Development of the Midgut
The midgut forms:
- distal duodenum
- jejunoileum
- caecum & appendix
- ascending colon
- proximal 2/3 of the transverse colon
All of these organs will receive blood supply from branches of the superior mesenteric artery
Physiologic Herniation of the Gut Tube
In the 6th week of development, the abdominal cavity is too small to accommodate the liver and ever expanding gut tube.
The midgut herniates into to umbilical cord.
The gut loop has cranial and caudal limbs that are attached at their midpoint to the yolk sac.
The superior mesenteric artery forms an axis around which this loop of bowel will ultimately rotate about 270o degrees before being retracted back into the abdominal cavity beginning in week 10 and completed by week 12.
In an embryo at about 8 weeks of development, the gut has herniated out of the abdominal cavity. Initially it rotates approximately 90o, completing an additional 180 o on its return to the abdomen in weeks 10 through 12.
This rotation of the gut pushes the duodenum and pancreas against the posterior body wall, causing them to become retroperitoneal.
Intraperitoneal vs Retroperitoneal
Following the return of the gut tube to the abdominal cavity, the colon elongates, resulting in a downward displacement of the cecum to the right iliac fossa. As a result of rotation of the midgut tube, the duodenum and pancreas lose their mesentery and become fixed to the posterior body wall. In addition, the ascending colon also becomes fixed to the posterior body wall. Organs that lose their mesentery and become fixed to the posterior body wall are said to be retroperitoneal. Organs that retain their mesentery and are freely movable within the abdominal cavity are said to be intraperitoneal.
Cecum, Appendix and Ascending Colon
By 12 weeks, the colon has elongated and the cecum and appendix are displaced downward to their normal location in the right iliac fossa. In the majority of cases, the appendix will be located in a retrocecal position (behind the cecum).
The cecum and appendix remain intraperitoneal, but the ascending colon loses its mesentery to become retroperitoneal.
During the 10th week of development, the rotated gut loop begins to return to the abdominal cavity. This is thought to be associated with an increase in the capacity of the developing abdominal cavity to accommodate the highly convoluted gut. When the gut returns to the abdominal cavity, the cecum and appendix are located just under the liver (subhepatic location).
The video on the right is a demonstration of a model that was made in the 1940's by a prominent anatomist of the time, and a faculty member at Hahnemann Medical College and Hospital, Dr. George Paff.
Dr. Paff's model on rotation of the GI tract is a useful tool for demonstrating this process and understanding the final displacement of the viscera in the abdominal cavity.