Toluidine blue staining is particularly associated with revealing metachromasia in which cellular structure?

• A. Mast cell granules
• B. Nuclei
• C. Cytoplasm
• D. Mitochondria

Answer: (A)

Toluidine blue exhibits metachromasia when it binds to highly sulfated acidic substances. Mast cell granules are packed with sulfated glycosaminoglycans (notably heparin), which create a strong negative charge environment. The cationic toluidine blue molecules organize around these granules and form metachromatic complexes, causing a color shift from blue to a purplish or reddish hue. This metachromasia is a distinctive feature that helps identify mast cell granules in tissue sections. Nuclei, cytoplasm, and mitochondria don’t contain the same concentrated sulfated polymers, so they stain the dye’s standard blue color rather than showing metachromasia.