metachromasia

The underlying mechanism for metachromasia is the presence of polyanions within the tissue.

The strongly acidic macromolecular carbohydrates of mast cells and cartilage are coloured red by the blue dye, a phenomenon called metachromasia.

The apparent lack of metachromasia is not because it is truly orthochromatic but because "it is already fully metachromatic" in aqueous solution.

Cell and tissue structures that have high concentrations of ionized sulfate and phosphate groups-such as the ground substance of cartilage, heparin-containing granules of mast cells, and rough endoplasmic reticulum of plasma cells-exhibit metachromasia.