A good fixative for central nervous tissue to be stained with silver or gold techniques is which of the following?

• A. Formalin ammonium bromide
• B. Bouin's solution
• C. Carnoy fluid
• D. Osmium tetroxide

Answer: (A)

The main idea is that silver or gold staining of nervous tissue needs a fixative that preserves fine neural structure without interfering with the metal deposition process. Formalin ammonium bromide provides a gentle, well-preserving fixation that yields good tissue morphology and compatibility with silver and gold impregnation. It fixes the tissue with formaldehyde while buffering helps prevent excessive hardening or masking of delicate structures, allowing the silver or gold to deposit where it should.

Bouin’s solution, with picric acid, can cause excessive shrinkage and background yellowing, which can interfere with staining. Carnoy fluid is very dehydrating and brittle, not ideal for delicate CNS tissue destined for metal impregnation. Osmium tetroxide preserves lipids well but is heavy metal-based and can complicate later silver/gold steps and is not typically preferred for CNS silver or gold techniques.