EFFECTS OF FIXATION ON CARBOHYDRATE HISTOCHEMISTRY

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Abstract
Even if complex carbohydrates (polysaccharides and so-called mucoproteins) are not "fixed", or made insoluble, they can be retained often in tissue sections. For example, injected dextran and soluble glycogens were colored by Schiff''s reagent after oxidation by periodic acid in alcohol, provided tissues were fixed and paraffin sections cut without exposure to water. The optimum demonstration of complex carbohydrates is required before differences in stained sections can be ascribed safely to fixation. For example, the Alcian blue and the colloidal iron stains are more sensitive for many acidic carbohydrates than metachromatic dyes. The treatment of microsections after staining can limit the final result; the mucinous capsules of Cryptococcus neoformans are severely shrunken by dehydration after staining but are thick and round when mounted in glycerin-jelly. Analysis of the effects of so-called pure enzymes on tissue sections requires caution. The removal of stainable substances from sections may result from enzyme action on associated material normally forming an insoluble complex with the former and not from specific action on the carbohydrates. Contaminant enzymes may produce misleading effects. Certain carbohydrates are dissolved by the enzyme solvent alone. Some fixatives modify the substrates so that applied enzymes have no effect. Regard enzymes as capable of acting on particular linkages rather than specific substances. Fixatives that contain much chromic acid may destroy 1, 2 glycol groups and lessen the periodic acid-Schiff reaction. Osmic acid fixation may produce oxidizable 1, 2 glycol groups from unsaturated groups, e. g., in myelin. Most neutral polysaccharides are adequately preserved in cold to very cold absolute ethanol. For acid mucopolysaccharides, fix in either ice cold neutral buffered formalin or in cold alcohol-formalin. The precise effects of various fixatives on particular reactive groups of tissue carbohydrates are not yet clear.