At least two classes of human megakaryocyte progenitor cells have been identified: the burst-forming unit megakaryocyte (BFU-MK) and the colony-forming unit megakaryocyte (CFU-MK). The BFU-MK is the most primitive progenitor cell committed to the megakaryocytic lineage. The CFU-MK appears to be a more differentiated megakaryocyte progenitor cell and is thought to be ultimately a descendant of the BFU-MK. A number of recombinant cytokines have recently been shown to be able to promote megakaryocyte colony formation in vitro. Recombinant GM-CSF and IL-3, in particular, have the ability to promote both CFU-MK- and BFU-MK-derived colony stimulatory formation. The activities of these two cytokines on in vitro megakaryocytopoiesis are also additive. Recent results of clinical trials in both primates and humans, in which these glycoproteins were administered in vivo, suggest that these cytokines, both alone and in combination, can enhance in vivo thrombopoiesis and therefore may be potentially useful in the treatment of thrombocytopenic disorders.
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