RE: [Histonet] Microwave Processing
Hi Terry & HistoNetters
Let me take a stab at this, although I am not a
chemist (Dick Dapson, where are you when I really
Microwave stabilization uses heat to harden the
tissue through a process of coagulation of the
proteins in the tissue. Kok & Boon liken this to
poaching an egg. Alcoholic "fixation" also
hardens the tissue by coagulation of the
proteins, but also removes the water (the higher
the percentage of alcohol, the more the hardening
effect). The use of alcohol as the primary
fixative results in a characteristic shrinkage
within the tissue. Aldehydes "fix" the tissue
through cross-linking of the proteins.
When I talk about microwave stabilization not
chemically "fixing" tissue, I mean to say that
the proteins are not cross-linked. In Leong's
work, the biopsies were microwave-stabilized in
saline (hardened), then further hardened in
graded ethanols on a conventional processor
(normally referred to as "alcoholic fixation"),
but still not cross-linked. With larger tissues,
I would worry about shrinkage with this method,
but Leong seemed to have success with it on
biopsies. I still don't think they would look
quite the same as formalin-fixed specimens.
I'm sorry if I didn't make myself clearer.
To muddy the waters further, some labs carry out
microwave stabilization of gross tissue specimens
in formalin at 50°C, rather than in saline at
65°C, but this still isn't fixation, because the
formalin doesn't penetrate the tissue
sufficiently to cross-link.
At 12:48 PM +0100 4/21/04, Marshall Terry Dr,
Consultant Histopathologist wrote:
>My confusion has not been dispelled.
>"The saline will not work
>alone as a fixation step, except in the case of
>very small biopsies (as written up in an article
>by Tony Leong)- ..."
>In the next post, we get:
>"Yes, in the Leong method for biopsies, the specimens are heat
>stabilized in the microwave in saline, and not really chemically
>fixed. They get fixed in the ethanol in a traditional processor ..."
>These statements are mutually incompatible.
>Moreover, what is the difference between "stabilisation" and "fixation".
>A further muddle is introduced by:
>"not really chemically fixed."
>Well, hell, are they fixed or no? Heat fixation
>is fixation - who cares whether chemically fixed?
>Of course, the major problem is that mechanisms
>of fixation are varied (with the fixative) and
>not well understood.
>(Speak for yourself do I hear someone say?)
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