum rotates 90° and the cecum rotates 180°. During this rotation, the cecocolic loop passes anterior and superior to the origin of the superior mesenteric artery. The cecocolic loop then continues toward the right and descends into the right lower quadrant where it becomes fixed to the retroperitoneum. As rotation is completed, the mesentery fuses from the ligament of Treitz to the right lower quadrant. The various forms of malrotation result from arrested or abnormal progression in the process of fetal intestinal rotation.
An omphalocele results from failure of appropriate return of the embryonic midgut to the coelomic cavity from the umbilical stalk. Failure of normal bowel rotation also occurs in these patients. Persistence of the bowel within the umbilical stalk results in a markedly enlarged umbilical cord that contains a variable segment of intestine. ...