Which stain uses formalin ammonium bromide as the preferred fixative?

• A. Cajal
• B. Holmes
• C. PTAH
• D. Bodian

Answer: (A)

The key idea is that some silver-based neural stains require a specialized fixative to preserve delicate neural structures and enable proper silver deposition. The Cajal stain is designed to visualize nerve fibers with high contrast, and using formalin ammonium bromide as the fixative provides the right chemical environment: it fixes tissue while maintaining permeability and reducing excessive cross-linking that could block silver impregnation. This combination allows silver to deposit along axons and dendrites, giving crisp, dark fibers against a lighter background. Other stains in this area typically use standard formalin or different fixatives and do not rely on formalin ammonium bromide, which is why this particular fixative is associated specifically with the Cajal method.