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The doubling potential of several hundred clones derived from WI-38 and WI-26 cell cultures has been determined. Clones were isolated at various population doubling levels (PDLs) during the finite in vitro life-span of the mass (uncloned) cultures. In all cases, there was a large variation in population doubling potential (or life-span) among the clones isolated from a single mass culture. When clones were isolated from mass cultures which had undergone eight or nine population doublings, only about 50% of the clones were capable of more than eight population doublings. This percentage was further reduced when clones were isolated from mass cultures at higher PDLs. Mass cultures appear to be composed of two subpopulation classes: one with a low population doubling potential, and the other with a higher population doubling potential. Nevertheless, the highest doubling potential observed in clones isolated from any single culture was about the same as the doubling potential of the mass culture from which single cells were taken.
Smith et al. (Mon,) studied this question.