This is really opening a barrel of worms but...

Re formaldehyde solutions.
If tissues are fixed for a few hours in standard formalin solutions then it is possible to reverse the fixation process by washing. It is a reversible process in that the bonds that have been formed are generally temporary and can be broken.
Technically if you wish for "adequate or true fixation" then Pearse provides the theory for why this should be for about 1 week. 
(The fixation process does in fact continue for years).

Of course no one fixes for one week except by accident. However the fixation for 24 to 48 hours is only an initial fixation. Fixation is generally carried on by the alcohols used for the processing.

A. G. E. Pearse "Histochemistry Theoretical and Applied: volume 1 Preparative and Optical Technology."
Churchill Livingstone. 

Barry 

-----Original Message-----
From: histonet-bounces@lists.utsouthwestern.edu [mailto:histonet-bounces@lists.utsouthwestern.edu] On Behalf Of GT Hebert
Sent: Tuesday, October 03, 2006 11:29 AM
To: histonet@lists.utsouthwestern.edu
Subject: [Histonet] Proof that fixation in formaldehyde solution can bereversible?

Hello,
   
  I am in a heated debate with an investigator regarding his samples.  They were shipped to me after (24-48hrs fixation - 10% NBF or 4% PF unsure exactly which one) and then switch to PBS for shipment.  They sat at room temperature for over 1 month before being processed routinely and embedded in paraffin wax.  
   
  Can someone tell me if indeed the fixation is reversible, or once they have been fixed for over 24hrs they remain fixed??  Can anyone refer me to books or papers that talk about this??  
   
  Also, what affect on antigenicity will such storage in PBS have on these samples?
   
  Thank you all so much for your help.
   
  G. H.
  Cambridge, MA
   
   
   
  Key words:
  NBF, Alcohol, 10%, paraformaldehyde, 4%, fixation, fix, ethanol, 70%, reverse, reversible.

 		
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