excelife at earthlink.net
Sat Aug 8 23:56:08 EST 1998
An excellent reply so far as it went.
Evidence is mounting that most cancers in re-producing somatic cells are the
result of the cells bypassing their senescence and crisis stages, (reached as
a result of a loss of the telomeric repeats at the end of the chromosomes),
by the extraneous expression of the enzyme telomerase. Most stem cells show
a low level of telomerase activity which is up-regulated during mitosis and
cellular differentiation which helps maintain their chromosomes' telomeric
length. Cancer is far more likely to be initiated in the differentiated
cells lacking telomerase activity since they will reach senescence and crisis
stage much sooner than the stem cells themselves.
Nerve cells, as we have in the brain, do not normally reproduce nor enter
senescence in the normal course of growth. They can sustain genetic damage
in quite a number of ways and if that damage re-activates telomerase
production then those cells too can become cancerous but this is a rare
occurrence and helps explain the discrepancies in tumor formation rates.
So far as why nerve and other non-reproducing cells usually stop dividing
after they "terminally differentiate" the jury is still out. Studies into
human nerve growth factor and age related genetic expression may help solve
this problem and a medline search at http://www.ncbi.nlm.nih.gov/PubMed will
provide some abstracts on the current research.
Lifeline Laboratories, Inc.
In article <#KuY##Sw9GA.302 at upnetnews03>, matthew_avison at email.msn.com
>Some tissues that constantly proliferate (like skin for example) do so
>because the tissues contain stem cells. These cells are constantly dividing
>and producing daughters who the differentiate into the main cells of the
>tissue (e.g. skin cells). Such tissues usually undergo a constant cell
>death at their surface (e.g. skin cells die at the surface and are shed) so
>the stem cells are essential to stop the tissue waring away, providing a
>constant supply of new cells. The brain as a tissue is very different. It
>is not designed to cope with constant waring away of differentiated cells
>like the skin is. Therefore there are very few neuronal stem cells to
>provide new cells to replace ones that do die. Hence eventually the brain
>starts falling apart. For most people, however, they are dead before this
>really happens. One of the holy grails of neuroscience is to try and get
>neurones to de-differentiate into stem cells, thus allowing a source of
>proliferation for people who have got problems with too much neuronal death.
>Watch this space.
>Interestingly, a reason why neuronal derived tumours are so rare and skin
>cancers relatively common is because it is a stem cell going wrong that
>usually results in a tumour and so the more stem cells there are, the more
>chance of getting a tumour. The most rapidly increasing cancer nowadays
>(particularly in men) is Colon cancer, the surface of which is very similar
>to skin in terms of its ability to regenerate.
>Hope some of this helps.
>Matthew B. Avison, University of Bristol, UK.
>JJ Miranda wrote in message ...
>>FOrgive the ignorance of this question... I'm a chemist...
>>I was wondering, does anyone know the cellular or molecular explanation
>>as to why brain cells don't grow back when killed? Also, I know thatsome
>>brain cells grow back but not others... What distinguishes between these
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