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Hepatic hematopoiesis in the mouse embryo is preceded by two hematopoietic waves, one in the yolk sac, and the other in the paraaortic splanchnopleura, the presumptive aorta-gonad-mesonephros region that gives rise to prenatal and postnatal blood stem cells. An homologous intraembryonic site of stem cell emergence was previously identified at 5 weeks of human gestation, when hundreds of CD34⁺⁺ Lin⁻ high-proliferative potential hematopoietic cells border the aortic endothelium in the preumbilical region. In the present study, we have combined immunohistochemistry, semithin section histology, fluorescence-activated cell sorting and blood cell culture in an integrated study of incipient hematopoiesis in the human yolk sac, truncal arteries and embryonic liver from 21 to 58 days of development. The chronology of blood precursor cell emergence in these distinct tissues suggests a pivotal role in the …