Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the
adult blood-forming system since hematopoietic stem and progenitor cells
(HSPCs) arise from ventral aortic endothelium in all vertebrate animals
studied. Work in zebrafish has demonstrated that arterial and venous
endothelial precursors arise from distinct subsets of lateral plate
mesoderm. Here, we profile the transcriptome of the earliest detectable
endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that
tissue-specific EC programs initiate much earlier than previously
appreciated, by the end of gastrulation. Classic studies in the chick
embryo showed that paraxial mesoderm generates a subset of somite-derived
endothelial cells (SDECs) that incorporate into the dorsal aorta to
replace HSPCs as they exit the aorta and enter circulation. We describe a
conserved program in the zebrafish, where a rare population of endothelial
precursors delaminates from the dermomyotome to incorporate exclusively
into the developing dorsal aorta. Although SDECs lack hematopoietic
potential, they act as a local niche to support the emergence of HSPCs
from neighboring hemogenic endothelium. Thus, at least three subsets of
ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic
ECs, and SDECs. Taken together, our findings indicate that the distinct
spatial origins of endothelial precursors dictate different cellular
potentials within the developing dorsal aorta.