Fetal bone marrow CD34⁺CD41⁺ cells are enriched for multipotent hematopoietic progenitors, but not for pluripotent stem cells

We have investigated the expression of CD41a (gpIIbIIIa) on a subpopulation of human fetal bone marrow (FBM) CD34⁺ progenitor cells. Human FBM CD34⁺Lin^- cells were subfractionated into CD41a⁺ and CD41a^- subpopulations by flow cytometry. All the megakaryocyte colony-forming cells (CFU-MK) and almost all the burst-forming units-megakaryocyte (BFU-MK) were found within the CD41a⁺ subpopulation. In addition, a 14-fold greater number of granulocyte-macrophage colony-forming units (CFU-GM) and a five-fold greater number of mixed lineage progenitor cells (CFU-mix) were observed within the CD34⁺Lin^-CD41a⁺ subpopulation compared to the CD34⁺Lin^-CD41a^- subpopulation. The high proliferative potential of CD34⁺Lin^-CD41a⁺ cells was demonstrated by their capacity to expand in in vitro culture containing human plasma and recombinant Mpl ligand (thrombopoietin [Tpo]) with production of over 80% CD41b⁺ (gpIIb⁺) MKs. However, in long-term bone marrow cultures, the CD34⁺Lin^-CD41a^- population contained a significantly higher frequency of cobblestone area-forming cells (CAFC) than the CD34⁺Lin^-CD41a⁺ population, indicating the presence of a primitive hematopoietic stem cell (HSC) population within the CD34⁺Lin^-CD41a^- subset. These data suggest that fetal CD34⁺Lin^-CD41a⁺ cells are enriched for MK progenitor cells (CFU-MK and BFU-MK), myeloid progenitors, and CFU-mix but do not contain the more primitive CAFC.