Name the two metachromatic dyes that stain mast cells.

• A. Toluidine blue and aldehyde fuchsin
• B. Hematoxylin and eosin
• C. Alcian blue and PAS
• D. Mucicarmine and Van Gieson

Answer: (A)

Metachromasia is the key idea: certain tissue components change the color of a dye, revealing specific granule contents. Mast cell granules are rich in sulfated glycosaminoglycans (like heparin), which cause metachromatic staining with selected dyes. Toluidine blue is a basic dye that binds to these granules and shifts color from the dye’s blue to a purple hue, making mast cells conspicuous. Aldehyde fuchsin also produces this metachromatic reaction with mast cell granules, yielding a purple color. This combination is classically used to identify mast cells in tissue sections. Other stains, such as hematoxylin and eosin or Alcian blue with PAS, target general tissue structures or mucopolysaccharides but do not produce the characteristic metachromatic staining of mast cell granules.