(676b) Imaging Primo Vascular System, a Newly Found Vascular System

Primo Vascular System (PVS) is a newly found organ in the body, which is composed of nodes (Primo Nodes; PNs) and thin vessels (Primo Vessels; PVs) branching out of PNs. The liquid inside the PVS (Primo Fluid) flows at an average velocity of 0.3 mm/s. PVs are mechanically elastic but rather fragile, requiring extensive care during harvesting. This system has been observed in most organs, forming an extensive network throughout the body, including the heart, intestine, liver, adipose tissue, brain, even in some blood and lymphatic vessels.

 PVS is extremely difficult to image to the people who are not trained for microsurgery, due to its semi-transparent optical property and small size. The diameter of rabbit PVs is in the range of only 20-50 micrometers and a PV is composed of a bundle of sub-vessels running in parallel. The size of the PN is in the range of 100-1000 micrometers. A few unique characteristics of PVS that could be used for imaging PVS are: that its outer vessel walls are more porous than those of blood or lymphatic vessels and that the nuclei of the PV endothelial cells are rod-shaped instead of circular shape. The first property can used to visualize it with the help of contrast agents because the agents are preferentially infiltrated to PVS, not to blood or lymph vessels. The latter can be used for imaging using the nucleus specific dye (e.g., DAPI) so that the vessels with rod shaped nuclei can be differentiated from that with circular neclei (e.g., blood or lymphatic vessels).

 Due to the short history of discovering this system, only limited information on the PVS roles and functions has been revealed, via various imaging techniques. Some of the important properties already reported are:

 * The abundant presence of PVS in the adipose tissue may be an important in the study of obesity;

 * Some cells transported via PVS present stem cell biomarkers, which may be important for the relationship between PVS and regeneration;

 * These stem-cell-like cells are in the PNs at a very high concentration, potentially implying that PNs are for storing these cells;

 * A PV runs from the brain via the spinal canal, indicating some type of communication between the brain and rest of body.  Also this PV may be used as a special conduit for drug delivery avoiding the known difficulties in the delivery through the blood-brain barrier;

 * According to the study results using xenografs of several cancer types (lung, ovarian, skin, gastric and breast cancers, leukemia, etc.), as the cancer is formed PVS is also formed at a high density around the tumor. In addition, the cancer cells were observed to be transported via PVS from the primary to the secondary tumors more actively than via lymph vessels. Optical imaging of cancer cell transport via PVS has been performed using quantum dot-electroporated cancer cells, which was another unique imaging technique.

With the findings listed above alone, PVS appeared to have fundamental roles in human life and image techniques with high resolution that can provide three dimensional PVS maps in the body (e.g., CT and MRI with gold and iron oxide nanoparticles, respectively) may change the paradigm of current medical practice. We feel that it is extremely important to elucidate the roles and functions of PVS as soon as possible, so that these findings can be used in human health management.