Dear Wendy,
Apart from the damage to the tissue morphology caused by the defrost cycle of the cryostat that has been mentioned a couple of times, there is another risk. Water crystals tend to re-crystallize from amorphous to needles between -1 and -30C. Depending on the temperature this process takes a couple of days and causes holes in the tissue. The lower the temperature, the slower this process will be. And this is exactly the reason why we are storing our frozen tissue at -70 and -80C. 
Don't leave your tissue in the cryostat!

Chris van der Loos, PhD
Dept. of Pathology
Academic Medical Center
Amsterdam - The Netherlands 

----- Original Message ----- 
>From  "Snyder, Wendy"  
Date  Wed, 12 May 2004 10:52:38 -0400 
To  "'histonet@pathology.swmed.edu'"  
Subject  [Histonet] Frozen sections 
Hello,
  I was wondering about a policy for the length of time a frozen section
block can safely remain in a cryostat before it is taken out and routinely
processed.  I have a pathologist that wants to leave the frozen tissue block
in the cryostat for up to 3 days in case he wants to do immuno stains from
it.  I would be interested to here of other policys from other institutions
regarding this issue.

Thank you,
Wendy Snyder
United Hospital Center
Clarksburg, WV



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