Kinetics of Megacaryocyte Proliferation. ∗

Conclusion

The kinetics of the mega-caryocytic cell line of the rat bone marrow were studied using tritiated thymidine as a cell label. The changes in the percentage of labeled cells as a function of time after injection of the tracer were registered separately for arbitrarily chosen successive recognizable stages of megacaryocytic differentiation. Emphasis is put on the development of initially labeled cells into a stage of maturation corresponding to initially non-labeling cell forms. The following results were obtained: 1. The transit time for the most immature recognizable stage of megacaryocytic development to megacaryocytic disintegration is approximately 40 hours. 2. Evidence was obtained that the recognizable megacaryocytic elements originate from unrecognized precursors which continuously synthesize DNA for a period of at least 1 to 3 days prior to maturation into recognizable megacaryocytic precursors. 3. The immature megacaryocytic cells able to synthesize DNA take up more H³ thymidine than the rest of the bone marrow cells. This is consistent with polyploidy. 4. The process of nuclear lobulation is not accomplished by the end of DNA synthesis, thus being comparable to nuclear segmentation in neutrophilic granulocytes. This latest phase of maturation is relatively long (approximately 25–30 hours) as compared to the phase during which recognizable megacaryocytic precursors are able to synthesize DNA (less than 15 hours).