MadSci Network: Medicine

Re: Humans duplicate and create new cell every instant. Then why do we grow old

Date: Mon Nov 27 01:42:26 2000
Posted By: Mike Crawford, Postdoctoral Fellow, Biology
Area of science: Medicine
ID: 974420738.Me

Hi Cynthia,

I turns out that probably all living cells (at least the ones that have been tested) are not 
created to live forever under normal circumstances.
Cells from mammals to common yeast will eventually die.  The term to describe this is called 'senescence'.
With senescence, our cells simply produce a certain number of daughter cells, then won't divide any more.
The number of divisions to senesence varies depending on what organ is involved (skin cells divide more
often than brain cells), and what animal you're talking about.  Lobsters, for example, live for hundreds of years,
and their cells are able to divide much more often than those of a mouse, which only lives for ~2 years.
Part of the reason for longevity of things like lobsters or turtles is that they (and their cells) are not
very active.  There is always a trade off for quantity of life and quality--species that live for a long time
are not able to eat or move much.

There are a couple of reasons for complicated mammals such as ourselves to kill off cells:

(1) The cell is no longer needed, and in fact might be harmful to the rest of the body.
(2) The possibility of cancer.

(1)  One such case involves our development from an egg to embryo on up to a fully developed human.
There are certain cells that are involved in starting eye formation, for example--we want these at only 
one stage of development, and at only two places on our body.  You can imagine what would
happen if these cells were allowed to survive for too long or were at the wrong place.
Another area where regulation of cell number is important involves the immune system, 
which expands to combat an infection, then needs to significantly reduce itself after the infection is removed,  
or else you'll be continually having fevers or runny noses.

Both of the instances above actually involve a process related to senescene called 'apoptosis', which an
outside stimulus causes a cell to undergo a genetically pre-programmed pathway to literally commit suicide.

(2)  Unfortunately, some people do have immortal cells in their body--it's called cancer.
One of the hallmarks of cancer cells is that they are able to divide infinitely without senesence
(or apoptosis) taking place.  Many researchers work with cancer cells (HeLa cells) from a woman 
(Henrietta Lachs) who died in the 1920s or 30s, but her cells are still growing strong in laboratories all over the world. 

What causes senescence and apoptosis is a highly active area of research at this point.
People are of course interested in prolonging human life while keeping the risk of cancer down.

As I mentioned before,  another problem is the quality of a prolonged life.
Indeed, mice that are fed just enough to survive and are not allowed to move much are
able to live significantly longer through something called the 'stringent response'.  Lobsters and turtles
are probably always in this 'stringent' mode, but probably aren't happy.

Anyways, I ramble too much.
Hope this helps.

Michael Crawford

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