Describe the Midgut and Rotation PDF

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Describe How the Midgut Develops, with Reference to the Role of Rotation.  Midgut begins caudal to liver bud and extends to the junction of the RHS 2/3 rd’s and L 1/3rd of the transverse colon.  Week 5 – midgut suspended by mesentery and communicates w. yolk sac via vitelline duct or yolk stalk.  Supplied by sup. Mesenteric aa.  Development characterized by rapid elongation of gut and its mesentery resulting in formation of 1’ intestinal loop.  @ loop apex = open connection w. yolk sac by vitelline duct.  Cephalic limb of loop develops into distal part of duodenum, jejunum, and part of ileum  Caudal limb  Lower portion of ileum and cecum, The appendix, asc. colon and proximal 2/3rd’s of

 Cephalocaudal and lat. folding portion of endoderm from gastrulation incorporated  primitive gut; yolk sac and allantois outside.

 Ventral layer of endoderm = gut tube  Physiological Herniation; o Result of rapid growth and expansion of the liver – abdominal cavity becomes temporarily too small for all intestinal loops so enter the extraembryonic cavity in the umbilical cord during week 6 of development. o Small intestine coiling and cecum formation occur during herniation  Midgut Rotation; o 1’ intestinal loop rotates around an axis formed by sup. Mesenteric aa. o Contraclockwise rotations of 270 – 90 degrees during herniation and return of intestinal loops into abdominal cavity remaining 180. o During rotation – elongation of the small intestinal loop continues, and jejunum and ileum form several coiled loops o Large intestine lengthens but doesn’t participate in coiling  Retraction of Herniated Loops; o Week 10 – herniated intestinal loops return to abdominal cavity o Proximal portion of jejunum first to renter on the LHS

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Returning loops settle more to right Week 6 - Cecal bud appears – dilation of caudal limb of 1’ intestinal loop = last part of gut to renter; temp. lies in UR quadrant below R lobe of liver and desc.  R iliac fossa placing asc. Colon and hepatic flexure on RHS. – during cecal bud forms appendix

 Mesenteries of Intestinal Loop; o Mesentery proper = mesentery of 1’ intestinal loop o When caudal limb of loop moves to RHS of abdominal cavity; dorsal mesentery twists around origin of sup. Mesenteric aa. o Asc. And desc. Portions of colon obtain definitive pos. their mesenteries press against peritoneum of post. abdominal wall. – after fusion; asc. And desc. Colons permanently anchored in retroperitoneal pos. o Appendix, lower end of cecum and sigmoid = free mesenteries o Transvers colon fuses w. post. abdominal wall of greater Omentum but maintains mobility o Mesentery of jejunoileal loops @ first cont. w. asc colon....