Mesoderm-Derived PDGFRA+ Cells Regulate the Emergence of Hematopoietic Stem Cells in the Dorsal Aorta

Mouse hematopoietic stem cells (HSCs) first emerge at embryonic day 10.5 (E10.5) on the ventral surface of the dorsal aorta, by endothelial-to-hematopoietic transition (EHT). We investigated whether cells with mesenchymal stem cell-like cell (MSC-LCs) activity that provide an essential niche for HSCs in the bone marrow reside in the aorta-gonad-mesonephros (AGM) and contribute to the structural development of the dorsal aorta and EHT. Using transgenic mice, we demonstrate a lineage hierarchy for AGM MSC-LCs and trace the aortic endothelium and HSCs to mesoderm-derived (Mesp1) PDGFRA+ cells. Mesp1/PDGFRA+ MSC-LCs dominate the sub-endothelial and ventral stroma in the E10.5–E11.5 AGM but by E13.5 are replaced by neural crest (Wnt1) MSC-LCs. Co-aggregating endothelial cells with Mesp1 but not with Wnt1 MSC-LCs resulted in EHT and generation of LT-HSCs that is interrupted by dose-dependent inhibition of PDGFRA signalling. This partnership between endothelial cells and AGM Mesp1 MSC-LCs could be harnessed to manufacture HSCs from endothelium. Single cell RNA sequencing was used to identify differences and similarities between the tested populations of fresh adult cardiac endothelium and co-aggregated culture with MesP1(der)/PSCs for 24, 48, 72 and 96 hours..