Extravascular endothelial and hematopoietic islands form through multiple pathways in midgestation mouse embryos

The de novo generation of hematopoietic cells occurs during midgestation when a population of endothelial cells called hemogenic endothelium transitions into hematopoietic progenitors and stem cells. In mammalian embryos, the newly formed hematopoietic cells form clusters in the lumens of the major arteries in the embryo proper and in the vascular plexus of the yolk sac. Small clusters of hematopoietic cells that are independent of the vasculature (referred to here as extravascular islands) were shown to form in the mesentery during vascular remodeling of the vitelline artery. Using three-dimensional imaging of whole mouse embryos we demonstrate that extravascular budding of hematopoietic clusters is a more widespread phenomenon that occurs from the vitelline and the umbilical arteries both proximal to the embryo proper and distal in the extraembryonic yolk sac and placenta. Furthermore, we show that there are several mechanisms by which hematopoietic clusters leave the arteries, including vascular remodeling and extrusion. Lastly, we provide static images suggesting that extravascular islands contribute to the formation of new blood vessels. Thus, extravascular islands may represent a novel mechanism of vasculogenesis whereby established vessels contribute endothelial and hematopoietic cells to developing vascular beds.