Fig. 6. Model of hematopoietic ontogeny in the zebrafish embryo. (A) Different regions of lateral plate mesoderm (LPM) give rise to anatomically distinct regions of blood cell precursors. Drawing depicts a dorsal view of a five-somite-stage embryo. (B) Embryonic hematopoiesis appears to occur through four independent waves of precursor production. Each wave is numbered based on the temporal appearance of functional cells from each subset. First, primitive macrophages arise from cephalic mesoderm and migrate onto the yolk ball before spreading throughout the embryo. Second, primitive erythrocytes begin to differentiate within the intermediate cell mass (ICM) before entering circulation at approximately 24 hpf. Third, the first definitive progenitors arise as EMPs are formed within the posterior blood island (PBI). EMPs generate the first definitive myeloid cells and a new wave of erythroid cells. Finally, definitive hematopoiesis culminates with the production of multipotent HSCs between the axial vessels in the zebrafish equivalent of the AGM region.