The versican-hyaluronan complex provides an essential extracellular matrix niche for Flk1+ hematoendothelial progenitors

•Versican- and hyaluronan-deficient mouse mutants fail to form yolk sac vasculature and blood.•Mouse embryonic stem cells with CRISPR-Cas9 Vcan mutation have impaired angiogenesis and hematopoiesis.•The versican-hyaluronan complex is strongly associated with emerging Flk1+ hematoendothelial precursors.•Versican and hyaluronan are each destabilized by the absence of the other.•The versican chondroitin sulfate chains sequester vasculogenic factors VEGF and Indian Hedgehog.