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The midgut elongates to form a ventral U-shaped loop of gut – the midgut loop or primary intestinal loop, which projects into the proximal umbilical cord as there is not enough room for it inside the abdominal cavity to accommodate its rapid growth (due to the large size of the liver and kidneys). This movement of the gut tube outside the abdominal cavity is known as the physiological umbilical herniation and is followed by rapid elongation of its mesentery.

The loop can best be considered as having two limbs – cranial (yellow) and caudal (orange), meeting at an apex where they attach to the vitelline duct which attaches the gut loop to the yolk sac.

The cranial limb grows rapidly, developing into the distal duodenum, the jejunum and part of the ileum.

The caudal limb changes very little, except for the growth of the caecal diverticulum , a small outpouching which forms the caecum and appendix. The remainder of the caudal limb forms the lower ileum, the ascending colon and the proximal two thirds of the transverse colon.

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Rotation of the Midgut Loop

The midgut loop, while in the umbilical cord, rotates 90 ° anticlockwise (when viewed from the anterior aspect) around the axis of the superior mesenteric artery. (i.e. the cranial limb swings down and right while the caudal limb swings up and left). This process is completed around week 8.

During rotation, the cranial limb of the midgut elongates and forms jejunal-ileal loops while the expanding caecum sprouts a vermiform appendix .

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Return to the Abdomen (Reduction of the physiological hernia)

The cranial loop (forming the small intestine) returns first, passing posterior to the superior mesenteric artery. Initially the loops pass to the left side of the back wall of the abdomen but later loops are deposited further to the right as the midgut loop undergoes a further 180 ° anticlockwise rotation.

The caecal diverticulum is the last part of the gut to reenter the abdominal cavity, temporarily lying in the right upper quadrant directly below the right lobe of the liver.

The caecal bud descends to the right iliac fossa, forming the appendix as it does so. This gives rise to the ascending colon and the hepatic (or right colic) flexure on the right hand side of the abdominal cavity. As the appendix forms during the caecum's descent, it frequently lies posterior to the caecum (retrocaecal) or posterior to the colon (retrocolic).

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Mesenteries

The rotation of the stomach causes the duodenum to fall to the back wall of the abdominal cavity, where it is now pressed by the transverse colon – this causes the duodenum (except for the first 2.5cm derived from foregut) to come to lie retroperitoneally .

The mesentery of the primary intestinal loop becomes very twisted with the movements of the bowel. The dorsal mesenteries of the ascending and descending colon shorten and fold, bringing them into contact with the posterior abdominal wall – making these structures secondarily retroperitoneal, however the appendix, caecum and sigmoid colon all retain their mesenteries.

The transverse mesocolon fuses with the posterior layer of the greater omentum, hence the transverse colon remains intraperitoneal.

After the mesentery of the ascending colon disappears, the fan-shaped mesentery of the small intestine (The mesentery) acquires a new line of attachment running from the duodenojejunal junction in an inferolateral direction to the ileocaecal junction.

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Go to Hindgut

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