I'm not an expert by any stretch of the imagination, but I've taken to lightly glazing tissue with a thin layer of OCT (or equivalent) just before snap-freezing over liquid nitrogen. We sometimes keep tissues in a -80C freezer for 12 months (or longer) before sectioning. Even wrapping the brains in Parafilm and storing in short, squat cryo-containers led to surface dehydration after a couple months. The thin coating of OCT seems to protect the tissue during prolonged storage.
FWIW - I float the tissue in a weigh boat in a Dewar that is half-filled with liquid N2 and covered to keep the vapor in. We work primarily with rodent tissues, often brain. When I dropped them directly in the N2, they tended to split or even brak into fragments. I also tried embedding smaller organs like thymus in OCT (and some of the thicker embedding compounds) using small disposable plastic molds, but it prolonged the freezing time considerably.
Hope this helps,
Date: Fri, 25 Jan 2008 11:39:00 -0000
From: "Mareike Heimann"
Subject: [Histonet] O.C.T. embedding vs. freezing of unembedded
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I have been wondering about the advantages (or disadvantages) of embedding tissue in O.C.T. before snap-freezing in liquid nitrogen when compared to snap-freezing the "naked" tissue.
I do use O.C.T. before freezing, but when recently asked why, I found no better answer than "it seems to be what most people do..."
I would be most grateful for any explanations!
(My tissue blocks are used for immunohistochemical stainings later on)
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