Andrew K. Groves (grovesa at starbase1.caltech.edu) wrote:
>> I disagree, and cite my examples of stem cells in bone marrow, gut and
> skin. These cells do not appear to have a limit on the number of times
> that they can divide in the living animal. ...
> ... There are dividing stem cells in the gut of 120 year old humans,
> and there is no reason to think that they have a limit on division.
>> I stress this by way of comparison to cells that have a "Hayflick" limit.
> Some cells in an animal DO have a seemingly pre-programmed number of
> divisions, and will eventually stop dividing. They won't necessarily die -
> they will just be unable to divide. Stem cells in the gut, bone marrow and
> skin do not behave like this, even though the differentiated progeny to
> which they give rise do.
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.
Domenick Venezia
ZymoGenetics, Inc.
Seattle, WA
venezia at zgi.com
