Regulation of human fibroblast growth rate by both noncycling cell fraction transition probability is shown by growth in 5-bromodeoxyuridine followed by Hoechst 33258 flow cytometry.

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Abstract
Growth of human diploid fibroblasts in the presence of 5-bromodeoxyuridine, followed by flow cytometric analysis of DNA-specific fluorescence with Hoechst 33258 dye, allows quantitation of the proportion of cells that have not cycled, as well as those in G1 and G2 of 2 subsequent cell cycles. This technique allows rapid and accurate quantitation of the growth fraction and G1/S transition rate of these cells. The cell cycle kinetics of human diploid fibroblasts at all population doubling levels reveal 2 components: cycling cells showing a probabilistic rate of G1/S transition, and a variable proportion of noncycling cells. Both the transition probability (rate of exit from G1) and the noncycling proportion of cells change systematically as a function of serum concentration and as a function of population doubling level. The data suggest the existence of an underlying heterogeneity in the population of human diploid fibroblasts with respect to the capacity to divide in the presence of a given concentration of mitogen. Models of cell cycle kinetics must be modified to include regulation of growth by changes in the fraction of cycling cells, as well as by changes in the rate of exit from G1.