Embryologic Rotation and Fixation of

The suspending dorsal mesentery of the distal foregut and midgut elongates considerably as the stomach and duodenum go through their complex rotation. This leads to the development of the lesser peritoneal sac and the greater omentum. As the dorsal bulge of the stomach becomes more marked, it carries the mesentery along with it to the left side of the abdomen. As this rotation takes place, the peritoneal space that originally lay to the right of the mesentery extends posterior to the stomach into the left hemiabdomen15 (Fig. 2-11). This eventually becomes the lesser sac. The elongated mesentery also doubles back on itself to form an apron that hangs down from the greater curvature covering the peritoneal cavity (Fig. 2-12). Eventually, the potential space within the omentum is obliterated. If the fusion is incomplete, an omental cyst may form. The differentiation of omental, enteric, mesenteric, and neurenteric cysts and/or dissecting pancreatic pseudocysts depends on their lining cell elements and wall constituents.16 The suspending dorsal mesentery of the transverse colon fuses with the greater omentum, forming the definitive transverse mesocolon5,9 (Fig. 2-12).

Relatively early in its development, the intestinal tract markedly elongates, reaching a length too great to be contained within the abdominal cavity. It therefore her-

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Fig. 2—9. Gastric duplication cyst.

(a) Contrast-enhanced CT reveals a bilobate low-density mass (D) along the greater curvature of the stomach.

(b) Sagittal sonogram confirms the cystic nature of this gastric duplication cyst.

Duplication Cyst Barium Enema

Fig. 2—10. Duplication of the colon.

(a) Barium enema study demonstrates a communicating colonic duplication (D) within the mesentery of the transverse colon and descending colon.

(b) Spot film clearly shows the features of the opacified duplication along the medial wall of the descending colon.

Fig. 2—10. Duplication of the colon.

(a) Barium enema study demonstrates a communicating colonic duplication (D) within the mesentery of the transverse colon and descending colon.

(b) Spot film clearly shows the features of the opacified duplication along the medial wall of the descending colon.

Barium Bowel Diagrams

Fig. 2—11. Diagram of the upper abdomen (as seen from below) during gastric rotation.

The suspending dorsal mesentery (mesogastrium) has elongated and is carried to the left of midline. This allows the right hemiperitoneum to extend posterior to the stomach, starting the formation of the lesser sac.

niates into the yolk sac. The superior mesenteric artery acts as the axis ofthis physiologic herniation. At the apex of this loop is the omphalomesenteric (vitelline) duct. This midgut elongation is predominantly composed of that segment that lies proximal to the duct, the prear-terial limb. In order to accommodate this increase in length, the small bowel is thrown into a serpentine pattern, an appearance it maintains into adulthood. The more distal segment, distal to the omphalomesenteric duct, is labeled postarterial and becomes the distal ileum, appendix, and large bowel proximal to the splenic flexure. The cecum starts as a small bud just distal to the apex of the loop and plays an important role in the reduction of the physiologic herniation (Fig. 2-13).

The herniated loop of midgut undergoes a 270° counterclockwise rotation (as seen from the front). The prearterial limb, which starts out superiorly, is carried first to the right and then inferiorly. Conversely, the distal postarterial limb is carried first to the left and then superiorly. Thus, the two limbs find themselves located 180° opposite to their original locations.

Transverse Mesocolon

Fig. 2—12. Greater omentum and transverse mesocolon.

(a) Longitudinal schematic drawing showing the fusion of the two leaves of the greater omentum with obliteration of the inferior recess of the lesser sac.

(b) Fusion of the greater omentum with the transverse colon and its dorsal mesentery gives rise to the definitive gastrocolic ligament and transverse mesocolon.

(From Javors BR, Sloves JH.4)

Fig. 2—12. Greater omentum and transverse mesocolon.

(a) Longitudinal schematic drawing showing the fusion of the two leaves of the greater omentum with obliteration of the inferior recess of the lesser sac.

(b) Fusion of the greater omentum with the transverse colon and its dorsal mesentery gives rise to the definitive gastrocolic ligament and transverse mesocolon.

(From Javors BR, Sloves JH.4)

Eventually, the body cavity enlarges sufficiently to allow the herniated bowel to return (Fig. 2-14). As it does so, the final 90° rotation is completed. The developing cecal bud hinders the return of the postarterial midgut, and therefore the prearterial limb returns to the abdomen first.9,17-19 As the final part of the rotation occurs, the prearterial limb is carried into the left upper quadrant, crossing beneath the superior mesenteric arterial axis. Therefore, the transverse duodenum is carried inferior to the superior mesenteric artery.

The postarterial midgut and hindgut are now forced to the periphery of the abdomen. The right colon, which develops from the distal limb of midgut, is carried in front of the superior mesenteric artery into the right upper quadrant (Fig. 2-15). It is further growth of the right colon, rather than any additional rotation, that carries it into the right lower quadrant.9 The suspending dorsal mesentery of the ascending and descending colons is eventually resorbed and united with the posterior abdominal wall. This results in the so-called retroperi-tonealization of those structures. In actuality, they maintain a peritoneal surface along their anterior aspect, similar to that of the duodenum.

The appendix develops from the cecal bud. It originates as a triangular projection with a wide orifice that is in line with the longitudinal axis of the right colon. The ileocecal valve impedes the growth of the colonic wall at its entry site, and the opposite wall continues to grow, effectively moving the appendix to the same side of the cecum as the valve.20 In addition, further increase

Omphalomesenteric Duct
FOREGUT-MIDGUT JUNCTION MIDGUT-HINDGUT JUNCTION

Fig. 2—13. Longitudinal view of the intestinal tract at 6 weeks of development.

The superior mesenteric artery (SMA) acts as the axis for midgut rotation. The omphalomesenteric duct (OMD) divides the midgut into pre- and postarterial limbs. Also seen is the physiologic herniation of the midgut through the umbilical orifice (UO). Heavy lines mark the foregut-midgut (/) and the midgut-hindgut (//) junctions. The celiac axis (CA) is the major artery of the foregut; the inferior mesenteric artery (IMA) supplies the hindgut. CB = cecal bud. (From Javors BR, Sloves JH.4)

Fig. 2-14. Frontal view of a 10-week fetus.

The elongated redundant prearterial limb has reentered the abdomen and crossed to the left of and behind the SMA. This displaces the hindgut to the left. Heavy lines mark the foregut-midgut (/) and the midgut-hindgut (//) junctions. CB = cecal bud; OMD = omphalomesenteric duct; UP = umbilical orifice. (From Javors BR, Sloves JH.4)

in the transverse diameter of the cecum, without further growth of the appendiceal lumen, brings about the more familiar vermiform (wormlike) appearance of the appendix.

This complex series of twists and turns and subsequent mesenteric resorption leaves the gastrointestinal tract prone to many and often complex errors of rotation and fixation.9,10,21,22

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