Hematopoietic stem cells derive directly from aortic endothelium during development

A major goal of regenerative medicine is to instruct formation of multipotent, tissue-specific stem cells from induced pluripotent stem cells (iPSCs) for cell replacement therapies. Generation of hematopoietic stem cells (HSCs) from iPSCs or embryonic stem cells (ESCs) is not currently possible, however, necessitating a better understanding of how HSCs normally arise during embryonic development. We previously showed that hematopoiesis occurs through four distinct waves during zebrafish development, with HSCs arising in the final wave in close association with the dorsal aorta. Recent reports have suggested that murine HSCs derive from hemogenic endothelial cells (ECs) lining the aortic floor1,2. Additional in vitro studies have similarly suggested that the hematopoietic progeny of ESCs arise through intermediates with endothelial potential3,4. In this report, we have utilized the unique strengths of the zebrafish embryo to image directly the birth of HSCs from the ventral wall of the dorsal aorta. Utilizing combinations of fluorescent reporter transgenes, confocal timelapse microscopy and flow cytometry, we have identified and isolated the stepwise intermediates as aortic hemogenic endothelium transitions to nascent HSCs. Finally, using a permanent lineage tracing strategy, we demonstrate that the HSCs generated from hemogenic endothelium are the lineal founders of the adult hematopoietic system.