Development of Midgut Dr. Rania Gabr

Objectives Give the extent of the midgut. Define Vitelline duct or yolk stalk. Discuss the formation of primary intestinal loop. Explain the mechanism of Midgut Development. Describe midgut abnormalities.

Extent of Midgut Extend from the duodenum (distal to the opening of bile duct) to the proximal 2/3rd of transverse colon. In 5th week midgut is suspended from posterior abdominal wall by a short mesentery. Midgut is connected with yolk sac by vitelline duct or yolk stalk.

Stages of Midgut Development Physiological umbilical hernia Rotation of the midgut loop Retraction of herniated midgut loops to Abdomen Fixation of intestines

Physiological herniation At the biginning of 6th week, the midgut elongates to form a venteral U-shaped midgut (intestinal) loop. Midgut loop communicates with the yolk sac by vitelline duct or yolk stalk. As a result of rapidly growing liver, kidneys & gut ,the abdominal cavity is temporarily too small to contain the developing rapidly growing intestinal loop. So ,Midgut loop projects into the umbilical cord …this is called physiological umbilical herniation (begins at 6th w.).

Midgut development Mid gut is supplied by the superior mesenteric artery. Elongation of the gut with its mesentery forms the primary intestinal loop. The intestinal loop has two limbs (cranial and caudal) At the Apex, the loop is connected with yolk sac by vitelline duct.

Midgut development Superior mesenteric artery runs in the axis, between the 2 limbs. The cephalic limb forms the distal part of the duodenum, jejunum and part of the ilium. The caudal limb forms the lower portion of the ilium, cecum, appendix, ascending colon, and proximal 2/3rd of transverse colon.

Rotation of midgut Midgut loop has a cranial limb & a caudal limb. Midgut loop rotates around the axis of the superior mesenteric artery. Midgut loop rotates first 90 degrees to bring the cranial limb to the right and caudal limb to left during the physiological hernia. The cranial limb of midgut loop elongates to form the intestinal coiled loops (jejunum & ileum). This rotation is counterclockwise and it is completed to 270 degrees, so after reduction of physiological hernia it rotates to about 180 degrees.

Retraction of herniated loops Begins during 10th week. It is called reduction of physiological midgut hernia. Factors responsible: Regression of the mesonephric kidney Reduced growth of the liver Expansion of the abdominal cavity The proximal part of the jejunum is the first part to re-enter & comes to lie on the left. The cecal bud (cecum) is the last part to re-enter.

90°

180°

270°

Development of cecum and Appendix Cecal bud appears during sixth week, lie at first in the right upper quadrant, then descends to the right iliac fossa. During its descent, appendix appears as a narrow diverticulum from the distal end of the cecal bud. Appendix may have different positions, retrocecal, retrocolic and pelvic appendix

Development of cecum and Appendix

Positions of Appendix

Mesenteries of the intestinal loops Mesentery proper Mesentery of ascending & descending colon (dorsal mesocolon) Appendix, lower end of cecum & sigmoid colon Transverse mesocolon

Fixation of various parts of intestine The mesentry of jejunoileal loops is at first continuous with that of the ascending colon. When the mesentry of ascending colon fuses with the posterior abdominal wall, the mesentry of small intestine becomes fan-shaped and acquires a new line of attachment that passes from duodenojejunal junction to the ileocecal junction.

Fixation of various parts of intestines The enlarged colon presses the duodenum & pancreas against the posterior abdominal wall. C & F Most of duodenal mesentery is absorbed, so most of duodenum ( except for about the first 2.5 cm derived from foregut) & pancreas become retroperitoneal. C & F Intestines prior to fixation Intestines after fixation

Mid gut Anomalies Mobile cecum: Persistence of a portion of mesocolon. It may lead to Volvulus or retrocoloic hernia. Omphalocele: Herniation of abdominal viscera through an enlarged umbilical ring, covered by amnion. It is due to failure of the bowel to return to the body cavity from its physiological herniation during 6th to 10th week. It is associated with other defects such as cardiac anomalies, neural tube defects & chromosomal abnormalities.

Gastroschisis: Protrusion of abdominal contents through the body wall directly into the amniotic cavity not covered by amnion or peritoneum. The bowel may be damaged by exposure to the amniotic fluid. This defect is due to the abnormal closure of the body wall around the connecting stalk. It occurs lateral to the umbilicus usually on the right.

Umbilical Hernia The intestines return to abdominal cavity at 10th week, but herniate through an imperfectly closed umbilicus It is a common type of hernia. The herniated contents are usually the greater omentum & small intestine. The hernial sac is covered by skin & subcutaneous tissue. It protrudes during crying,straining or coughing and can be easily reduced through fibrous ring at umbilicus. Surgery is performed at age of 3-5 years.

Vitelline Duct Abnormalities 2% of people, 2 -feet away from Ileo-cecal valve, 2 -inches long. Persistence of Vitelline duct forms: Meckel’s diverticulum or ileal diverticulum Other related structures are enterocystoma or vitelline cyst, umbilical fistula or vitelline fistula and vitelline ligaments

Abnormal rotation of gut

Gut Atresia and Stenosis