The mesenteries fixate the intestinal tube to the posterior wall of the abdominal cavity. The blood vessels and nerves utilize the mesenteries to reach the viscera. Since portions of the mesenteries partially adhere again to the posterior abdominal wall, the vessels and nerves that course through them also lie secondarily retroperitoneally.
In the region of the stomach and small intestine up to the liver bud exists a ventral fixation, the ventral mesogastrium all the way to the liver. Therein courses later the portal vein, the hepatic artery and the bile duct. A dorsal "meso" fixates the viscera here to the dorsal abdominal wall.
AIn stage 13, ca. 32 days, to the right of the intestinal tube in the region between the esophagus and lungs a niche arises, the recessus pneumato-entericus, that extends down into the peritoneal cavity. With the spreading of the diaphragm it becomes subdivided into a supra-diaphragmatic cavity, the subcardiac bursa, and an infra-diaphragmatic cavity, the omental bursa. On the left a similar niche arises that, however, soon atrophies or is suppressed by the rotation of the stomach.
With the rotation of the stomach the omental bursa spreads further out and delimits a space behind the stomach with a small opening into the peritoneal cavity, the epiploic foramen. In the front it is bounded by the ventral mesogastrium or omentum minus.
With the further development the dorsal mesogastrium extends over the large curvature of the stomach in the caudal direction and forms a mesenterial fold: the greater omentum, which in effect is nothing more than an extended omental bursa. With time the two layers of the greater omentum , adhere together and further fuse with the transverse mesocolon.
In the region of the small intestine the mesenterial relationships are somewhat more complicated due to the intestinal rotation. The mesenterium of the entire small intestine appears to originate at a point that lies above the inferior part of the duodenum. At the level of the large intestine, the mesenterium of the ascending and descending colon becomes positioned on the posterior abdominal wall, adheres to it, and these two parts become secondary retroperitoneal in the fetal period.
The root of the mesentery rises and runs transversely over the posterior abdominal wall from the cecum to the duodeno-jejunal bend and corresponds to the attachment of the part of the mesentery of the ascending colon that is fused with the posterior abdominal wall. Only in the region of the sigmoid flexure does the adhesion not occur and this part remains intraperitoneal with a mesosigmoid.
The rectum is located dorsal and caudal to the peritoneal cavity and thus retroperitoneal.