Evaluation of a New Vasculature by High Resolution Light Microscopy: Primo Vessel and Node

In the 1960’s Bong Han Kim discovered and characterized a new vascular system. He was able to differentiate it clearly from vascular blood and lymph systems. He demonstrated that this system is composed of nodes and vessels, and it was responsible for tissue regeneration. These findings remained dormant for many years until recently when a new interest in this anatomical vascular system appears. The system is now termed primo vascular system. In this work, we characterized microstructure of rodent primo nodes and vessels using a high resolution light microscopy. The most typical primo nodes are oval-shape of 0.1-0.5 mm over the short and 0.5-1 mm over the long axis. Both node’s ends are linked to primo vessels of 3-6 cm long and 40-100 μm in diameter. The primo vessel is composed of 1–20 subvessels of 3–25 μm in diameter. The external envelope of the subvessel is composed of two layers: the wall of endothelial cells with a rod-shaped nucleus of 15-20 μm and the outer membrane containing spindle-shaped cells with ellipsoidal nucleus of 13-27 μm long and 4-5 μm thick, that are similar to smooth muscle cells. The subvessels are surrounded by fine, longitudinal and circular fibers crossing each other. The bundle of subvessel of the primo vessel is laid into an external jacket composed of endothelial cells with 6–12 μm round or oval nuclei. The node is heterogeneous in nature, composed of twisted subvessel bundles that fill up nearly the entire node volume. The enlarged subvessel inside the node harbors microcells that express stem cells and stem cell niche markers. We conclude that these microcells are progenitors of multipotent stem cells and the nodes serve as the stem cell niches outside the bone marrow.