Histochemistry and Routine Staining

Histochemistry uses chemical reactions on a tissue section to give optical contrast to structures that would otherwise be nearly transparent. Routine staining — above all the hematoxylin and eosin (H&E) pair — provides the everyday colour map of tissue, while special stains and histochemical reactions selectively reveal particular substances such as carbohydrates, lipids, fibres, or microorganisms.

Definition

Histochemistry is the use of chemical reactions in situ to localise and identify specific chemical constituents of cells and tissues; routine staining is the application of standard dye combinations, principally hematoxylin and eosin, to provide general structural contrast in tissue sections.

Scope

This topic covers the principles of dye binding and histochemical reactions, the H&E method as the routine workhorse, and the families of special stains used to demonstrate specific tissue components. It is a methodological reference and does not provide clinical interpretation guidance.

Core questions

• How do dyes bind selectively to tissue components to create contrast?
• What do the hematoxylin and eosin components each stain, and why?
• How do histochemical reactions demonstrate specific substances such as carbohydrates or lipids?
• When is a special stain chosen over routine H&E?

Key concepts

• Dye-tissue affinity and charge interactions
• Basophilia and acidophilia
• Hematoxylin and eosin (H&E)
• Mordants and metachromasia
• Carbohydrate histochemistry (e.g. PAS)
• Connective-tissue and lipid stains
• Enzyme histochemistry

Mechanisms

Staining depends on selective interactions between dyes and tissue constituents. Many basic (cationic) dyes such as hematoxylin bind to negatively charged components — nucleic acids and acidic structures — making them basophilic, while acidic (anionic) dyes such as eosin bind to positively charged proteins, making them acidophilic; this charge logic underlies the familiar blue nuclei and pink cytoplasm of H&E. Some dyes require a mordant (often a metal ion) to bridge dye and tissue, and certain dyes shift colour when bound to particular substrates (metachromasia). Histochemical methods go further by exploiting defined chemical reactions in situ — for example oxidising vicinal diols to aldehydes that are then revealed by Schiff reagent to demonstrate carbohydrates, or enzyme-substrate reactions that deposit coloured product at sites of enzyme activity. The theoretical basis of these reactions is treated in standard histochemistry references (Pearse, 1980; Lillie & Fullmer, 1976).

Clinical relevance

Routine and special staining produce the slides interpreted across diagnostic and research histology, and recognising what each stain demonstrates is part of reading tissue evidence. This entry explains the methods conceptually and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

Staining principles and protocols are consolidated in long-standing histotechnology and histochemistry references (Suvarna et al., 2018; Kiernan, 2015; Pearse, 1980; Lillie & Fullmer, 1976). Laboratory quality programs additionally define controls and acceptance criteria for stain performance, treated within the quality-assessment topic.

History

Tissue staining grew rapidly after synthetic aniline dyes became available in the second half of the nineteenth century, and the hematoxylin-eosin combination became established as the routine method. Through the twentieth century histochemistry developed into a systematic discipline of in-situ chemical reactions, codified in reference works by Lillie and Fullmer (1976) and Pearse (1980), extending staining from purely structural contrast to the demonstration of defined chemical groups and enzyme activities.

Key figures

• A. G. Everson Pearse
• Ralph Lillie

Seminal works

• pearse-1980
• lillie-1976

Frequently asked questions

What do hematoxylin and eosin each stain?

Hematoxylin, with a mordant, stains acidic/basophilic structures such as cell nuclei blue-purple, while eosin stains basic/acidophilic components such as cytoplasm and many extracellular proteins shades of pink.

How does histochemistry differ from routine staining?

Routine staining gives general structural contrast through dye affinity, whereas histochemistry uses defined chemical reactions in situ to localise specific substances, such as carbohydrates, lipids, or enzyme activities.