Cumulative population doublings as the determinant of chick cell lifespan in vitro

Chick embryo fibroblasts were maintained at confluency for up to 35 days in medium containing 0.5% or 0.75% fetal bovine serum or 2.5% or 5.0% horse serum. At weekly intervals cells were subcultured and serially propagated in medium containing 10% FBS until their replicative lifespans were completed. The results showed that the replicative lifespan of embryonic chick fibroblasts was dependent on the cumulative number of population doublings undergone by the culture and was not related to the calendar time cells were in culture. Further characterization of 0.75% FBS maintained chick cells returned to 10% FBS medium showed that cells had protein contents and incorporated ³H-thymidine into DNA at a rate that resembled that of young cells, despite an advanced chronological age.