Figure 1.

A schematic representation of the processes that lead to the establishment of cellular senescence. The progression of senescence has been separated into several components: (1) triggering events; (2) initiation of the senescence response; (3) entry into senescence; and (4) a further deepening of senescence phenotypes. Stages 2 and 3 can be separated by a period of attempted repair, which may result in recovery and survival in a healthy postmitotic state, or even resumption of cell proliferation. Entry into senescence is likely the result of the acquisition of irreparable damage, followed by an extended period during which additional degenerative changes can take place, evolve, and accumulate. On the right are illustrated some of the many molecular phenotypes, or biomarkers, that have been associated with cellular senescence. This listing is not meant to be comprehensive and similarly, the order is not meant to imply the chronological acquisition of these features. The construction of such a timeline we believe constitutes an important challenge for the field going forward. DDR, DNA damage response.