The typical growth of primary fibroblast cultures is characterized by three different phases. After initiation of the culture (phase 1), cells rapidly proliferate (phase 2). Hereafter the growth rate starts to decline (phase 2b), followed by degeneration of the culture (phase 3). The commitment theory explains the finite lifespan and predicts that cells become senescent with some fixed probability after a specific number of cell divisions. The population growth rate drops when the first committed cell stops dividing and the culture enters senescence if the last cell becomes postmitotic. Causal relationships regarding to the different phases, especially concerning the onset of senescence, are unclear.
We obtained 68 skin biopsies (3 mm punch biopsies) from participants of the 85-Leiden plus study, aged 90 years. Up to now we cultured these fibroblasts under standard conditions for more than 350 days with striking differences in growth kinetics and cell morphology.
Dividing the growth curves into above-mentioned phases with corresponding inflexion points, we found significant interindividual differences in the onset of cellular senescence (phase 2b/3). The phase transitions were, independently of previous interindividual differences in cell morphology, accompanied with senescent cell phenotypes (increase of cell volume, bizarre forms, granular shape) and an increase of senescence specific markers, e.g. beta-galactosidase. First preliminary results regarding to underlying causes showed no influence of gender or common age related diseases.
Even in a quite homogenous study population of relatively healthy elderly the replicative capacity differs widely. Further research will focus on determinants, which have influence on our observations on cellular level.