Involvement of non-vascular stem cells in blood vessel formation.

Blood vessels clearly act as conduits for blood flow, but recently the concept that they are also involved in organ maintenance, especially by providing a niche for organ-specific stem cells, has begun to emerge. Moreover, several lines of evidence suggest that hematopoietic stem cells can differentiate directly into cells composing blood vessels. Recently, cancer stem cells (CSCs) have also been assigned these roles in the cancer microenvironment. Although anti-angiogenic drugs have been developed and are utilized in the clinic for their anti-tumor activity, their suppressive effects on tumor growth have been disappointing. This may be caused by transferring drug resistance from CSCs to endothelial cells. It has been suggested that CSCs localize in the peri-vascular niche. Therefore, it is extremely important to know how the vascular niche maintains CSCs, as such knowledge may enable us to develop promising new approaches to cancer treatment.