OBJECTIVE: To demonstrate the entire course of the human vitelline vein (VV) in specimens after degeneration of the yolk sac.
METHODS: Sagittal and horizontal histological sections from 8 embryos and 19 fetuses (gestational age approximately 6-12 weeks; crown-rump length 11-61 mm) were examined.
RESULTS: Two types of VV remnants were observed: a long VV on the right superior side of the mesentery of the jejunum (VV1) and a short VV on the left inferior side of the mesentery (VV2). The VV1, observed in 12 specimens, was 20-30 microns in diameter and ran dorsally between the right liver lobe and the jejunum, subsequently merging with an initial superior mesenteric vein on the pancreatic head immediately below the superior portion of the duodenum. The VV2, observed in four specimens, passed dorsally between loops of the ileum on the left side of the mesentery of the ileum and connected to the mesentery. Many of the VVs did not originate from the umbilical cord but suddenly started in the sack of physiological herniation. At 10-12 weeks, after herniation, the VVs originated from the umbilicus and were involved by the expanding greater omentum.
CONCLUSIONS: The right-sided and left-sided VVs seemed to correspond to right and left VV remnants, respectively, and both took an upstream course outside the mesentery of the jejunum and ileum. The right VV upstream portion was likely to disappear later than the left one, but the timing of degeneration varied greatly among individuals, depending on the topographical relationship between the right liver lobe and the jejunum.

Because most malrotations of the small intestine are thought to occur during repackaging, the location of the intestine should vary less during physiological herniation than afterward. Examination of serial sagittal sections of 27 embryos and fetuses (gestational age 6-9 weeks; crown-rump length 15-45 mm) during herniation showed that the jejunum and ascending colon passed through a small opening of the hernia sac at the levels of the stomach and pancreas in 16 specimens. Below the pancreas, a definite mesentery extended between the ascending and descending colon in the abdominal cavity. In the other 11 specimens, the descending colon passed through an opening of normal size and ran posteriorly along the urinary bladder, so the entire ilium, ascending colon, and transverse colon entered the sac. In these specimens, the duodenojejunal junction was usually situated in a window of the mesentery of the colon (internal herniation). The descending colon was observed at an outside location more frequently in earlier specimens. In contrast to our working hypothesis, the locations of the intestine were abnormal in 40.7% (11/27) of samples. In addition, no abnormal colon was observed in any of the seven specimens after repackaging. An outside location of the descending colon was not directly associated with malrotation because recovery was likely. However, the delayed development of the inferior mesenteric arterial branches could cause failure, including death in utero, during or after the repackaging associated with physiological herniation. Clin. Anat. 31:583-592, 2018. © 2017 Wiley Periodicals, Inc.