From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Sun Oct 27 2002 - 18:28:10 MST
On Sun, 27 Oct 2002, Charlie Stross wrote:
> On Sat, Oct 26, 2002 at 12:51:01PM -0700, Jeff Davis wrote:
> >
> > --- Charlie Stross <charlie@antipope.org> wrote:
> >
> > > Most of your muscle tissue will have been completely
> > > replaced by November
> > > 30th, 2003. Our tissues are not permanent, they turn
> > > over regularly;
> >
> > This is an oft-repeated factoid which I think this is
> > not quite accurate, or in need of some clarification.
I haven't been following this discussion, but it is reasonably
accurate to say that "most" tissues turnover at a rate varying
from days to several hundred days.
> What do ribosomes produce?
Proteins.
> What is apoptosis?
Programmed cell death that results in the elimination of cells
with a minimal amount of inflamation.
> What are stem cells,
Cells that have a limited ability to repair and replace other
tissues (at least in adults). In contrast, embryonic stem
cells are totipotent and probably have the ability to generate
any tissue if they receive the proper signals.
> and why do we have them?
Because without them, significant injuries (presumably common
when one is hunting mammoths or saber toothed tigers) would be
fatal. Also because many tissues from blood to the brain need to
have a constant supply of new cells to function properly.
While Charlie's numbers are probably likely accurate for material *within*
muscle cells, I question whether they are accurate with regard
to the extracellular matrix (which has a much lower turnover rate).
The slow turnover rate of the extracellular matrix is probably
one of the reasons we become "stiffer" with age.
One "factoid" which is very well supported from the scientific
literature is that protein turnover rates *do* decrease with age.
See: http://www.deamidation.org and papers by N. E. Robinson
in PubMed. The recycling rate for proteins may be "optimal" for
young individuals, but it suboptimal for older individuals.
In short -- the genetic program does not adapt well to "aging".
Robert
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