Aging of human fibroblasts is a succession of subtle changes in the cell cycle and has a final short stage with abrupt events

The proliferation of diploid human embryonic lung fibroblasts was analysed at different population doubling levels with growth curves, autoradiography after tritiated thymidine ([³H]TdR) labelling and staining of mitoses after incorporation of bromodeoxyuridine (BrdU). The parameters analysed allow for the first time the distinction between different changes in cell growth that occur during in vitro aging. The percent of slowly and rapidly dividing cells is steady during most of the lifespan; the number of cells capable of synthesizing DNA during a 24-h period is always high with a subtle decline during the second half of the lifespan up to the last 4-5 population doublings when the fall becomes pronounced. There is a constant decline of the rate of entrance into DNA synthesis and a stepwise increase in the sensitivity to cell cycle inhibition during cell crowding. Towards the middle of the lifespan, the population becomes more sensitive to low inocula which fail to accelerate the rate of entrance into the cycle. These changes lead to a final, abrupt stage of profound disorganization of proliferation taking place during the last 4-5 doublings.