Cytological Fixation by Chromic Acid and Dichromates

Abstract

ABSTRACT 0·07 M (1 per cent.), 0·17 M (2·5 per cent.), and 0·34 M (5 per cent.) solutions of analytical-grade potassium dichromate have pH values of 4·10, 4·05, and 3·85 respectively. The values for corresponding solutions of chromic acid are 1·20, 0·85, and 0·70. The oxidation potential for potassium dichromate is 0·76 V, and for chromic acid, 1·10 V. During fixation for 18–20 hours at room temperature, with either reagent, there is a small decrease in hydrogen-ion concentration but no appreciable change in oxidation potential. During postchroming for 48 hours at 37° C., there is a further decrease in hydrogen-ion concentration, but only in the case of chromic acid is there a decrease in oxidation potential. The morphological features of the fixed cells are determined almost entirely by the fixing reagent. Postchroming can influence staining properties. There are two characteristic fixation ‘pictures’ depending upon the pH of the reagent: (1) with chromic acid and the more acidic dichromates (barium, calcium, mercuric, or silver) there is destruction of the mitochondria and disorganization of the cytoplasm and nuclear contents, resisted only by the nucleolus, and (2) with potassium dichromate and the other less acidic dichromates (ammonium, lithium, or sodium), the mitochondria, cytoplasm, and nucleus are well fixed. With various tissues from mice, the transition between the two types occurs around pH 3·4–3·8. The chemical mechanism of cytological fixation by anionic chromium reagents remains to be elucidated. Oxidative reactions do occur. Probably more important are ionic interactions and complex formations, often with associated precipitation, involving various tissue constituents and the several chromium ions.