Long G1 or G0 state: A method of resolving the dilemma for the cell cycle of an in vivo population

Abstract

A technique is described using standard autoradiographic procedures whereby it is possible to study an in vivo cell population and to decide whether the cells in the population have a long G1 or whether they pass into an out-of-cycle G0 state. The method has been applied to the epithelium of the hamster cheek pouch and involves continuous infusion (or repeated injections) of tritiated thymidine into the animals for a time comparable to the turnover time of the cell population. The population is sampled after various period of infusion and the proportion of cells labelled is compared with the proportion that would be predicted by the G1 and G0 models. Using this technique it is concluded that the proliferating cells of the hamster cheek pouch have a mean cell cycle time of approx. 140 h. More than 90 per cent of this period is occupied by a finite pro-synthetic G1 phase, and few, if any, of the cells are in an out-of-cycle G0 state.