Which dye is appropriate for staining mast cell granules due to metachromasia?

• A. Toluidine blue
• B. Hematoxylin
• C. Eosin
• D. Silver stain

Answer: (A)

Metachromasia is when a dye changes color upon binding to certain charged tissue components, allowing specific structures to stand out. Mast cell granules are packed with highly sulfated glycosaminoglycans (like heparin), which are strongly acidic. A basic dye such as toluidine blue binds to these granules and, because of the dense sulfates, the dye forms aggregates at the site that shift the color from blue to a purple-red hue. This makes the mast cell granules visibly distinct against the blue-stained background, highlighting the cells themselves. Hematoxylin stains nuclei, eosin stains cytoplasm, and silver stains target other structures, but none produce the metachromatic color change seen with toluidine blue in mast cells.