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The page below is a sample from the LabCE course Theoretical and Practical Aspects of Routine H&E Staining. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Mechanisms of Action

Another way dyes are classified is by their mechanism of action.

  • Adsorption: A physical reaction dependent on both the charge of the dye and the charge of the tissue.

  • 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. This results in the desired degree and selectivity of staining. This is also referred to as progressive staining.

  • Indirect (regressive) staining: Indirect staining is also referred to as regressive staining. Tissue is overstained and then, using a a process called differentiation, the excess stain is removed until a crisp delineation of the desired parts is achieved. Monitoring of this differentiation process is done microscopically.

  • Mordant: Employs a metal to strongly attach the dye to a tissue. The attachment of mordants to dyes forms a "dye-lake" and is accomplished by means of a covalent and a coordinate bond. A covalent bond is a type of chemical bond involving the sharing of electrons between atoms in a molecule. A coordinate bond is a type of covalent bond in which both the shared electrons are contributed by one of the two atoms.