The embryonic form is simple; the adult form becomes elongated, coiled, and fused.
Gut is straight tube with dorsal mesentery behind (containing 3 gut vessels), ventral mesentery in front of foregut (containing L umbilical vein in free inferior margin) attached down to level of umbilicus.
- the dorsal mesentery is further called (where gut vessels run) dorsal mesogastrium, dorsal mesentery and dorsal mesocolon respectively
- the liver grows into the ventral mesentery, bifurcates, and grows in two lobes
- the pancreas grows into both ventral and dorsal mesenteries, fuse, but keep their 2 ducts (main and accessory in adult)
- the spleen grows in the dorsal mesogastrium (not truly a foregut structure though)
Midgut herniates through umbilicus (gut elongation and liver growth means everything no longer fit inside); until abdominal walls have grown enough to accommodate it by week 10.
- Apex (longest point) of the loop is at the vitellointestinal duct (site of MeckelÕs); arteries grow off SMA inferiorly (ileocolic and colic) and superiorly (ileal, jejunal)
Occurs as gut returns to abdominal cavity during week 10. Distal part of midgut loop goes up & to L, proximal part goes down & to R; from perspective of anteriorly-placed observer gut rotates anticlockwise. Rotation occurs around axis of SMA.
- the commencement gets plastered to the posterior wall as duodenum
- the position of the dorsal mesentery means it gets plastered from the start to the apex of the final positions of the gut; the colic vessels come to lie anterior to everything in the retroperitoneum.
- Most distally, the midgut dorsal mesentery does not fuse ˆ transverse mesocolon
- caecum initially high up near midline Ð grows down and right taking ileum with it.
Swing across on dorsal mesentery to L to lie on parietal peritoneum of L paracolic gutter; \ L colic vessels lie anterior to everything else on posterior abdominal wall. Sigmoid mesentery does not fuse.
Livers grows fast, so that ventral mesogastrium pushed down and left umbilical vein (ˆ ligamentum teres). This ÔsagsÕ the ventral mesogastrium into two parts: falciform ligament (between liver and anterior abdominal wall) and lesser omentum (between liver and stomach).
lesser omentum runs back behind ligamentum teres, lodges the remnant of ductus venosum
Liver grows & swings to R, stomach swings to L (grows more posteriorly, hence curvature).
- the lesser omentum becomes stretched between them across the abdo cavity
Duodenum swings to R & is plastered down onto posterior abdominal wall;
- asymmetric growth of its walls carries entry of CBD & PD onto medial aspect.
Meanwhile the dorsal mesogastrium hinges to L to front of left kidney; hangs off now ÔgreaterÕ curvature of stomach as the start of the greater omentum.
- splenic growth divides this part of the greater omentum into gastrosplenic and lienorenal ligaments.
- Caudally the greater omentum hangs down over the transverse colon as a double layer which fuses as a fatty apron.