THE transition probability model of the cell cycle1,2 was proposed in order to account for the high variability of intermitotic times always observed in cell populations. According to this model, the cell cycle is divisible into an A state of indeterminate length, and a B phase of fixed length during which the activities of the cell are directed towards division. Cells enter the A state sometime after mitosis. The transition from the A state to the B phase in G1 is a probabilistic event, and it is the stochastic nature of this transition that generates the observed variation. Although the model provides an adequate explanation of the proliferation kinetics in vitro2–6 and in vivo1,7,8, there is direct experimental support, time-lapse cinematography studies of intermitotic times only for those in vitro2–5. Therefore, the applicability of the new model in vivo is still controversial5. We show here that an original kinetic parameter, the mitotic coincidence of neighbouring cells (M c o), can be predicted accurately, on the basis of the new model, using independently measurable variables, for cell populations growing in vivo. We present the theoretical basis for the computation of this parameter according to the classical model and according to the new one. Our results in chick embryo hepatocytes, in close agreement with the latter, are direct evidence of the applicability of the transition probability model in vivo.
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