Re: APES slides & stained glass

From:"J. A. Kiernan"

"clarke.ian" wrote:
>              ... A colleague of mine has told some of the trainees
> that they where not to put sections onto Apes coated slides as this would
> lead to contamination and background staining, particularly when doing a PAS
> and some of the silver stains.  ... I would like to know if this is the
> case.   ...  I havn't noticed any background from the special stains.

Geoff McAuliffe wisely advised you to try it and see. That's
what you should do. BUT you should think about mechanisms! 

For what it's worth:  There is no reason to expect APES-treated 
glass to be PAS-positive. Periodic acid generates aldehyde groups
from certain sugars; Schiff's reagent reacts with aldehyde groups
to make a red product. APES (aminopropylethoxysilane) reacts with
glass but no sugar-like products or aldehyde groups are

Glass can be envisaged as a macromolecular form of sodium
The APES treatment covalently binds aminopropyl groups 
(H2N-(CH2)3-) to oxygen atoms of silicate ions. The resulting 
-Si-O-(CH2)3-NH2 groups on the surface of the glass become
positively charged by protonation (-NH3+) when immersed in
a liquid whose pH is below a certain critical value (which I 
don't know, but suspect is above 7). 

You would expect the positively charged surface of APES-treated 
glass to stain with anionic dyes such as eosin Y and the dyes 
used in trichrome methods etc. I've looked for that and have not 
seen it. I hazard a guess that the dye binds to the positively
charged glass but a layer with the thickness of only one dye 
molecule is not coloured strongly enough for my eye to detect it. 

(Staining of the silicate ions of ordinary glass by some cationic 
dyes such as alcian blue 8G is easy to see, as anyone who does
washing-up in a histo lab knows. Dyes of this kind have flat
molecules that are known to be able to accumulate in stacks, and
stacking amplifies staining.)
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London,   Canada   N6A 5C1

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