Meziane, M.; Litz, M. P. H.; Chandrasekaran, P.; Frank, D. B.; Li, P.

How endothelial cells from distinct developmental sources are integrated into a single continuous vascular system remains unresolved. Here, using the developing mouse lung, we identify a mesenchymal progenitor population that generates endothelial cells de novo and incorporates them into the expanding vasculature through a mechanism we term integrative vasculogenesis. Genetic lineage tracing shows that these progenitors contribute directly to the pulmonary endothelium, defining a source distinct from endothelial cells of the major vessels. Live imaging and single-cell tracking reveal that newly specified angioblasts exhibit high motility, dispersing through stochastic migration before integrating into pre-existing vascular networks. Cell ablation demonstrates that pre-existing networks are required to support the migration, proliferation and survival of nascent endothelial cells. Integrative vasculogenesis is thus distinct from classical vasculogenesis and angiogenesis, providing a framework for how endothelial populations of different origins are assembled into a functional circulatory system.