Contribution of Sclerotome to the Hematopoietic Stem Cell Niche and Vascular Smooth Muscle

Hematopoietic stem cells (HSCs) are a self-renewing population that sustains production of blood lineages throughout life. Vertebrate HSCs are first specified in the embryo, arising from hemogenic endothelium in the ventral wall of the dorsal aorta (DA). The cell types within this three-dimensional environment and the signals they provide to promote HSC development are not well defined. A better understanding of native embryonic HSC specification niche could inform efficient directed differentiation of true HSCs from induced pluripotent stem cells for clinical and research applications. We previously showed that wnt16 is required for development of zebrafish HSCs, as well as normal patterning of the ventral somite, the sclerotome. We hypothesized that normal sclerotome patterning is required for HSC specification by contributing to the cellular environment that presents inductive signals. To follow sclerotome development in living embryos, we developed transgenic reporter zebrafish where fluorophore expression is driven by regulatory elements from the sclerotome gene, pax1a. We show that sclerotome-derived cells migrate to and make contact with hemogenic endothelium of the DA shortly before initiation of the definitive hematopoietic program, demonstrating that they are present at the right time and place to present specification signals. By live imaging and profiling of sorted cells, we show these cells also give rise to arterial vascular smooth muscle cells. Loss-of-function analyses demonstrate a requirement for multiple sclerotome genes, for both patterning of sclerotome, and subsequent specification of HSCs and downstream hematopoietic lineages. Our data indicate that sclerotomal cells contribute to the HSC specification niche of the early embryo, and form a basis for defining the niche architecture and signals that drive definitive hematopoietic programming.