Alcian Blue staining at pH 2.5 stains carboxylated acid mucopolysaccharides.

• A. True
• B. False
• C. It stains sulfated mucopolysaccharides only
• D. It stains glycogen

Answer: (A)

Alcian blue is a positively charged dye that binds to negatively charged acidic mucins. At pH 2.5, both sulfated and carboxylated mucopolysaccharides carry negative charges and thus readily bind the dye, so carboxylated acid mucopolysaccharides are stained. Neutral mucins and glycogen don’t have those acidic groups to the same extent, so they don’t stain under these conditions. If the pH were lowered further, staining would favor sulfated mucopolysaccharides only. So the statement is correct.