Continuous observation of living cells by phase-contrast time lapse microcinematography revealed different origins of multinuclear cells and cells with lobed nuclei in the hematopoietic progenitor cell series, which must be classified as megakaryoblasts, the determinated progenitor cells of the megakaryopoietic series. Hematopoietic progenitor cells with single nuclei of nuclear diameter 5–13 μ m, the promegakaryoblasts, may become polyploid by various means: by postmitotic cytoplasmic fusion, by consecutive nuclear fusion, by endomitosis, or by nuclear amitosis or lobe formation presumably after endoreduplication and nuclear growth. Including amitotic nuclear division in the mitotic rate of normal progenitor cells nearly the half of all these cells in G2 phase (0.4) produce in vitro octoploid megakaryoblasts, respectively osteoclasts. Thrombopoietin-rich sera in the medium enhance the mitotic index of the hematopoietic progenitor cell line from 0.1 to 0.25, but do not change the proportion of polyploid mitoses.

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