[Characterization of hematopoietic progenitor cells during the human embryonic development]

In a search for assays that might facilitate identification of pluripotent stem cells with extended potentialities, we analysed the properties of hematopoietic progenitor cells detected in the extraembryonic yolk sac and in the intraembryonic part of human embryos between approximately 28 and 45 days of development. Cells from the yolk sac, the liver rudiment and the remainder of the embryo were plated in semi solid methylcellulose colony-assays supplemented with combinations of cytokines. Large BFU-E-derived colonies as well as granulocytic colonies were detected in every yolk sac sample. Interestingly, progenitor cells were also detected in the intraembryonic part, outside the liver and a subclass of these progenitors were detected that generated large granulomacrophagic colonies capable of generating secondary colonies when replated. These were preferentially located in the embryo. Colony-assays initiated with CD34+ cells sorted from the different tissues confirmed these data. These results first indicate that embryonic progenitors exhibit unique phenotypic features, and second, analysis of the distribution of progenitors between the different tissues may suggest the existence of other sites of hematopoietic production. More detailed analysis of the potentialities of these progenitors should now be assessed in vitro in cocultures assays and in vivo by reconstituting immunodeficient mice.