Asymmetric distribution of DNA between daughter cells with final symmetry breaking during aging of human fibroblasts.

: Human fibroblasts proliferating in vitro go through functional modifications, lose progressively their capacity to divide, and enter finally a post-mitotic state. These events are supposed to reproduce the developmental steps taking place in vivo during aging of the organism. The gradual changes occurring through proliferation are incompatible with an even distribution of the genetic material during cell division. We measured the amount of DNA on pairs of daughter cells at different population doubling levels of human fibroblasts. It was found that at each doubling in a significant fraction of cells, the distribution of DNA between sister cells is asymmetric. The cell system is in a steady state through the different phases of the fibroblast population life span; then during the last mitoses when the cells enter the terminal phase IV there is symmetry breaking with a phase transition, the cells settling into a new state.