Fig. 1

Schematic representations of embryonic hematopoiesis in vivo. There are 3 waves of hematopoietic cell generation, including primitive, pro-definitive, and definitive HSC. Among them, the primitive hematopoietic cells, including erythrocytes, megakaryocytes and macrophages, emerge in the extraembryonic yolk sac blood islands; the pro-definitive wave of hematopoiesis occurs in the yolk sac and generates definitive erythro-myeloid progenitors (EMPs) and lymphoid-primed progenitors (LMPPs) in the yolk sac; the definitive HSCs arise in the intraembryonic aorta–gonad–mesonephros region (AGM) and migrate into the fetal liver for maturation, expansion and differentiation, then initiate mobilization to the bone marrow to supply life-long hematopoiesis