DEVELOPMENT OF STOMACH AND GREATER OMENTUM
The foregut begins as a simple, midline, tubular structure lined by epithelium derived from endoderm. While the endoderm creates the lining of the stomach, the visceral mesoderm that surrounds it will form the muscles, connective tissues, and mesenteries that are associated with the organ. The portion of the foregut that will become the stomach first starts to expand in the sagittal plane, ballooning outward on its anterior and posterior surfaces. However, the expansion of the posterior surface quickly outpaces the other side and the stomach begins to bend. The enlarged expansion of the posterior side will become the stomach’s greater curvature, and the anterior side will become the lesser curvature. As this is happening, the presumptive stomach rotates so that the posterior side shifts toward the left of the body and the right side shifts to the right. The rotation and expansion of the posterior side are what give the stomach its characteristic shape, with the esophagus entering just to the right of the fundus and greater curvature, whereas the outlet of the stomach, the pyloric region, shifts to the right and slightly superior to the greater curvature. This moves the stomach from a superior/inferior axis to more of a right/left axis within the abdomen. The inner, circular layer of muscle at the terminus of the stomach enlarges significantly to form the pyloric sphincter.
The rotation and expansion of the stomach do not occur in isolation. The foregut is attached to the posterior body wall by a dorsal (posterior) mesentery, in which the spleen and part of the pancreas will develop. The section of this mesentery between the developing spleen and the stomach will become the greater omentum. Anteriorly the stomach is connected to the liver, and thereafter, to the anterior body wall by a ventral (anterior) mesentery. The section of the ventral mesentery that attaches the liver to the anterior body wall will become the falciform ligament, and the section between the liver, stomach, and duodenum will form the lesser omentum. As the stomach’s posterior surface expands and rotates to the left, the attached mesentery follows, so that the spleen lies along the left side of the abdominal cavity. The dorsal mesentery between the stomach and spleen expands, folding onto itself and creating a large pocket between the two folds. and expansion of the greater curvature bring this double-layered “apron” to extend inferiorly from the stomach, falling anterior to the transverse colon and small intestines. The space between the two folds is termed the inferior recess of the omental bursa; however, this space typically disappears as development proceeds and the two folds adhere and create a single greater omentum. As the liver grows, the stomach shifts to the left side of the abdomen and the liver shifts to the right side. This brings the omental bursa to lie anterior to the pancreas, inferior to the inferior surface of the liver, and posterior to the stomach and lesser omentum, which can be subdivided into the hepatogastric and hepatoduodenal ligaments. Occasionally, the omental bursa can extend superiorly and posteriorly to the liver as the superior recess of the omental bursa. In its mature form, the omental bursa is isolated from the rest of the abdominal cavity except for a small opening called the omental foramen, located immediately posterior to the right edge of the hepatoduodenal ligament.