Derivation of the frequency distributions of cycle durations from continuous labeling curves

In cell populations that are continuously exposed to radioactive thymidine over a long period, all proliferating cells become labeled as they pass through their DNA-replicating phase. The continuous labeling curve (CLC) shows the percentage of labeled cells versus time. Expected CLCs are calculated for cell populations with arbitrary frequency distributions of cycle durations. By optimizing the parameter values of a general probability function in the formula for CLC, frequency distributions of cycle durations are estimated from experimental CLCs. The analysis of several fetal rat tissues shows that the cycle durations vary over quite a wide range within the same tissue.