DNA synthesis in the human diploid cell strain WI-38 during in vitro aging: an autoradiography study

The percent of cells in a WI-38 cell population which did not incorporate tritiated thymidine (3H-TdR) was determined by autoradiography from the measurement of percent nonlabeled nuclei and the ratio of total cell numbers at the initiation and at the termination of exposure to 3H-TdR. This percentage was minimally affected by factors influencing the proliferation of labeled cells, but was dependent on the population doubling level (PDL). The results suggest the presence, in a proliferating WI-38 population, of subpopulation(s) with an extremely slow rate of S phase entrance. The parameter was useful in estimating, empirically, the doubling potential of a cell population.