The limited in vitro lifetime of human diploid cell strains

The time at which human diploid cell strains can be expected to cease dividing in vitro (Phase III) is not a function of the number of subcultivations but rather of the number of potential cell doublings. Each clonable cell within the population is endowed with the same doubling potential (50 ±10). Cells of the same strain, but with different “doubling potentials”, were mixed. Phase III in such mixed populations occurs at that time when the “youngest” cell component is expected to reach Phase III. The “older” component has no effect on the time at which Phase III is expected to take place in the “younger” component. An ancillary conclusion that Phase III cannot be explained by the presence of a latent virus, mycoplasma or media composition is confirmed. Human diploid cell strains derived from adult lung have a significantly lower doubling potential in vitro than do fetal strains. The Phase III phenomenon may be related to senescence in vivo. The cellular theory of aging must be related to normal cells in vitro and not to heteroploid cell lines. The former have a finite period of multiplication; the latter are indefinitely cultivable. In vivo experiments also indicate that transplanted normal tissue has a finite lifetime. Chromosome anomalies occurring in Phase III may be related to such anomalies occurring in the cells of older animals, including man. The survival curves obtained with human diploid cell strains are comparable to “multiple-hit” or “multiple-target” curves obtained with other biological systems where an initial threshold dose is required before an exponential form of the curve is established. Whatever cell component(s) may be involved in the finite lifetime of human diploid cell strains, the ultimate accumulation of nondividing cells could be the result of accumulated damage to a single cellular target or to inactivation of many targets.