AORTA-ASSOCIATED CD34(-CELLS IN THE EARLY HUMAN EMBRYO() HEMATOPOIETIC)

Hematopoiesis is established from circulating blood stem cells that se ed the embryonic rudiments of blood-forming tissues, a basic notion in developmental hematology. However, the assumption that these stem cel ls originate from the extraembryonic mesoderm, where primitive hematop oiesis is initiated by intrinsic precursors, has been reconsidered aft er analysis of blood cell development in avian embryo chimeras: yolk-s ac-derived stem cells do not contribute significantly to the definitiv e blood system, whose first forerunners develop independently along th e ventral aspect of the embryonic aorta. Recently, the homologous intr aembryonic tissues of the mouse have been submitted to sensitive in vi vo and in vitro assays, which showed that they also harbor multipotent ial hematopoietic stem cells. We have now identified a dense populatio n of hematogenous cells, marked by the surface expression of the CD34 glycoprotein, associated with the ventral endothelium of the aorta in the 5-week human embryo. Therefore, we extend to the human species the growing evidence that intraembryonic hematopoietic cells developing i ndependently of the yolk sac might be the real stem of the whole blood system. (C) 1996 by The American Society of Hematology.