A cell is 'senescent' when is has permanently left the cell-cycle. This can be caused by stresses, or by reaching the 'Hayflick limit' (the cell has reached its replicative lifespan, as defined by its telomeres).
Cells cultured in vitro can be used as models to study senescence (or sometimes 'ageing', although the distinction there is not necessarily within scope of this question) by growing the same population of cells for a long time (many passages). A common method to identify a senescent cell population is to use beta-galactosidase staining.
I was wondering to what degree senescent cells in culture (identified by beta-gal staining) are actually comparable to those you might expect in vivo. I ask because it seems to me that a terminally senescent cell may not actually survive long in an organism, so what we refer to as senescent cells in vivo are actually pre-senescent cells? I haven't got any basis for this, just a feeling. (Very scientific I know).