To determine the origin and role of circulating endothelial progenitor cells (EPCs) contributing to postnatal vasculogenesis, this study used a mouse model to conclude: “postnatal neovascularization does not rely exclusively on sprouting from preexisting blood vessels (angiogenesis); instead, EPCs circulate from bone marrow to incorporate into and thus contribute to postnatal physiological and pathological neovascularization, which is consistent with postnatal vasculogenesis.”