In article <3m63gq$482 at mserv1.dl.ac.uk>, <W.G.VAN.DOORN at ATO.AGRO.NL> wrote:
> In a discussion about the definition of immortality Oliver Bogler wrote:
> > But to me that is no different from saying that the stem cell that divides
> > at age 120 is immortal because by definition it is derived from a cell
> > that was onboard at birth. You only get cells from other cells - they are
> > not ever made de novo. So the experiment would be to see whether there are
> > any dividing stem cells at age 120 - and that is all.
> I have serious difficulty with this definition. We do not know how long an
> individual stem cell lives. The parent cell may divide and then fully
> differentiate, i.e. until death. The daughter cell does the same, etc. Each
> individual cell may live for a few days only. Yet you call the cell immortal.
What is an individual cell? Cell A divides to give cells B and C. Where is
cell A now? Is it dead, just because you can not point to it? If each cell
lives only a few days the whole organism will die soon.
> In this definition a cell is immortal when its descendants keep multiplying.
> As someone else has said, this would imply, logically, that the only immortal
> cell was the first one.
This argument would lead to a useless definition of the term immortal -
see below.
> Bacteria, which will go on multiplying when regularly placed on a new
> substrate, would in this line of thinking also be called immortal, although
> we know for a fact that each cell only lives for a day or two.
>> Also human beings would then be immortal, since they have ever been
multiplying
> (with some success). This shows that the definition defies common sense.
Again you reveal your confusion in terms of what 'immortality' means in
the context of cellular senescence.
>> I could agree with calling a cell culture immortal, but there is some
hybris in
> the word. Isn't it a bit exaggerated? How could we prove it? Things that
> cannot be proven or disproven are outside the realm of science. The term,
> therefore, is unscientific.
Immortal in the normal sense means to live forever, and I agree with you
this is an unscientific term. However there is a scientific definition of
the immortal - which I thought we were all using. In the scientific term a
cell is termed immortal if it generates a lineage of descendants that do
not undergo senescence at the time when sister cell clones do. We tend to
be a little sloppy, if you will, in that we call cells immortal - of
course as individual cells they never are, because they need to keep
dividing to be termed immortal. As soon as they divide they are gone. What
is immortal is in fact the clone which they give rise to.
One needs to think differently about organisms and cells. Organisms give
rise to descendants without losing their identity. Therefore it is easy to
see that they are all individually mortal, while their species may live
much longer, or even be immortal (meaning the species lives many orders of
magnitude longer than any inividual). Not so for cells. If all cells have
a limited life at the time that they are generated then they wouldn't last
long. If you think of an example: let us consider the first ever cell:
lets call it A. It divides to make B and C.If B and C are now programmed
with a finite life span, and their descendants inherit that limit then
soon all the cells will be dead. (This is exactly what happens when you
observe fibroblasts senesce - cells derived from the same cell stop
dividing and senesce at the same time.) Since bacteria, for example have
lived for millions of years, you could say that they are immortal - all
the bacteria alive today are related - they all eventually come from the
first bacterial cell division - they are immortal.
Oliver
--
Oliver Bogler
obogler at ucsd.edu
