Haematoxylin

Haematoxylin is a natural dye long used as a nuclear stain in light microscopy. It is obtained from the heartwood of the logwood tree (Haematoxylum campechianum), a source of the natural pigment that forms the basis of the staining reagent. In histology, haematoxylin is typically used after oxidation to hematein and combination with a metal mordant, most often aluminum, to produce a haematoxylin–aluminum complex that binds to nucleic acids.

The staining mechanism is based on the affinity of the complex for acidic components such as DNA and RNA. When bound to these structures, the nuclei appear blue to purple, a property described as basophilic staining. The intensity and shade can vary with concentration, oxidation state, and the specific mordant or differentiation steps employed. Haematoxylin is frequently used as the principal nuclear stain in the widely used hematoxylin and eosin (H&E) staining method; eosin then stains the cytoplasm and extracellular matrix.

Commercially, several formulations of haematoxylin are in common use, including Harris, Mayer, and Gill types; these differ in preparation, oxidation level, and pH. Variants such as Weigert's hematoxylin are used for elastic fibers and other specialized staining protocols. Typical protocols involve deparaffinization, staining with haematoxylin, differentiation and bluing steps to optimize nuclear contrast, and subsequent counterstaining with eosin or other dyes.

Historically, haematoxylin-based stains emerged in the 19th century and have remained foundational in diagnostic histology due to their reliable nuclear contrast and compatibility with numerous tissue types. Safety considerations include standard handling of laboratory dyes and reagents, with appropriate PPE and waste disposal.