Which term describes a color change of a dye staining tissue that differs from the dye solution due to polymerization?

• A. Metachromasia
• B. Orthochromasia
• C. Counterstain
• D. Differentiation

Answer: (A)

Metachromasia is the phenomenon where a dye changes color when it binds to certain tissue components, due to interactions that cause dye molecules to polymerize or form aggregates. This spectral shift makes the tissue stain a different color than the dye solution itself. A classic example is toluidine blue staining of mast cell granules or cartilage matrix, where the blue dye appears purple or red-purple because the dye polymers in the tissue environment. This is distinct from orthochromasia, where the tissue color matches the dye solution, or from a counterstain or a differentiation step, which serve different purposes in staining procedures.