Sclerotome-derived vascular smooth muscle progenitors contribute to the hematopoietic stem cell specification niche

Haematopoietic stem cells (HSCs) are the self-renewing progenitors that continuously populate the haemato-immune cell lineages throughout life, and constitute the therapeutic component of bone marrow transplants. A major biomedical goal has been to understand the native specification of HSCs during embryonic development as a means to inform in vitro directed differentiation of pluripotent stem cells. Across vertebrate phyla, HSCs derive from haemogenic endothelium in the ventral floor of the primitive dorsal aorta (DA), also known as the descending aorta in mammals. Competent HSC-fated cells in the endothelium likely receive instructive signaling from neighbouring cells that constitute a "specification niche." We previously showed that experimental manipulations leading to defects in the most ventral compartment of the somite, the sclerotome, are correlated with HSC defects, raising the possibility that sclerotome patterning is required for HSC specification. Here we show that in zebrafish, specific sclerotome-derived cells contact the DA immediately prior to the emergence of HSCs. These cells subsequently give rise to vascular smooth muscle cells (VSMCs). When sclerotome patterning is disrupted, VSMCs are diminished, and HSC specification fails. We conclude that sclerotome-derived VSMC progenitors contribute to the embryonic HSC specification niche, most likely by providing unknown HSC inductive signals.