The human embryo, but not its yolk sac, generates lympho-myeloid stem cells: Mapping multipotent hematopoietic cell fate in intraembryonic mesoderm

We have traced emerging hematopoietic cells along human early ontogeny by c ulturing embryonic tissue rudiments in the presence of stromal cells that p romote myeloid and B cell differentiation, and by assaying T cell potential in the NOD-SCID mouse thymus. Hematogenous potential was present inside th e embryo as early as day 19 of development in the absence of detectable CD3 4(+) hematopoietic cells, and spanned both lymphoid and myeloid lineages fr om day 24 in the splanchnopleural mesoderm and derived aorta where CD34(+) progenitors appear at day 27. By contrast, hematopoietic cells arising in t he third week yolk sac, as well as their progeny at later stages, were rest ricted to myelopoiesis and therefore are unlikely to contribute to definiti ve hematopoiesis in man.