Chapter 85. The Midgut: Fixation, Cecum and Appendix - Review of Medical Embryology Book

The proximal portion of the colon lengthens, giving rise to the hepatic flexure and ascending colon as the cecum descends from the upper to the lower right side of the abdomen into the right iliac fossa
As the intestines settle into their final positions, their mesenteries press against the back wall of the abdominal cavity

The mesentery of the ascending colon fuses with parietal peritoneum and disappears; thus, the ascending colon becomes retroperitoneal
The duodenum (except for its foregut portion) also becomes retroperitoneal
The rest of the midgut loop derivatives (jejunum and ileum) keep their mesenteries, which at first are attached to the midline of the posterior abdominal wall, but during midgut rotation they twist around the origin of the superior mesenteric artery. When the mesentery of the ascending colon disappears, the intestinal mesentery gets a new line of attachment from the duodenojejunal junction down to the ileocecal junction

DUODENAL FIXATION: with rotation of the duodenum and stomach, the duodenum and pancreas fall to the right and meet the dorsal abdominal wall; the adjacent layers of peritoneum fuse and disappear; and most of the duodenum and head of the pancreas become retroperitoneal
THE CECUM AND APPENDIX

The cecal diverticulum appears in week 6 and is the primordium of the cecum and the vermiform appendix

The diverticulum is seen as a conical pouch on the antimesenteric border of the caudal limb of the midgut loop just beyond the apex of the loop
The distal end of the blind sac does not grow as fast, thus the appendix, which is a vestige of the incomplete development of the cecum, develops
As the proximal portion of the colon elongates, the cecum and the appendix descend on the right side of the abdomen. The position of the appendix can be variable

The appendix grows in length, so that at birth, it is long and worm-shaped, or vermiform
After birth, the cecal wall grows unequally and the appendix comes to lie on its medial side