Developmental Biology of the Blood System

Hematopoietic stem and progenitor cells (HSPCs) in the bone marrow are derived from a small population of hemogenic endothelial (HE) cells located in the yolk sac and major caudal arteries of the mammalian embryo. HE cells undergo an endothelial to hematopoietic cell transition, giving rise to HSPCs that accumulate in intra-arterial clusters before colonizing the fetal liver. To examine the molecular transitions between endothelial cells, HE, and intra-arterial cluster cells, and the heterogeneity of HSPCs within the intra-arterial clusters, we profiled ~40,000 cells from the caudal arteries (dorsal aorta, umbilical, vitelline) of embryonic day 9.5 to 11.5 mouse embryos by single-cell RNA sequencing (scRNA-seq) and single-cell chromatin accessibility sequencing (scATAC-Seq). A continuous developmental trajectory leads from endothelial cells to intra-arterial cluster cells, with identifiable intermediate stages between endothelial cells and HE. The intermediate endothelial stages most proximal to HE are characterized by elevated expression of genes regulated by GATA and SOX transcription factors. Developmental bottlenecks separate endothelial cells from HE cells, with the efficiency of transit through one of the last bottleneck regulated by RUNX1 dosage. Distinct developmental trajectories within intra-arterial cluster cells result in two populations of CD45⁺HSPCs; an initial wave of multi-lineage committed progenitors followed by precursors of hematopoietic stem cells (pre-HSCs). These and other insights gained from single cell analyses of HSPC formation from arterial endothelium will be presented.