The action of chromium(III) in fixation of animal tissues

Chromic salts have been studied as fixatives of mammalian tissues for light microscopy, and the binding of the metal has been examined histochemically. Tissues bind chromium(III) from aqueous solutions less acid than pH 2.5; the metal attaches mainly to collagen and basement membranes. Solutions containing chromium(III) as the only active ingredient cannot be used as fixatives because they destroy cytoplasm and cause great structural distortion. When mixed with other fixative agents, however, chromic salts can bring about considerable improvement in structural preservation. In aqueous mixtures more acid than pH 2, and in aqueous-methanolic solutions in the pH range 4.0-5.3, a chromic salt provides only a nonspecific osmotic effect: little or no metal is bound to the tissue, and an aluminium or a sodium salt can be effectively substituted. In less acid (pH 2.3-3.2) aqueous mixtures, the beneficial action of chromium(III) cannot be imitated by aluminium or sodium ions. Chromium(III) forms coordinate bonds that cross-link ionized carboxyl groups of macromolecules. The reaction occurs so slowly that such cross-links can internally strengthen a tissue only after the structure has been stabilized by rapidly acting fixative agents. Thus, a valuable future use of chromic salts may be in a post-fixation treatment to protect specimens against the adverse effects of embedding in paraffin wax. Chromium(III) might also be useful for enhancing the opacity of collagen fibrils in electron microscopy.