Evidence for the Primo Vascular System above the Epicardia of Rat Hearts

We for the first time reported evidence for the existence of a novel network, a PVS, abovethe epicardium of the rat heart. (1) We were consecutively able to visualize the PVs and the PNs above the epicardial spaces of five rats' hearts by using Cr-Hx spraying or injection. (2) Hematoxylin and eosin (H&E) and toluidine blue staining of the PVs and the PNs showed that they consisted of a basophilic matrix; specifically the PNs contained several mast cells, some of which were degranulating into pericardial space. Also, 4′, 6-diamidino-2 phenylindole (DAPI) images of the PVs and the PNs showed that they contained various kinds of cells. (3) Transmission electron microscopic (TEM) longitudinal image of the PVs showed that the sinuses contained many granules with high-electron-density cores in parallel with putative endothelial cells. (4) TEM images of the PNs demonstrated that they consisted of lumen-containing cells surrounded by fibers and that they had mast cells that were degranulating toward the epicardium of the rat heart. The above data suggest that mast-cells-containing novel network exists above the epicardium of the rat heart.