Physiologic retinal vascularization. (A) Schematic representing the approximate time course of retinal vascularization in a C57Bl/6 mouse. The astrocytic template (green) precedes nascent vessels and is fully laid out at birth. The superficial vascular network grows between birth and P8, the deep plexus forms approximately between P7 and P12, whereas the intermediate plexus forms during the third week of life. (B) Model of vascular growth during retinal development. Astrocytes express PDGFR-α and invade the developing retina from the optic nerve head, ahead of the vascular front. They travel on top of PDGF-A–expressing RGCs. Nascent vessels follow the astrocytic template and form R-cadherin junctions with proximal astrocytes.