The effects of division rate-limiting amounts of fetal bovine serum on the proliferation and aging of cultured chick cells.

Chick embryo fibroblasts serially propagated in media containing division rate-limiting amounts of fetal bovine serum underwent premature culture senescence as illustrated by accelerated declines in the number of cells incorporating 3H-thymidine, increased population doubling times, reduced cell densities at subcultivation, and reduced replicative life-spans compared to cells grown in medium containing non-rate-limiting amounts of serum. Low serum serially propagated "senescent" cultures returned to 10% serum containing medium had proliferative rates, incorporated 3H-thymidine, and attained saturation densities at confluency similar to younger cells. "Senescent" cells serially propagated in low serum and returned to 10% serum, achieved life-spans similar to cells continuously grown in the presence of 10% serum. The results of these and other studies show that cells serially propagated in the presence of division rate-limiting amounts of fetal bovine serum, or at high inoculation densities, accumulate a substantial number of cells in the population during exponential growth conditions that are not senescent but are prevented from entering DNA synthesis because of mitogen limitations. Our results indicate that the amount of serum mitogen in the growth medium affects only the rate at which cells express their genetically predetermined replicative potential and not the replicative life-span per se. These results are discussed in relation to the techniques that should be employed for studying cellular aging and the mechanism of senescent cell formation.