When used as a counterstain for Luxol Fast Blue, cresyl echt violet must be used in an acid solution.

• A. True
• B. False
• C. Only in alkaline conditions
• D. Must be neutral

Answer: (A)

Acidic conditions are required for cresyl echt violet when used as a Luxol Fast Blue counterstain because the acid medium drives the dye to selectively bind to the Nissl substance in neuronal cell bodies, producing a purple contrast against the blue-stained myelin. This acidified counterstain differentiates neuronal cytoplasmic RNA-rich material from the myelin, giving clear visualization of cell bodies without overlap on the blue myelin. Using a neutral or alkaline solution would lead to more non-specific staining and poorer contrast, making it harder to distinguish neurons from surrounding tissue.