I initially started to study the development of vascular system by focusing on the interaction of hematopoietic cells and vascular cells (Takakura et al. Cell 2000 and so on). We have identified apelin/APJ system to regulate caliber size of capillary and arterio-venous alignment (Kidoya et al. Dev Cell 2015). Recently, we have identified endothelial stem cells in the pre-existing blood vessels and showed critical roles of this cell population for the maintenance of vascularity (Wakabayashi et al. Cell Stem Cell 2018). We are now working on the analysis how this stem cell population can be utilized for vascular regeneration therapy.