Regressing vitelline vein and the initial development of the superior mesenteric vein in human embryos

The superior mesenteric vein was considered to develop in situ in the midgut mesentery secondary to regression of the left vitelline vein. We revisited the morphology using serial sections of 20 embryos at 5-6 weeks (CRL 9-15 mm). The regressing vitelline vein provided a long peritoneal fold in the immediately superior side of the midgut mesentery containing the thick superior mesenteric artery. Notably, in a half of specimens, there were tissue clefts along the superior mesenteric artery in the mesentery and they were communicated with the left vitelline vein at the superior end of the peritoneal fold. The tissue clefts appeared not to carry the endothelial lining. We considered the cleft as the initial superior mesenteric vein. Conversely, the initial vein seemed not to develop from budding or venous plexus.