The extent to which mouse megakaryocyte progenitor cells (colony- forming unit-megakaryocyte, CFU-M) can proliferate in semisolid cultures prior to endomitosis, and conditions that may regulate that differentiation step, were investigated. The proliferative capacity of CFU-M was determined by estimating the number of megakaryocytes per colony. A bimodal distribution was observed (modal values, 10–15 and 25- 30 cells/colony), indicating that separate megakaryocyte progenitor cells may be biased in their capacity for proliferation versus endomitosis. Differences were observed in the cell cycle characteristics of CFU-M as determined in vivo and in vitro that suggest that maturation of CFU-M into megakaryocytes may be regulated within the marrow by control of the cell cycle of the megakaryocyte precursor cell.

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