IDENTIFICATION OF CYTOPLASMIC BASOPHILIA (RIBONUCLEIC ACID) BY FLUORESCENCE MICROSCOPY

1. The fluorochrome, acridin orange (AO), allows identification of basophilic cytoplasmic inclusions in the supravital state. 2. The fluorescence picture of hepatic cells after treatment with AO shows the hyaloplasm faint gray-green; flake-like or granular structures in the cytoplasm, bright red; the nuclei, with chromatin structures, green; the central part of the nucleoli red, their periphery green. 3. The red-fluorescent cytoplasmic inclusions correspond to those observed with the toluidine-blue technique, and shown, by the RNase test, to consist mainly of RNA. The same is demonstrated by the changes of AO fluorescence after application of RNase to fresh or fixed tissues. 4. The fluorescence picture given by other cell elements and tissues in the rat liver is described. 5. Changes in the AO fluorescence of liver tissue with varying pH, dye concentration, staining time, action of salts, prolonged U.V. illumination, fixation, as well as the fluorescence of some biologically important substances (RNA, DNA, heparin, albumin, glycogen), treated with AO, are indicated. 6. The advantages and limitations of the method, and the theories of AO staining are discussed. 7. As compared to conventional histochemical methods, the applicability to living or supravital tissues of the fluorochrome technique decreases errors due to fixation, dehydration, embedding and related procedures, and the low concentrations applied in fluorochrome staining further safeguard against histochemical artifacts.