Freeze-drying technique fixation

If no other histochemical techniques are needed, fresh tissue can be used. The block must be frozen quickly without cracking. Tissue is cut in small blocks with a new razor blade (washed in soap to remove oil). An alternative to the method below is freezing tissue on aluminum foil on top of dry ice and surrounding with crushed dry ice. However, the perimeter of the tissue is usually poorly preserved with this method. For RNA detection, DEPC is included at 0.02% in the subbing and fixation solutions. Poly-L-lysine coating may be superior at temperatures > 50°C since gel from gel-coated slides may detach at higher hybridization temperatures. It is convenient to use only some of the slides for immediate hybridization and store the rest for future reference. 

An alternative to freezing or preservation in liquid fixahon/preservation is imprinting tissue on nitrocellulose paper (30,31). Cross-sections of fresh tissue are directly imprinted onto nitrocellulose paper. This imprints cells onto the paper from the tissue and also preserves localization within the cross-section. This technique provides a rapid and inexpensive means to preserve dried cells without further fixation or refrigeration. Both DNA and RNA can be recovered from these imprints. 

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