Hematopoietic stem cells (HSCs) underlie the continued production of blood and immune cell lineages for the lifetime of an organism. In all vertebrate embryos examined, HSCs arise from the unique transdifferentiation of hemogenic endothelium comprising the floor of the dorsal aorta during a brief developmental window. To date, this process has not been replicated in vitro from pluripotent precursors, both because the full complement of required signaling inputs remains to be determined and because the lineage relationships among each subtype of arterial endothelium remain to be elucidated. Our recent efforts to elucidate the signaling pathways required to generate HSC fate have demonstrated that Notch signaling is needed iteratively, with both environmental and intrinsic requirements. The earliest known requirement is present within the somite to generate the sclerotome, the ventromedial domain of each developing somite. The sclerotome, in turn, is necessary to present Notch ligand to the shared vascular precursors of HSCs as they migrate across the ventral face of the somites. These studies provide the earliest known molecular cue required for HSC development. We have followed upon this work to demonstrate additional roles of the somite in patterning HSC formation and function. Wnt9a is produced from the somites and received via Frizzled9b / EGF receptors present upon HSC precursors, which regulates the first proliferative events within nascent HSCs. Finally, we have also shown that the somite produces endothelial cells that migrate and incorporate into the developing dorsal aorta. Thus, it appears that three different classes of endothelial cells are present at the time of HSC emergence – hemogenic endothelium, arterial endothelium, and somite-derived endothelial cells. What regulates production of each subset, and how each contributes to HSC generation are topics of our ongoing studies.
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