In article <3lmk3b$o03 at news.halcyon.com>, venezia at zgi.com (Domenick
Venezia) wrote:
> Andrew K. Groves (grovesa at starbase1.caltech.edu) wrote:
>> Is senescence not a probabilistic process with a presumably gaussian
> distribution? Don't we lose individual stem cells one at a time? The
> stem cells left in a 120 year old are simply those few on the extreme
> right of the distribution. Couldn't this be the reason our skins get
> thinner, and our digestion gets poorer as we age? The number of stem
> cells regenerating these tissues is less, and the tissue regeneration is no
> longer as robust? So I think the Hayflick limit is active in human stem
> cells, and isn't that the whole implicit assumption of the
> telomere/telomerase work? That shortened telomeres lead to senescence
> of stem cells and that the result of that senescence is the Hayflick
> limit? If you have data that suggests that human stem cells in vivo are
> in fact immortal I'd like to have a look at it, because it would completely
> change the way myself and many others think about aging.
>
If you observe cell sensescence in culture, it certainly is not a
probabilistic process - virtually all cells in a culture will drop out of
the cell cycle within a few divisions of each other. The Hayflick limit
has not been measured (to my knowledge) in human stem cells, partly
because a) they are extremely difficult to identify and b) they are
extremely difficult to propagate in culture in the way that Hayflick and
others did for fibroblasts.
I would say that the fact that 120 year olds have an intact gut at all
would suggest that their crypt stem cells are working just fine - the
human gut sheds an enormous number of cells every day (I think it's been
estimated as 10 to the power of 11), and so any impairment of stem cell
function would certainly compromise the integrity of this tissue very
quickly indeed.
--
Andy Groves
Division of Biology, 216-76
California Institute of Technology
