University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

403

Pages:

3
Author:

Mariam A. A.

Embryology of the Midgut Midgut: distal duodenum, rest of SI, appendix, caecum, ascending and proximal transverse colon. Normally the yolk sac will be incorporated into the body stalk with the umbilicus, however in the disease gastroschisis, organs will be left outside the body as the yolk sac enters the amniotic cavity instead. Beginning in week 6, gut move into extraembryonic coelom with a sagittal orientation of primitive loop [1] (abdominal organs are growing faster than abdominal cavity itself which then forces primary intestinal loops to herniate through the umbilicus into umbilical cord). This herniation continues from week 6 to the early 8th week. As this herniation occurs, the loop undergoes a 90 degree, counter-clockwise rotation about superior mesenteric artery [2]. By the 8th week the cranial limb is now located on the right side of the body, whereas the caudal limb is located on the left side of the body. During the 8th through 10th weeks of development, primary intestinal loop continue to grow and elongate. The caudal limb also develops an enlarged caecal bud which will form the caecum [3]. By week 10 of development, abdominal cavity grown sufficiently and now can reincorporate the intestines. Hence, the primary intestinal loop retracts back into abdomen during 10th-11th weeks of development [4] As the loop is retracting it undergoes another 180 degrees counter-clockwise rotation about the superior mesenteric artery. [4] Cranial limb retracts first because caecal bud on caudal limb acts like a knot, preventing the caudal limb from moving first. The cranial limb forms most of small intestine. The caudal limb form distal ileum, cecum, appendix, ascending colon + proximal 2/3 of the transverse colon. Formation of the large intestines: By the 11th week the transverse colon now rests in front of the duodenum. The caudal limb structures (the large intestine) are situated to the right of the cranial (small intestine) structures Initially caecum and a short ascending colon rest underneath the liver. From the 11th week onward, the ascending colon elongates and caecum descends until the caecum and appendix lie in the lower right quadrant. [5] During the fetal period the vitelline duct regresses and disappears [6] 5 6 Once the final position of intestines is attained, ascending and descending colon become secondarily retroperitoneal (a process by which mesenteries The Vitelline duct disappears fuse to the posterior abdominal wall) → intestine development complete. Clinical Corner – Midgut • Omphalocele o If there is no reincorporation back of the intestines then there will be intestines left outside. This is different from gastroschisis and is not covered by the amniotic cavity. • Meckel’s diverticulum The vitelline duct does not regress properly, leaving a vestigial remnant → causing obstruction, pain etc. Non-rotation of the gut occurs if the intestine does not rotate 180 degrees as it returns into the abdominal cavity. In this case the large intestine is located towards the left of the body and the small intestine is on the right side. o This leaves the gut more prone to twisting as it is not anchored to normal structures o •