Some cell types do seem to divide in vivo far more often than anyone has
persuaded them to divide in vitro. This may be because of unidentified
shortcomings of the culture conditions, or it may be a purely statistical
consequence of the size of laboratory cultures (as suggested by Kirkwood in
1975 [1]). However, it is rather brave to extrapolate this to all cell
types. Over the past few years there have been reports of trace telomerase
activity in certain rapidly-dividing somatic cell types, including blood
stem cells [2] and epithelial stem cells [3]. If telomere length has
anything to do with replicative senescence, and if most cell types do not
exhibit even trace telomerase activity (because they divide rarely enough
that they can manage a human lifetime without it), then we still have a
meaningful phenomenon in vivo which, though perhaps exaggerated by in vitro
studies, is not purely an artifact of those studies. This is not to say
that it has anything to do with aging, of course: only that the possibility
cannot be eliminated on these grounds.
Aubrey de Grey
[1] Kirkwood, T.B.L. and R. Holliday (1975) J. Theor. Biol. 53: 481-496
[2] Broccoli, D. et al. (1995) Proc. Natl. Acad. Sci. USA 92: 9082-9086
[3] Yasumoto, S. et al. (1996) Oncogene 13: 433-439
