Skin vasculature cross-talking with hair follicle stem cells (HFSCs) during hair cycle may control key steps of adult skin homeostasis but is poorly understood. We have described a novel vasculature structure composed primarily of blood vessels that undergoes remarkable remodeling during hair cycle in both mouse and human skin. In particular, a horizontal(H) plexus(P) located under(u) the secondary hair(H) germ(G) at telogen might enforces mouse HFSC quiescence. This may potentially occur through cell signaling from endothelial cells towards the primed HFSC in the hair germ. If true, this finding would prove for the first time the skin endothelial cells as novel dermal signaling niches for HFSCs. The cell signals at play in the communication between endothelial cells and HFSC, as well as the nature and regulation of hair cycle vasculature remodeling, remain currently unknown. Here we will rigorously address these questions in two specific and complementary aims, with special emphasis on our novel vasculature structure, the HPuHG. First, we will examine distribution and dynamic remodeling of different endothelial cell lineages and vascular structures during the hair cycle and explore the genetic control of this remodeling (Aim1). Furthermore, we will systematically characterize the molecular makeup of EC lineages and identify putative pathways of cross-talking with the hair follicle epithelium. In particular, we will focus on the HPuHG interaction with the hair germ harboring the primed HFSCs, which initiate the new hair growth cycle. Second, we will explore the mechanisms at play in the communication from skin endothelial cells towards HFSCs by exploiting a relevant mouse model (the Alk1 endothelial-specific knockout) novel to hair cycle regulation (Aim2). We showed that in this mouse model the vasculature near the hair germ (in the HPuHG) is increased at telogen and this is accompanied by delayed HFSC activation. These phenotypes rationalize the exceptional fitness of this mouse model for further inquiry into the nature of the cross-talking between endothelial cells and HFSCs. Using a candidate as well as an unbiased approach we will explore mechanisms downstream of Alk1 in endothelial cells that may influence HFSC activation timing. Collectively, this work will characterize remodeling of skin vasculature in hair cycle and illuminate the communication between skin endothelial cells and HFSCs to coordinate tissue homeostasis. It will have future broad relevance for human skin regeneration studies and for more in depth understanding of skin vasculature disease.