Staining Formalin-Fixed Nerve Tissue with Mercuric Nitrate

Experiments were made to test the impregnating effect of Hg(NO3)2 on nervous tissue that had been fixed and chromated with solutions of known pH. Brains of cats, kittens, rats and mice were fixed by the pulsating-perfusion method of Haushalter and Bertram (1955), after first washing out the blood with saline-acacia solution, at pH 7.0, then followed by a 10% formol-saline-acacia fixative of the same pH. The removed brains were sliced to 3 mm thickness and further fixed 1-2 days in 10% formalin whose pH was also adjusted to 7.0. Chromation with acidified ZnCrO4 at pH 3.1 for 1 day followed by impregnation for 2 days in a saturated solution of Hg(NO3)2 at pH 5.5-6.0 effected the staining. Dehydration, paraffin embedding and sectioning completed the process. Some moderately successful stains were made with mercuric salts with no chromation, but it was found that fixation at pH 7.0-7.2 followed by chromation at pH 3.1, and later by impregnation in Hg(NO3)2 at pH 5.8-6.0 was optimum for best staining of nerve cells for their processes. The advantages of the technique are: (1) selective staining of nerve cells, especially the axonic details; and (2) a relatively short time needed for its completion.