The blue staining at pH 2.5 is used to visualize which mucosubstances?

• A. Alcian blue pH 2.5
• B. PAS
• C. Crystal violet
• D. Colloidal iron

Answer: (A)

Acidic mucosubstances are visualized with Alcian blue at pH 2.5. This cationic dye binds to the negatively charged sulfate and carboxyl groups in mucopolysaccharides, so tissues rich in acidic mucins—such as goblet cell secretions and basement membranes—appear blue. The pH 2.5 setting is chosen because it stains both sulfated and carboxylated mucosubstances; at more extreme pH levels, the staining becomes more selective (e.g., sulfated mucosubstances alone) and neutral mucins remain unstained. In contrast, PAS targets neutral mucopolysaccharides (and glycogen) and would not give the blue result for acidic mucins; crystal violet stains cellular components like nuclei, not mucosubstances; colloidal iron is another method for acid mucosubstances but is less commonly used for routine mucin visualization compared with Alcian blue at pH 2.5.