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There is now evidence that the immediate cause of the loss of proliferative capacity in senescent cells is mediated by a specific inhibitor. If this tentative interpretation is correct, the next hurdle will be to determine mechanism(s) that regulate this putative senescence cell inhibitor that would, in effect, be the determinant of proliferative life span. One previously proposed hypothesis predicts that the decline of replicative activity is analogous to a checkpoint response to accumulated chromosomal damage (Rosenberger et al., 1991). Advances in our basic understanding of the nature of DNA damage, DNA repair mechanisms, and the response of eukaryotic cells to accumulated DNA damage provide a solid rationale for a reassessment of the causal role of the accumulation of chromosomal damage in cell senescence in vitro.

In kinetic studies with human diploid fibroblast-like (HDFL) cells carried out in heterokaryons and in monokaryons, we have observed a first-order relationship between the level or concentration of DNA polymerase alpha and the rate of initiation of new rounds of DNA synthesis. Because cell size is inversely proportional to the concentration of DNA polymerase alpha and presumably other replication factors, it is inversely related to the initiation of new rounds of DNA synthesis. An inverse relationship between cell size and clonogenic activity was also observed in both unsorted HDFL cells and in HDFL cells sorted on the basis of size. Experimental enlargement of cells by serum deprivation at low density resulted in changes in colony-forming ability that would be predicted by these studies. A causal relationship between the observed increase in cell size with advancing passage level and the loss of proliferative activity is suggested by these studies; in addition, cell size may be a useful biophysical marker for cellular aging.