Colony formation and colony size do not reflect the onset of replicative senescence in human fibroblasts

Replicative senescence of human fibroblasts in vitro has been used as a model for in vivo aging. The onset of replicative senescence varies between several months to years. A colony formation assay, critically dependent on growth speed, can be performed within weeks, and has been reported being an indicator for the onset of replicative senescence. Earlier we could not find a correlation between growth speed in mass cultures and onset of replicative senescence of human fibroblast strains. Therefore, we studied the colony formation assay in 23 fibroblast strains that varied widely in their replicative capacity. Neither the number nor the size of colonies was related to the onset of replicative senescence. The number of cells within the colonies was modestly correlated to the growth speed of the mass cultures. We conclude that the colony formation assay does not reflect the onset of replicative senescence in human fibroblasts.