The proportion of cells labeled with tritiated thymidine as a function of population doubling level in cultures of fetal, adult, mutant, and tumor origin

The proportion of human fibroblast-like cells in culture able to incorporate tritiated thymidine ([³H]TdR) into their nuclei was shown autoradiographically to decrease with increased population doubling level, but variations were found in the pattern with which this occurred. Cultures of fetal origin (WI38) demonstrated consistently high proportions of labeled cells through 40 population doublings, after which a sharp decrease in the proportion of labeled cells was followed by culture termination. A diploid culture derived from an adult donor exhibited a more gradual decrease in the proportion of labeled cells beginning at low population doubling levels. Six Xeroderma pigmentosum cultures of four complementation groups showed decreases in the proportion of labeled cells intermediate between those of the fetal and adult cultures as expected from their donor ages. Diploid Fanconi's anemia and progeria cultures from young donors exhibited decreases in the proportion of labeled cells at low population doubling levels, and demonstrated sinusoidal fluctuations in the proportion of labeled cells as a function of population doubling level. A diploid culture derived from a benign tumor in an adult donor differed from the HeLa cell line in that it exhibited an accumulation of unlabeled cells with increased population doubling level, although this occurred at greater population doubling levels than in the fetal culture.