A variety of established methods for protecting mitochondria were tested on rat duodenal epithelium during the histochemical assay for succinic dehydrogenase. The use of sucrose at isotonic or hypertonic concentrations, 7.5 per cent polyvinylpyrrolidone, divalent cations, physiological salt solutions, phenazine methosulfate, coenzyme Q10, and menadione failed to improve the quality of the histochemical preparation once fresh frozen sections were prepared. However, preservation of mitochondrial integrity with little diminution in succinic dehydrogenase activity was obtained by fixing tissue slices (less than 1 mm. in thickness) in 8 per cent unneutralized, aqueous formaldehyde from 8 to 16 minutes at from 5° to 10°C. prior to freezing. To offset the inhibition of enzymatic activity it was necessary to extend the incubation period by 10 to 15 minutes. Two-micron-thick sections were easily obtained from the frozen blocks of such fixed tissue and incubated in the unmodified Nitro—BT-succinate medium. Once the optimum conditions for fixation of intestinal epithelium were determined, many other tissues were subjected to the same procedure. From the morphological standpoint the appearance of the mitochondria in these histochemical preparations compares favorably with the results obtained using the classical Regaud iron-hematoxylin staining procedure. With most tissues, the results are superior to those with fresh frozen sections. However, results with muscle, sperm, and kidney tubular epithelium are not as strikingly improved as with gut and liver.

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