Embryology Appendix

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MIDGUT

The derivatives of the midgut are

The small intestine, including the duodenum distal to the opening of the bile duct

The cecum, appendix, ascending colon, and the right one half to two thirds of the transverse colon

These midgut derivatives are supplied by the superior mesenteric artery, the midgut artery.

Wk 6 = Midgut elongates, it forms a ventral, U-shaped loop of gut (the midgut loop of the intestine) that projects into the remains of the extraembryonic coelom in the proximal part of the umbilical cord. At this stage, the intraembryonic coelom communicates with extraembryonic coelom at the umbilicus. This midgut loop of the intestine is a physiologic umbilical herniation and occurs because there is not enough room in the abdominal cavity for the rapidly growing midgut.The loop communicates with the umbilical vesicle through the narrow omphaloenteric duct (yolk stalk) until the 10th week. The shortage of space is caused mainly by the relatively massive liver and the kidneys that exist during this period of development.

The midgut loop of intestine has a cranial (proximal) limb and a caudal (distal) limb and is suspended from the dorsal abdominal wall by an elongated mesentery. The omphaloenteric duct is attached to the apex of the midgut loop where the two limbs join.The cranial limb grows rapidly and forms small intestinal loops, but the caudal limb undergoes very little change except for development of the cecal swelling (diverticulum), the primordium of the cecum, and appendix.

Rotation of the Midgut Loop

While it is in the umbilical cord, the midgut loop rotates 90 degrees counterclockwise (looking from the ventral side) around the axis of the superior mesenteric artery. This brings the cranial limb (small intestine) of the midgut loop to the right and the caudal limb (large intestine) to the left. During rotation, the cranial limb elongates and forms intestinal loops (e.g., primordia of jejunum and ileum).

Return of the Midgut to the Abdomen

During the 10th week, the intestines return to the abdomen (reduction of the midgut hernia). It is not known what causes the intestine to return; however, the enlargement of the abdominal cavity, and the relative decrease in the size of the liver and kidneys are important factors. The small intestine (formed from the cranial limb) returns first, passing posterior to the superior mesenteric artery and occupies the central part of the abdomen. As the large intestine returns, it undergoes a further 180-degree counterclockwise rotation (see C1 and D1). Later it comes to occupy the right side of the abdomen. The ascending colon becomes recognizable as the posterior abdominal wall progressively elongates (see Fig. E).

Fixation of the Intestines

Rotation of the stomach and duodenum causes the duodenum and pancreas to fall to the right. The enlarged colon presses the duodenum and pancreas against the posterior abdominal wall; as a result, most of the duodenal mesentery is absorbed (Fig. C, D, and F). Consequently, the duodenum, except for approximately the first 2.5 cm (derived from foregut), has no mesentery and lies retroperitoneally. Similarly, the head of the pancreas becomes retroperitoneal (posterior to peritoneum). The attachment of the dorsal mesentery to the posterior abdominal wall is greatly modified after the intestines return to the abdominal cavity. At first, the dorsal mesentery is in the median plane. As the intestines enlarge, lengthen, and assume their final positions, their mesenteries are pressed against the posterior abdominal wall. The mesentery of the ascending colon fuses with the parietal peritoneum on this wall and disappears; consequently, the ascending colon also becomes retroperitoneal.

Other derivatives of the midgut loop (e.g., the jejunum and ileum) retain their mesenteries. The mesentery is at first attached to the median plane of the posterior abdominal wall. After the mesentery of the ascending colon disappears, the fan-shaped mesentery of the small intestines acquires a new line of attachment that passes from the duodenojejunal junction inferolaterally to the ileocecal junction

The Cecum and Appendix

The primordium of the cecum and wormlike (Latin, vermiform) appendix-the cecal swelling (diverticulum)-appears in the sixth week as an elevation on the antimesenteric border of the caudal limb of the midgut loop (Fig. A). The apex of the cecal swelling does not grow as rapidly as the rest of it; thus, the appendix is initially a small diverticulum of the cecum (see Fig. B). The appendix increases rapidly in length so that at birth it is a relatively long tube arising from the distal end of the cecum (see Fig. D). After birth, the wall of the cecum grows unequally, with the result that the appendix comes to enter its medial side. The appendix is subject to considerable variation in position. As the ascending colon elongates, the appendix may pass posterior to the cecum (retrocecal appendix) or colon (retrocolic appendix). It may also descend over the brim of the pelvis (pelvic appendix). In approximately 64% of people, the appendix is located retrocecally (Fig. E).