A NEW METHOD OF ESTIMATING THE CELL CYCLE TIME IN EPITHELIAL TISSUES OF LONG GENERATION TIME

1. Using standard autoradiographic techniques, the percentage of labelled mitoses and the numbers of labelled cells in the various layers of the epithelium of the hamster cheek pouch have been studied at various times after a single injection of ³H]thymidine. 2. A new method for determining the cell cycle time of the basal layer cells is described and compared with the techniques previously used. This method is based on the rate at which labelled cells leave the basal layer and enter the spinous layer above it. 3. Using this technique, the mean cell cycle time of the basal cells has been estimated as 130 ± 7 hr; a value which compares favourably with the estimate of 135 ± 17 hr determined with similar assumptions from the length of the S period and the labelling index. 4. The technique enables an estimate to be made of the fraction of cells in the basal layer that are proliferating (the growth fraction). This is found to be indistinguishable from 1. 5. The mean time taken by cells in travelling from the basal layer to the keratin, i.e. the transit time through the spinous layer, has been found to be approximately 186 hr.