Primo-Vessels as New Flow Paths for Intratesticular Injected Dye in Rats

After intratesticular injection of a chromium hematoxylin and fluorescent nanoparticle solution, we found a novel flow path in the abdominal cavity consisting of primo-vessels and primo-nodes. This flow path formed a network that crossed over the surfaces of abdominal organs, and generally linked to the greater omentum and adjacent visceral peritoneum. Some of these structures terminated at organs such as the small intestine and the urinary bladder; occasionally, the network entered the parenchyma of organs. The semitransparent primo-vessels and nodes were wholly or partially stained dark-blue by chromium hematoxylin. Injected nanoparticles were also observed in primo-vessels and nodes as well as the parenchyma of organs which were the site of primo-vessel termination. Transmission electron microscopy showed that the primo-vessels consist of many sinuses (4–6 μm), surrounded by collagen fibers, specific granules, cellular remnants, immune cells, extracellular matrices, and hematopoietic cells. These histological features are completely different from blood and lymph vessels indicating that primo-vessels are novel structures that allow the flow of dye.