Recent studies have shown that nascent hematopoietic stem cells (HSCs) derive directly from the ventral aortic endothelium
(VAE) via endothelial to hematopoietic transition (EHT). However, whether EHT initiates from a random or predetermined subpopulation
of VAE, as well as the molecular mechanism underlying this process, remain unclear. We previously reported that different
zebrafish stem cell leukemia (scl) isoforms are differentially required for HSC formation in the ventral wall of the dorsal aorta. However, the exact stage
at which these isoforms impact HSC development was not defined. Here, using in vivo time-lapse imaging of scl isoform-specific reporter transgenic zebrafish lines, we show that prior to EHT scl-β is selectively expressed in hemogenic endothelial cells, a unique subset of VAE cells possessing hemogenic potential, whereas
scl-α is expressed later in nascent HSCs as they egress from VAE cells. In accordance with their expression, loss-of-function studies
coupled with in vivo imaging analysis reveal that scl-β acts earlier to specify hemogenic endothelium, which is later transformed by runx1 into HSCs. Our results also reveal a previously unexpected role of scl-α in maintaining newly born HSCs in the aorta-gonads-mesonephros. Thus, our data suggest that a defined hemogenic endothelial
population preset by scl-β supports the deterministic emergence of HSCs, and unravel the cellular mechanisms by which scl isoforms regulate HSC development.