Primo vascular system in human umbilical cord and placenta.

The primo vascular system (PVS) has been observed in various animals such as mice, rats, rabbits, dogs, swine, and cow, but not in humans. In this work, we report on the observation of a human PVS on both the epithelial fascia and inside the blood vessels of the umbilical cord (UC). The main morphological characteristics of the primo vessels (PVs) and primo nodes (PNs) from the human UC were in agreement with those of the PVS in various animal organs, including the thicknesses and the transparency of the PVs, the sizes of the PNs, the broken-line arrangement of the rod-shaped nuclei, the sparse distribution of nuclei, and the presence of hollow lumens in the central inner parts of the PNs. It was rather surprising that the human PV was not thicker than the PVs from small animals. The difference between the PVS and blood/lymph vessels was confirmed using immunofluorescence staining of von Willebrand factor, CD31, LYVE-1, and D2-40. The positive expression of the PVS to proliferating cell nuclear antigen, a cell-proliferation marker, was consistent with the recent finding of very small embryonic-like stem cells in the PVS of mice.