A) Microglia originate from myeloid precursors in the yolk sac, which migrate into the neuroepithelium by E10. They associate with radial glia and with blood vessels (also ingressing into the brain from the pial surface) where they may promote fusion of vascular tip cells in the periventricular vascular plexus (PVP). Arrow indicates progressive development from mouse embryonic day (E) 7 to E10. (Modified from .) B) Bottom: Reduced vascular branching in the brains of microglia-deficient Pu.1−/− mouse embryos. Flat mounted brains from embryonic day 12 mice stained for endothelium (CD31, blue) and microglia (F4/80, red). Branch points quantified from 4 mutants and controls. Students T Test P < 0.005.