Another way dyes are classified is by their mechanism of action.
Adsorption: A physical reaction dependent on the dye's charge and the tissue's charge.
Direct (progressive) staining: The dye is differentially absorbed into the tissue during a pre-determined staining time. When this time is reached, the process is stopped, resulting in the desired degree and selectivity of staining. This is also referred to as progressive staining.
Indirect (regressive) staining: Indirect staining is also called regressive staining. Tissue is overstained, and then, using a process called differentiation, the excess stain is removed until a crisp delineation of the desired parts is achieved. This differentiation process is monitored microscopically.
Mordant: Employs a metal to attach the dye to a tissue strongly. The attachment of mordants to dyes forms a "dye-lake" and is accomplished using a covalent and a coordinate bond. A covalent bond is a chemical bond involving the sharing of electrons between atoms in a molecule. A coordinate bond is a type of covalent bond in which one of the two atoms contributes both the shared electrons.
