Don't do anything without knowing the reason! Of the four liquids you list, the first three will extract most of the lipids (PAS +ve and others) from the sections. Formalin will not do this (unless it's diluted with a large excess of an organic solvent such as alcohol). You didn't say if the pieces of lung had been fixed before freezing. If not, brief treatment with formalin won't fix them, though it will probably reduce autolysis and delay putrefaction. The three lipid solvents that you list will all rapidly provide good fixation at the micro-anatomical level (e.g. X40 dry objective), as long as you're not interested in lipids or enzyme activity histochemistry. 

John Kiernan
Anatomy, UWO
London, Canada  
---
----- Original Message -----
From: Mark Elliott 
Date: Sunday, October 21, 2007 22:09
Subject: Re: [Histonet] PAS for frozens
To: John Kiernan 

> Thanks very much for the info, it is much appreciated.  
> Have received a number of methods with various pre-fixes-none, 
> acetic ethanol, ethanol, Carnoys and formalin.  Will try a 
> variety.
> Mark
> 
> >>> John Kiernan  10/18/2007 8:22 AM >>>
> If your frozen sections are mounted on slides, the periodic acid-
> Schiff PAS procedure is the same as for paraffin sections. 
> Lipids, especially hydrophilic ones, are PAS-positive to a 
> variable extent. This is partly on account of the sugars in 
> glycolipids and partly due to atmospheric oxidation at 
> unsaturated bonds. 
> 
> ----- Original Message -----
> From: Mark Elliott 
> Date: Tuesday, October 16, 2007 13:58
> Subject: [Histonet] PAS for frozens
> To: histonet@lists.utsouthwestern.edu 
> 
> > Does anyone have a procedure for doing a PAS on frozen 
> > tissue??  I assume we need to shorten the times in the 
> > various regents, but by how much?  Any tips/tricks would 
> be 
> > greatly appreciated.  We are staining human lung tissue.
> > 
> > Thanks
> > Mark in dreary Vancouver BC
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