Which fixative is recommended for the fixation of tissue for staining with the Cajal method for astrocyte demonstration?

• A. Formalin ammonium bromide
• B. Acetone
• C. Picric acid
• D. 95% alcohol

Answer: (A)

The technique used to visualize astrocytes with the Cajal (Golgi-type) silver impregnation relies on a fixative that preserves delicate cell morphology without blocking silver deposition. Formalin ammonium bromide fits this need because formaldehyde cross-links proteins to maintain tissue architecture, while the ammonium bromide component supports the chemical environment that enhances the subsequent silver impregnation needed to reveal fine astrocyte processes. In contrast, strong dehydrating fixatives like acetone or 95% alcohol can cause shrinkage and poor impregnation, and picric acid can yield background staining or tissue fragility, making it harder to obtain clear, specific astrocyte staining. Therefore, the recommended fixative is formalin ammonium bromide.