Dear Frouwke,
To my opinion, the storage of cryostat tissue sections is something
different as storage of frozen tissue blocks.
After cutting a cryostat section you let it dry for a while. Upon storage
you don't want to let it become wet again. However, I don't understand the
use of silacagel for keeping your cryostat sections dry. I think this is
quite overdone because the environment in your -80C freezer is already as
dry as the dessert. Remember how dry your skin starts feeling when only -10C!

A frozen tissue block still contains a lot of (frozen) water. You don't
want to let it dry out (freeze-drying effect; the white spots at meat as
you described). For this reason, you need to put a few drops of water into
the cryotubes next to your tissue block for real long storage at -80C.
After closing the cryotube, the frozen water ensures a certain (very low)
vapor pressure, preventing the tissue block from freeze-drying effects.

I hope this contributes to solving your dilemma.
Chris van der Loos
Academic Medical Center
Dept. of Cardiovascular Pathology
Amsterdam, The Netherlands

Frouwke wrote:
 >Date: 5 Mar 2003 09:20:54 -0600
 >From: Frouwke Kuijpers 
 >Subject: storage frozen tissue -80C
 >Hallo everybody
 >I have a question about storage of frozen tissue.
 >We have no experience with that and we are forced to it now because the mice
 >which we use for our experiments are sick and it takes at least a year
 >before we can get new ones. So we have to be very economical with the tissue
 >we have left. We have the fixed sections now in autoclaved PBS and So-azide
 >at 4 C.
 >People suggested we can store the left sections at - 80 C after mounting
 >them on slides. I have read about it in the archives of the Histonet and
 >most people advice to put silicagel in the slide boxes to prevent the
 >sections become too humid. Is that for the ice crystals?
 >But now my problem: other people of our University told me to put wet tissue
 >in the slideboxes, to prevent the sections are drying out? (Those white
 >spots you see when you left your frozen meat too long in the fridge).
 >So, I am in a dilemma now, I have two complete opposite advices, what should
 >I do??
 >Frouwke Kuijpers
 >F.J.Kuijpers-Kwant
 >Dept. Cellular Animal Physiology
 >University of Nijmegen
 >Toernooiveld 1
 >6525 ED Nijmegen
 >frouwke@sci.kun.nl