Skin fibroblasts from aged fischer 344 rats undergo similar changes in replicative life span but not immortalization with caloric restriction of donors

We have compared the in vitro replicative life span and characteristics of immortalization of skin flbroblast cultures derived from ad libitum-fed and caloricrestricted Fischer 344 rats of 6, 24, and 29 months of age. Cells from all 6-, 24-, and 29-month-old animals showed a gradual decline in proliferative potential as evidenced by decreases in harvest density, in the fraction of cells initiating DNA synthesis, and in the number of population doublings per passage. These declines were accompanied by morphological changes including cell enlargement. The replicative life span prior to immortalization decreased significantly with donor age (P < 0.0001), while caloric restriction had no effect on the cumulative population doubling level. Prior to immortalization mitotic cells from all cultures showed a normal rat karyotype. Postcrisis cultures tended to have more polyploid cells but there were no characteristic or specific chromosomal changes found in the cells with an immortalized phenotype. Interestingly, fibroblasts derived from caloric-restricted animals had a significantly slower growth rate through the tenth week after immortalization (P < 0.0057). When these cultures were seeded at one-quarter the normal seeding density, to favor the outgrowth of the fastest growing cells, a population with a more "transformed" phenotype emerged.