In cultures of murine neoplastic mast cells, the duration of different phases of the division cycle (G1, S, G2, and mitosis [M]) was determined under optimal and several well-defined suboptimal growth conditions. Two methods of evaluation were applied to the same culture system: first, the relative number of G1, S, G2, and M cells was determined by pulse labeling of samples with thymidine-3H and subsequent radioautography in conjunction with a microfluorometric technique permitting rapid measurements of cellular DNA content; second, after pulse labeling with thymidine-3H, the variations with time of the mitotic labeling index were analyzed. Suboptimal culture conditions were obtained by reducing the concentration of single essential medium components (leucine, glucose, or serum) or by the addition of specific metabolic inhibitors (actinomycin D, amethopterin). Growth-limiting culture conditions resulted in increased generation times. Even under control conditions, the cell number doubling time exceeded the generation time, and this difference was more pronounced in suboptimal media. Under most of the suboptimal conditions tested, the increase in generation time was attributable primarily to an extended duration of the G1 phase. Under certain growth-limiting conditions, however, other phases were also prolonged. In addition, the variabilities of the generation time and of certain cell cycle phases were increased under suboptimal culture conditions. Results obtained by the two methods of evaluation were, in general, in good agreement with each other. Some differences were, however, observed and interpreted in terms of cell death and/or asymmetric frequency distributions of cell cycle parameters.

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