We for the first time report evidence for existence of novel structures, prima vessels (PVs) and prima nodes (PNs) floating inside the venous sinuses of rat brains. For this purpose, we applied a chromium-hematoxylin (Cr-Hx) solution to stain the PVs and the PNs floating inside the venous sinuses (superior sagittal sinus, strait sinus, and transverse sinus) of seven rats' brains preferentially compared to the blood clots that easily form during surgery. Cr-Hx-stained PVs and PNs were examined by light and transmission electron microscopies: (1) we were consistently able to visualize the PVs and the PNs in the venous sinuses of all seven rats' brains. (2) The PVs and PNs consisted of rod-shaped and some round-shaped cells, respectively, as demonstrated by using 4',6-diamidino-2-phenylindole (DAPI). (3) Cross sections of the PVs showed that the sinuses contained loose fibrous materials and clusters of nano-sized granules enveloped by the cortex. The above data imply that thrombus of the venous sinuses may be related with these novel floating structures. However, the functions of the PVs and PNs floating in the venous sinuses remain to be investigated in terms of normal or thrombus-provoked diseases. (C) 2012 Elsevier Ireland Ltd. All rights reserved.