The preferred fixative for sections to be stained by the Cajal technique is

• A. Formalin ammonium bromide
• B. Neutral buffered formalin
• C. Bouin's solution
• D. Glutaraldehyde

Answer: (A)

The key idea is that the fixative chosen for a Cajal (Golgi) staining procedure must preserve tissue architecture while remaining compatible with the silver impregnation steps that reveal neurons and their processes. Formalin ammonium bromide does this well: the aldehyde fixative (formalin) preserves morphology without excessive crosslinking, and the ammonium bromide provides bromide ions that interact with the silver ions used in the impregnation process. This interaction helps form silver deposits specifically within neurons, producing clear, high-contrast staining with less background.

Neutral buffered formalin, Bouin’s solution, and glutaraldehyde are less favorable for this technique. Neutral buffered formalin can fix more rigidly, which can hinder the diffusion and selective deposition of silver ions. Bouin’s solution and especially glutaraldehyde create stronger crosslinking and pigments or residues that interfere with the delicate silver impregnation and can increase background staining or obscure fine neuronal details.

So, the preferred fixative supports both preservation of delicate neuronal structures and compatibility with the silver-based impregnation steps, making formalin ammonium bromide the best choice for Cajal staining.