Comparative view of human developmental hematopoiesis in human embryo in vivo and during PSC differentiation in vitro. Top schematic diagram represents developmental hematopoietic waves in humans. The primitive wave composed of primitive erythroid, megakaryocytic, and macrophage progenitors is followed by a transient definitive wave consisting of multiple hematopoietic progenitors (YSPMs and LMPs; Figure 1). These progenitor waves also give rise to long-lasting tissue-resident macrophages and unique γ/δ T-cells. At CS14/CS16, the definitive hematopoietic wave in the embryo gives rise to HSC-forming HE and the HSC lineage. An arterially specified EC in the aorta, characterized by active Notch, WNT, and transforming growth factor βsignaling and patterned by retinoids, undergoes EHT by inhibiting WNT and transforming growth factor β pathways and activating hematopoietic signaling. HSC functional maturation is attained in the liver during following weeks via yet unknown niche signals. It has been possible to recapitulate the early steps of the distinct hematopoietic waves to differentiate PSCs to both YS-like and AGM-like HE precursors and their progeny (bottom). BMP4-mediated mesoderm induction with sequential addition of other morphogens and cytokines, including small molecule–mediated WNT activation, activin A inhibition, and use of retinoids, have been optimized for the generation of diverse intra and extraembryonic-type mesodermal precursors. However, the generation of functionally mature and robustly engraftable human HSCs in culture requires further optimization of the culture microenvironment. SCF, stem cell factor; TPO, thrombopoeitin; VEGF, vascular endothelial growth factor.