Preparation of Frozen Sections for Analysis

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Fresh tissue must be preserved in some manner before analysis, because the substances to be tested in the tissue are often labile and cannot withstand analytical procedures without first being preserved. The best method for antigen preservation in immunocytochemical analysis is freezing. Freezing is a very suitable means for preserving antigens that lose their immunoreactivity (1). Fresh tissue, after being obtained, is quick frozen in a flask of liquid nitrogen for a few seconds depending on the size of the sample. The rapid introduction of ultracold temperatures prevents soluble materials from degrading and reinforces the structural components, steadfastly holding them in place. This method of preservation does not specifically alter the tissue in any way other than to cause some labile or low-concentration solutes to degrade slightly or lyophilize. In performing assays on frozen specimens, there is always a bit of denaturation as the sections are being prepared since the cut section thaws in order to adhere to the glass slide. There is also the risk of lyophilization of important components as well as the threat of freeze/thaw conditions occurring in the freezer after the specimens are preserved. However, this method of producing frozen specimens for analysis by antibody labeling is the closest one can come to in vivo conditions since no chemical changes have been forced on the tissue. Instead, the tissue is bathed in the cold liquid form of nitrogen gas until frozen. The quickness with which this occurs is the reason this process is so good for preservation. Sometimes it is desirable to speed up the process even further, and an alcohol is used to bathe the specimen directly. In this case, penetration occurs more quickly than with liquid nitrogen alone. The opinions or assertions herein represent the personal views of the author, and are not to be construed as official or as representing the views of the Department of the Army or the Department of Defense.