Megakaryocytes increase in size within ploidy groups in response to the stimulus of thrombocytopenia.

Experiments were done to determine if sizes of megakaryocytes within a defined maturation stage were strictly determined by amount of nuclear DNA. Normal mice, mice recovering from an acute episode of thrombocytopenia induced by a single injection of heterologous antiplatelet serum (APS), and mice with sustained thrombocytopenia from daily injections of APS were examined. Areas of mature megakaryocytes were measured in bone marrow smears stained with polychromatic stains. The nuclear DNA content of the same cells was then measured microspectrophotometrically after staining by the Feulgen reaction. Normal megakaryocytes showed a trimodal, lognormal distribution of nuclear chromophore, corresponding to 8n, 16n, and 32n with smaller numbers of 4n and 64n cells; 16n was the predominant ploidy class. In response to thrombocytopenia, ploidy values shifted: the proportions of 8n and 16n cells decreased; 32n and 64n cells increased; 128n megakaryocytes occasionally appeared. These shifts were accompanied by an increase in the average size of all megakaryocytes. In addition to shifts to higher ploidy values, megakaryocytes within ploidy groups became larger than normal megakaryocytes of the same ploidy especially in the mice with sustained thrombocytopenia. These findings show that megakaryocyte size in thrombocytopenic mice is influenced by factors other than the ploidy and maturity of the cell.