Re: Plant and animal cell replacement question

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.