Proto-oncogene c-mpl is structurally homologous with the hematopoietic growth factor receptor superfamily. The ligand for c-mpl was purified and its gene cloned. Extensive functional studies revealed that the ligand for c-mpl plays a crucial role in megakaryocytopoiesis and platelet production, hence, this ligand proved to be the long-sought hematopoietin, thrombopoietin (TPO). We briefly review here the role for TPO in early hematopoiesis, based on our in vitro data obtained using a serum-free culture system. TPO in combination with the ligand for c-kit (SF) or interleukin-3 (IL-3) but not TPO alone supported the growth of murine primitive hematopoietic progenitors. Studies on lineage expression indicated that the progenitors supported by TPO plus SF or TPO plus IL-3 are multipotential. Delayed addition experiments demonstrated that TPO has the potential to effectively support the survival of primitive hematopoietic progenitors. TPO also hastened IL-3-dependent growth of progenitors by shortening the time required for cell-cycling. While size of the colonies did not differ between colonies supported by IL-3 alone and those supported by IL-3 plus TPO, the incidence of megakaryocyte-containing colonies in cultures supported by IL-3 plus TPO was higher than that in cultures supported by IL-3 alone. Taken together, TPO as a single factor can support the survival of hematopoietic progenitors and TPO synergizes with SF or IL-3 to be active on early multipotential hematopoietic progenitors. These findings suggest that the function of TPO initially thought to be restricted to the megakaryocytic lineage proved to be greater in hematopoiesis. Reports from other laboratories regarding the involvement of TPO in early hematopoiesis are also discussed.
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