A study of staining for electron microscopy using collagen as a model system—IX. The effect of tannic acid fixation

Electron optical examination of reconstituted collagen fibrils exposed to tannic acid reveals marked changes in staining behaviour. Positive staining in solutions of tungstate salts, where uptake of the heavy metal anions occurs on the positively charged side-chains of arginyl, lysyl and hydroxylysyl residues, reveals some reduction in stain uptake following tannic acid treatment. This reduced uptake is consistent with inhibition of staining at residues known, from independent biochemical evidence, to interact with tannic acid. Negative staining shows that tannic acid introduces some additional bulk into the fibril structure, although this is not as great as that induced by glutaraldehyde fixation. Staining patterns from doubly fixed specimens, together with thermal stability measurements on collagen gels, show that tannic acid fixation does not preclude a subsequent reaction with glutaraldehyde.