Primo vascular system and its potential role in cancer metastasis.

The primo vascular system (PVS) is a newly found organ, which is distributed throughout the entire body. The system is composed of nodes storing many small cells and thin vessels branching out from the nodes. Inside the vessel there are multiple subvessels. The PVS is found in and on most organs, including the brain, and interestingly inside some lymph and blood vessels. The PVS is normally difficult to visualize due to its semitransparent optical property and its small size, which may be the main reason why it was not discovered until recently. The diameter of primo vessels (PVs) is in the range of 20-50 μm and the size of a primo node (PN), 100-1,000 μm. The outermost layer of the PVS is more porous than that of blood or lymph capillary vessels, and the nuclei of the PVS endothelial cells are rod shaped. Important PVS properties reported are: in the fluid inside the PVS, there are cells presenting stem cell markers CD133, Oct4, and Nanog, which may imply that this system has a role in regeneration. Another very important finding is its potential relevance to cancer. According to results from an animal study using xenografts of various cancer types (lung, ovarian, skin, gastric cancer, and leukemia), as the tumor grows, the PVS is formed in a high density in the vicinity of the tumor. In addition, it was shown that PVs connect the primary and secondary tumors and that cancer cells were transported via the PVs in an active manner. In this report, we illustrated the formation of the PVS in breast cancer, and using the green fluorescent protein-expressing gastric cancer cell lines, we observed the cancer cell movement from the primary to the secondary sites during the cancer progression.