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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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Classification of Biological Stains

In order to understand histology it is important to learn the science behind it. There are several different dyes used in the study of tissue and they each have distinguishing features which determine how they can be utilized.
A classification system, based on these features, is outlined in the following chart:
Methods by Which Dyes are ClassifiedDistinguishing Factor
  • Natural
  • Synthetic (can also be referred to as artificial)
Chemical structure
  • The molecular structure correlates to the color of the dye
Physical or chemical properties
  • Basic: has a pH above 7 and has the affinity for acidic tissue components (acidophilic)
  • Acidic: has a pH less than 7 and has the affinity for basic tissue components (basophilic)
  • Neutral: has a pH equal to 7
  • Metachromatic: multiple colors with one dye
  • Fluorescent: can fluoresce when exposed to ultraviolet light
Staining application
  • Vital: staining living tissue
  • Postmortem: staining tissue that has been fixed
Mechanism of action
  • Absorption: dependent on both the charge of the dye and the tissue
  • Direct staining: dye differentially absorbed into the tissue
  • Indirect staining: tissue is over-stained and then a process called differentiation is used to remove the excess stain
  • Mordant: employs a metal to strongly attach the dye to a tissue