Human yolk sac-like haematopoiesis generates , and/or dependent blood and -positive endothelium.

The genetic regulatory network controlling early fate choices during human blood cell development are not well understood. We used human pluripotent stem cell reporter lines to track the development of endothelial and haematopoietic populations in an model of human yolk-sac development. We identified SOX17CD34CD43 endothelial cells at day 2 of blast colony development, as a haemangioblast-like branch point from which SOX17CD34CD43 blood cells and SOX17CD34CD43 endothelium subsequently arose. Most human blood cell development was dependent on RUNX1. Deletion of only permitted a single wave of yolk sac-like primitive erythropoiesis, but no yolk sac myelopoiesis or aorta-gonad-mesonephros (AGM)-like haematopoiesis. Blocking GFI1 and/or GFI1B activity with a small molecule inhibitor abrogated all blood cell development, even in cell lines with an intact gene. Together, our data define the hierarchical requirements for RUNX1, GFI1 and/or GFI1B during early human haematopoiesis arising from a yolk sac-like SOX17-negative haemogenic endothelial intermediate.