Which fixative is recommended only for CNS tissue, especially Cajal's astrocyte procedure, very acidic, lyses RBCs, and gives a direct positive Schiff reaction due to Fuelgen hydrolysis during fixation?

• A. Formalin Ammonium Bromide
• B. Glutaraldehyde
• C. Mercuric Chloride
• D. Acetate Formalin

Answer: (A)

The key idea here is a CNS-specific fixative that creates an acidic environment to both clear red blood cells from delicate neural tissue and enable a Schiff-positive reaction directly through fixation-driven hydrolysis (a Fuelgen-like effect). Formalin ammonium bromide fits this niche because it is used in CNS work, particularly for methods like Cajal’s astrocyte procedure, where the acidic conditions and components promote RBC lysis and allow a direct Schiff reaction to occur during fixation. This combination helps reveal fine glial structures without interference from blood cells and without needing additional DNA-detecting steps after fixation.

The other fixatives don’t provide this exact combination. Glutaraldehyde and mercuric chloride are strong crosslinkers best suited for high-resolution structural preservation (often for electron microscopy) but they don’t promote the acid-induced Schiff reaction during fixation and tend to hinder such histochemical reactions. Acetate formalin is acidic and can lyse RBCs, but it is not the fixative specifically described for the CNS-Cajal technique that yields the direct Schiff positivity during fixation.