Dominance of the Senescent Phenotype in Heterokaryons Between Replicative and Post-Replicative Human Fibroblast-Like Cells

In heterokaryons between senescent and young diploid fibroblast-like cells, dominance of the former with respect to nuclear DNA synthesis (incorporation of ³H]thymidine) was demonstrated. For identification of the respective partners, double-layer autoradiography was used after the old cells were labeled with ³H]methionine and the young cells were labeled with ¹⁴C]thymidine. Synchrony of nuclear labeling (i.e., all nuclei in a cell labeled with ³H]thymidine) was observed in the majority of di- and polykaryons during the second and third of three 24-hr periods of labeling with ³H]thymidine. The results are compatible with either terminal differentiation or error theories of clonal senescence.