Definitive hematopoietic stem cells first develop within the major arterial regions of the mouse embryo

The aorta-gonad-mesonephros (AGM) region is a potent hematopoietic site wit hin the mammalian embryo body, and the first place from which hematopoietic stem cells (HSCs) emerge. Within the complex embryonic vascular, excretory and reproductive tissues of the AGM region, the precise location of HSC de velopment is unknown. To determine where HSCs develop, we subdissected the AGM into aorta and urogenital ridge segments and transplanted the cells int o irradiated adult recipients. We demonstrate that HSCs first appear in the dorsal aorta area. Furthermore, we show that vitelline and umbilical arter ies contain high frequencies of HSCs coincident with HSC appearance in the AGM, While later in development and after organ explant culture we find HSC s in the urogenital ridges, our results strongly suggest that the major art eries of the embryo are the most important sites from which definitive HSCs first emerge.