Re: Plant and animal cell replacement question

Thank you for this. Do I understand you correctly in saying that the
proteins and organelles would change even in those animals that
retained exactly the same cells during their adult life? Would this be
a general truth?

Thanks,
David Oderberg

r norman <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
> On 26 Nov 2003 06:27:28 -0800, [EMAIL PROTECTED] (David Oderberg)
> wrote:
> 
> >Hello, I have been trying to get the answer to the following question,
> >with no luck:
> >
> >Is it the case that all plants and animals, including humans, turn
> >over every single one of their cells during the course of their mature
> >life? One hears stories to the effect that human cells change in their
> >totality every 7 years, or (on the contrary) that certain neurons do
> >in fact remain in the human being until the person's death, etc. But
> >I've never seen an authoritative statement of the facts.
> >
> >Can anyone help?
> >
> 
> Nerve cells, in particular, are not ordinarily replaced.  Until
> recently, it was believed that mammalian neurons were not produced in
> adults at all, ever.  Now, it is known that there are least a few
> brain centers where at least a few new neurons are generated.  Still,
> the large bulk of the billions of neurons in our brain do not get
> replaced.
> 
> Many invertebrates have identfiable and named neurons, each with a
> specific function.  Some vertebrates have some similar cells --
> Mauthner cells in fish and amphibians, for example.  The lobster
> cardiac ganglion has nine neurons.  These things are constant
> throughout life -- the cells don't change.
> 
> Some invertebrates have absolutely fixed cell number.  The roundworm
> Caenorhabditis elegans, for example, has 556 cells in the newly
> hatched larval stage and 959 somatic cells in the adult.  These are
> not replaced during the lifetime of the animal.
> 
> It is quite likely, though, that te cells themselves completely turn
> over during their life so that a cell that remains is made of
> completely new proteins and organelles.  The DNA does not turn over,
> though.