Hematopoietic stem cells localize to the endothelial cell layer in the midgestation mouse aorta

The emergence of the first adult hematopoietic stem cells (HSCs) during mammalian ontogeny has been under intense investigation. It is as yet unresolved whether these first HSCs are derived from intraembryonic hemangioblasts, hemogenic endothelial cells, or other progenitors. Thus, to examine the spatial generation of functional HSCs within the mouse embryo, we used the well-known HSC marker, Sca-1, and a transgenic approach with an Ly-6A (Sca-1) GFP marker gene. Our results show that this transgene marker is expressed in all functional HSCs in the midgestation aorta. Immunohistology of aorta-gonads-mesonephros (AGM) regions show that GFP(+) cells are specifically localized to the endothelial layer lining the wall of the dorsal aorta but not to the mesenchyme, strongly suggesting that HSC activity arises within a few cells within the endothelium of the major vasculature.