What is the connection between senescence and apoptosis?
They are both mechanisms of damaged cells to avoid replication.
Apoptosis is the process in which a cell decides to kill itself. Senescence is an irreversible arrest of cell proliferation while the cell maintains metabolic function (often associated with cellular ageing).
Both apoptosis an senescence are induced when a cell senses that the DNA in the cell is damaged . When a cell would keep on proliferating despite of DNA damage, it is very well possible that the newly generated cells don't function properly and even derail becoming cancer cells (with unlimited proliferative capacity).
Interestingly, senescent cells are able to resist apoptosis; they only stop proliferating, but are still able to secrete molecules that influence neighbouring cells. These secreted factors can even have tumor-promoting properties .
It appears that senescence evolved alongside apoptosis to suppress formation of tumors. However, apoptosis does a better job at this and it is still poorly understood why senescence hasn't been competed out by apoptosis. Senescence must have some advantage over apoptosis, but this is still unclear.
If you want to read more about this very interesting subject, I can recommand the article of Child et al. 2014 in EMBO reports .