Effects of pulse labelling with tritiated thymidine on the circadian rhythmicity in epidermal cell-cycle distribution in mice

The influence of pulse labelling with 50 microCi tritiated thymidine ( [3H]TdR) (2 microCi/g) on epidermal cell-cycle distribution in mice was investigated. Animals were injected intraperitoneally with the radioactive tracer or with saline at 08.00 hours, and groups of animals were sacrificed at intervals during the following 32 hr. Epidermal basal cells were isolated from the back skin of the animals and prepared for DNA flow cytometry, and the proportions of cells in the S and G2 phases of the cell cycle were estimated from the obtained DNA frequency distributions. The proportions of mitoses among basal cells were determined in histological sections from the same animals, as were the numbers of [3H]TdR-labelled cells per microscopic field by means of autoradiography. The results showed that the [3H]TdR activity did not affect the pattern of circadian rhythms in the proportions of cells in S, G2 and M phase during the first 32 hr after the injection. The number of labelled cells per vision field was approximately doubled between 8 and 12 hr after tracer injection, indicating an unperturbed cell-cycle progression of the labelled cohort. In agreement with previous reports, an increase in the mitotic index was seen during the first 2 hr. These data are in agreement with the assumption that 50 microCi [3H]TdR given as a pulse does not perturb cell-cycle progression in mouse epidermis in a way that invalidates percentage labelled mitosis (PLM) and double-labelling experiments.