A few of points on this issue:
1) Are you positive that fixation is the problem? In my experience the vast
majority of pathologists assume that any problem in processing is
"fixation." They hand the slide from a poorly processed sample to you and
say, "this isn't fixed." What they usually mean is that it didn't process.
In fact the sample is usually so poorly processed that evaluating specimen
fixation from the slide alone is impossible.
2) The cassettes are not permeable.
3) Tissue that touches the support cross on both sides of the cassette has
surface area reduced by 5-10% (not all brands of cassette have this raised
cross on the inside of the cassette).
4) Tissue that completely fills the cassette has surface area reduced by
5) Results are even worse if tissue "squeezes" out of the slots.
These reductions in surface area caused by tissue too large for the
cassette account for almost all processing problems in an established
processing schedule that is working fine for most tissue. The difference
between a 4 and 5mm section is a 25% increase in thickness. Since fluids
diffuse from both sides of the section one can estimate a 12% increase in
processing time from a schedule optimized for 4mm sections to one optimized
for 5mm sections. The reduction in surface area caused by the contact with
the cassette throws this estimate out the window. Instead you will need to
triple processing time or cut thinner sections.
From: Mike Black [SMTP:firstname.lastname@example.org]
Sent: Wednesday, November 14, 2001 9:31 AM
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