Integration of vascular progenitors into functional blood vessels represents a distinct mechanism of vascular growth.

During embryogenesis, the initial vascular network forms by the process of vasculogenesis, or the specification of vascular progenitors de novo. In contrast, the majority of later-forming vessels arise by angiogenesis from the already established vasculature. Here, we show that new vascular progenitors in zebrafish embryos emerge from a distinct site along the yolk extension, or secondary vascular field (SVF), incorporate into the posterior cardinal vein, and contribute to subintestinal vasculature even after blood circulation has been initiated. We further demonstrate that SVF cells participate in vascular recovery after chemical ablation of vascular endothelial cells. Inducible inhibition of the function of vascular progenitor marker etv2/etsrp prevented SVF cell differentiation and resulted in the defective formation of subintestinal vasculature. Similar late-forming etv2+ progenitors were also observed in mouse embryos, suggesting that SVF cells are evolutionarily conserved. Our results characterize a distinct mechanism by which new vascular progenitors incorporate into established vasculature.