Which stain differentiates carboxylated from sulfated mucopolysaccharides by staining pattern?

• A. Alcian Blue 2.5 and colloidal iron
• B. Alcian Blue 1.0
• C. PAS
• D. Congo Red

Answer: (A)

The staining principle here relies on the negative charges of acidic mucopolysaccharides (mucins) and how Alcian Blue binds to them under different conditions. Alcian Blue at a permissive pH (such as 2.5) binds to acidic groups present in both sulfated mucopolysaccharides and carboxylated mucopolysaccharides, giving a blue stain. However, to tell these two apart, you bring in colloidal iron, which has a strong affinity for sulfate groups. Sulfated mucopolysaccharides will bind iron and show an additional or distinct iron-associated staining pattern, whereas carboxylated mucopolysaccharides lack sulfate groups and do not bind the iron. So, when you use Alcian Blue at pH 2.5 together with colloidal iron, sulfated and carboxylated mucopolysaccharides stain differently by pattern (one with the iron-associated signal, the other without), allowing differentiation between them.