A mechanism of intracellular timing and its cooperation with extracellular signals in controlling cell proliferation and differentiation, an amended hypothesis.

Various observations suggest that an intracellular timer is involved in the control of cell proliferation and differentiation that supplements control by extracellular signaling and depends on quantitative relations between cytoplasm and nucleus. To further elucidate the mechanism of this timer, we examined the results of experiments with mice in which cell cycle regulating genes were inactivated: the inactivation of negative cell cycle regulators extends cell proliferation, whereas inactivation of positive regulators decreases cell proliferation. We conclude that this is caused in the former case by shortening of G1 which decreases the cytoplasmic growth rate per cell cycle, whereas in the latter case this rate is increased due to G1 prolongation. This is consistent with our hypothesis according to which the cytoplasmic/nuclear ratio must increase to a certain level to induce end stage differentiation and cell cycle arrest. A new basis of this hypothesis is the fact that end stage differentiation requires large quantities of membranous cytoplasmic structures that the cells are unable to produce de novo. Embryonic cells, however, possess only few of these structures. The only feasible way to multiply these structures is by growing more cytoplasm per cell cycle than needed for a doubling so that successively, the level of the cytoplasmic/nuclear ratio is reached that is required for differentiation. A consequence is that the cytoplasmic growth rate per cell cycle determines the number of amplification divisions. We suggest that the differentiation signal may be triggered when a differentiation-preventing protein (for example Bcl-2) is diluted out by the expansion of cytoplasmic membrane structures, thus simultaneously determining the cell size. The intracellular timer and extracellular signals cooperate in adjusting cell production to the organism's need and in determining when and how the cells respond to extracellular signals or transmit extracellular signals.