Primo Vascular System in Human Umbilical Cord and Placenta

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