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Fast penetration --> rapid killing
Smooooooooth texture in finished product.
Secret mechanisms of REAL action that keep everyone coming back.
Reasons for using HCHO.
Habit
Protocols for 100+ years
Easy to use and relatively safe to keep as paraformaldehyde
VERY wide range of concentrations that work (2-~40%)
Works as gas (dry) and in aqueous solution.
Habit
Progressive fixative - some of the chemistry is fairly clear
Reversibility is relevant to IHC-Ag recovery, but not to much else. But, I
repeat, the progressive addition stage of HCHO fixation does not occur in
significant degree, according to some, until 12+ hours after exposure
begins, and the earlier process might have more to do with the reductive and
toxic (whatever that means mechanistically, I do not know) capacities of
HCHO than on the question of addition and/or subtraction.
[Biological systems are well-balanced thermodynamic systems, but HCHO is
most likely so popular, because it is effective in rapidly diffusing
throughout the system, poisoning both anabolic and catabolic systems, and
SLOWLY thereafter reacting directly with adjacent macromolecules to provide
crosslinks that progressively preserve the structural relationships of those
cellular and tissue components.]
[In urology it is common, for some chronic irritative bladder dysfunctions,
to instill 50% DMSO (for a little while under anesthesia-I think) as a
palliative. One of the early papers, while reporting on the therapeutic
efficacy of various concentrations of DMSO, notes that 50-100% DMSO is also
a very good histologic fixative for bladder epithelium.]
Summary: Rapid penetration, killing and halting degenerative changes are
not mutually exclusive criteria for a good chemical fixative. It is
interesting that, except in pathology and electron microscopy, HCHO has
almost replaced all of the old, heavy metal fixative formulations.
Cheers,
Fred Monson
-----Original Message-----
From: Guy Cox [mailto:[log in to unmask]]
Sent: Tuesday, April 15, 2003 4:04 AM
To: [log in to unmask]
Subject: Re: formaldehyde fixation reversible?
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Scott Snyder wrote:
>What I have often wondered is why formaldehyde is so popular given its
>variability >from batch to batch, reversibility, pH dependence, and
>potential for forming unwanted fluorescent adducts.
Nobody in this field uses bottled formaldehyde, precisely because of
this variability. We prepare formaldehyde freshly from paraformaldehyde
and hence obtain a fresh and uniform product. When permanent
crosslinking is required glutaraldehyde is the fixative of choice -
the great advantage of formaldehyde is its ability to provide
a milder and less permanent stabilisation. If you could offer
rivals to these two which didn't induce fluorescence I'm sure a lot
of people would listen!
About the only other protein cross-linking agent I know that has
achieved any wide use is MBS (m-maleimidobenzoyl N-hydroxysuccinimide
ester) introduced by Sonobe & Shibaoka to stabilise actin (Protoplasma
148, 80-86 (1989)). A similar (and I can't face typing it) substance
has been used to stabilise microtubules. I've certainly used MBS
in the past and I reckon it works, but I'm unclear why one needs a
different reagent for a different protein (tubulin). Maybe that's
why the aldehydes remain popular?
Guy
Assoc. Prof. Guy Cox, ooOOOOOOoo
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