Must an AGING PROCESS be universal?

Mon Apr 3 04:03:44 EST 1995

     The next question is how old do individual cells really get. I would like
     to ask for sound proof, derived from individually tagged cells, showing
     their age. Andy thinks that the stem cells in the intestine live as long
     as the individual human being. If this is true the maximum age of an 
     individual cell is at least 120 years. Can anyone add to this list, but 
     please only when also quoting the paper in which it has been 
     published. For me the list has not yet started, as I would like to know
     whether the intestine stem cells live that long for a fact.          
> We are dealing with a problem of definition here. [We] are using a set of 
  precise definitions (which I will outline below) that are used by biologists 
  working on the problems of cell division. 
> If you take a population of embryonic rat fibroblasts .. and grow them in a 
  culture dish, you will notice that the cells
> divide and rapidly fill up the area on the dish. At this point, they will
> stop dividing. They are said to be QUIESCENT. If you dissociate the cells
> on the dish, and replate them at a lower density, the cells will start
> dividing again. You can repeat this process between four and six times (by
> which time each of your founder cells will have divided probably between
> 20 and 30 times). At this point, the cells once more stop dividing, and
> become noticeably flattened. They cannot now be induced to divide any
> more. They are said now to be SENESCENT. Senescent cells do not die - they
> just sit there and carry on metabolising but cannot divide any further. We
> would interpret this phenomenon by saying that the cells we isolated from
> the rat embryos have a finite limit to the number of divisions they can
> undergo. This will tend to be roughly the same number in every experiment
> you perform.
> It is possible to carry out manipulations whilst these cells are still
> dividing that will override their limit to cell division. This can be done
> by, for example, introducing  certain oncogenes such as v-myc, or the SV40
> large T antigen. The cells will now continue to divide in culture
> indefinitely and will not respect their normal limited number of
> divisions. We call these cells IMMORTAL, in that they can be propagated
> indefinitely in culture, and the oncogenes that cause these effects
> IMMORTALISING ONCOGENES. Note that this is a narrow but precise definition
> of the word immortal used by biologists.
> My point here is that biologists tend to define immortal cells NOT as
> cells that 'live forever', but as {cells} that have no intrinsic limit to
> the number of divisions they can undergo. This is in stark contrast to
> most normal cells, which appear to have a pre-programmed number of cell
> divisions. Stem cells do not appear have a limit to the number of
> divisions they can undergo. 
     As to this last paragraph, Andy, do you really mean cells or cell lines.
     See the word cells that I placed between marks.
     Best regards,
     Wouter van Doorn
     ATO-DLO, Wageningen, Holland
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> Date: Sun, 02 Apr 1995 00:06:30 -0800
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> From: grovesa at starbase1.caltech.edu (Andrew K. Groves)
> Subject: Re: Must an AGING PROCESS be universal?
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