The study investigates the development of hematopoietic stem cell (HSC) activity in the mouse embryo, focusing on the aorta-gonad-mesonephros (AGM) region. The authors demonstrate that the AGM region contains potent and high-frequency HSC activity, capable of long-term, complete, and functional hematopoietic repopulation in adult irradiated recipients. This activity is observed before the appearance of yolk sac and liver stem cell activity. The AGM-derived cells produce all mature blood lineages, including erythroid, lymphoid, and myeloid cells, and can repopulate secondary and tertiary recipients. The results suggest that the AGM region may be the initial source of long-term repopulating cells, possibly migrating to the fetal liver by 11 days postcoitum (dpc). The study also discusses the potential mechanisms underlying the maturation of HSC activity in the mouse embryo, including the role of specific microenvironments and the sequence of hematopoietic activities in the yolk sac and AGM region.The study investigates the development of hematopoietic stem cell (HSC) activity in the mouse embryo, focusing on the aorta-gonad-mesonephros (AGM) region. The authors demonstrate that the AGM region contains potent and high-frequency HSC activity, capable of long-term, complete, and functional hematopoietic repopulation in adult irradiated recipients. This activity is observed before the appearance of yolk sac and liver stem cell activity. The AGM-derived cells produce all mature blood lineages, including erythroid, lymphoid, and myeloid cells, and can repopulate secondary and tertiary recipients. The results suggest that the AGM region may be the initial source of long-term repopulating cells, possibly migrating to the fetal liver by 11 days postcoitum (dpc). The study also discusses the potential mechanisms underlying the maturation of HSC activity in the mouse embryo, including the role of specific microenvironments and the sequence of hematopoietic activities in the yolk sac and AGM region.