Characterization of the mammalian prodefinitive angio-hematopoietic lineage

Mammalian hematopoietic cells arise from mesodermal progenitors in a close developmental relationship with endothelium and along three distinct cell lineages known as primitive, prodefinitive, and definitive hematopoiesis. However, the developmental hierarchies between early mesodermal progenitors, endothelium, and blood cell lineages are incompletely understood. Here, fate-mapping, cloning, and genetic experiments identified a population of KDR⁺CXCR4⁺ prodefinitive angio-hematopoietic progenitors (PDAPs) in murine primitive streak stage mesoderm and human induced pluripotent stem cell (hiPSC) cultures. PDAPs gave rise to yolk sac (YS) and rostral (cephalic) endothelial cells and RUNX1-dependent prodefinitive hematopoietic cells, including erythro-myeloid progenitors (EMPs) and tissue macrophages, via NOTCH1-independent hemogenic endothelial cells. Notably, PDAPs did not give rise to primitive erythropoiesis or to caudal endothelium and definitive hematopoiesis. These results identify a previously unrecognized layer of lineage segregation in early mammalian mesoderm that characterizes the prodefinitive angio-hematopoietic lineage and the origin of tissue macrophages and rostral blood vessels that may be of interest for pathophysiology and cell-based therapy efforts.