Primo Vascular System in the Subarachnoid Space of the Spinal Cord of a Pig

The primo vascular system was recently observed in the central nervous systems of rabbits and rats, but no investigations in large animals have been reported. In the present work we found a putative primo vascular system in the spinal cord of a pig. We obtained spines from four healthy pigs and fixed them with paraformaldehyde. The primo vessels were expected to lie in the subarachnoid space between the pia mater and the arachnoid mater. The composite of three membranes (the pia, the arachnoid, and the dura maters) wrapping the spinal cord was peeled off, isolated from the spine, and put on a slide glass. This composite was stained with 4′,6′-Diamidino-2-phenylindole (DAPI) and phalloidin to show the nuclei and the f-actin, respectively, in the cells of the primo vessels. We observed eleven pieces of the putative primo vessels in the subarachnoid space of the spines at the thoracic spinal nerve area. They had the typical rod-shaped nuclei distributed in a broken line, and f-actin signals around nuclei. The lengths of the nuclei were 12–15 μm, and the thicknesses of the primo vessels were 8～20 μm, which were consistent with other primo vessels that had been observed in the various organs of rabbits, rats, and mice. In addition, we observed branching of the primo vessels, which is again an expected result from previous works. In conclusion, a primo vessel was observed in the subarachnoid space of the spinal cord of a pig. This was the first observation of a primo vessel in a large animal, and the staining method used to observe the primo vessel in a fixed sample was newly developed in this work.