Emergence of intraembryonic hematopoietic precursors in the pre-liver human embryo

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 prenata l and postnatal blood stem cells, An homologous intraembryonic site of stem cell emergence was previously identified at 5 weeks of human gestation, wh en hundreds of CD34(++) Lin(-) high-proliferative potential hematopoietic c ells border the aortic endothelium in the preumbilical region, In the prese nt study, we have combined immunohistochemistry: semithin section histology , fluorescence-activated cell sorting and blood cell culture in an integrat ed study of incipient hematopoiesis in the human yolk sac, truncal arteries and embryonic liver from 21 to 58 days of development, The chronology of b lood precursor cell emergence in these distinct tissues suggests a pivotal role in the settlement of liver hematopoiesis of endothelium-associated ste m cell clusters, which emerge not only in the dorsal aorta but also in the vitelline artery Anatomic features and in vitro functionality indicate that stem cells develop intrinsically to embryonic artery walls from a presumpt ive territory whose blood-forming potential exists from at least 24 days of gestation.