During angiogenic sprouting, endothelial cells adopt two specialized phenotypes; tip cells that spearhead the angiogenic sprout, and stalk cells that follow the tip cells and proliferate to form the vascular lumen. Tip cells have a unique gene expression profile that allow them to migrate and lead the growing sprout. In the lab, we use the mouse postnatal retina as a model to study angiogenic sprouting. During the first postnatal week of retina vascularization in mice, tip cells migrate from the optic stalk on the retinal surface to form the superficial vascular plexus. Once they reach the retinal periphery, vertical sprouts emerge and invade vertically the neuroretina to give rise to the deep vascular layers. Using single-cell RNA sequencing, we compared the identities of superficial vs. diving tip cells and found that diving (D) tip cells have a distinct molecular signature compared to superficial (S) tip cells and acquire blood-retina-barrier (BRB) properties. We are presently investigating a number of signaling pathways that are responsible for D-tip cell identity acquisition, and how they affect retinal vascular development. These studies combine genetic mouse models to study vessel development and function in vivo, and signaling experiments using primary endothelial cells in vitro.