Toluidine blue, also known as TBOortolonium chloride (INN) is a blue cationic (basic) dye used in histology (as the toluidine blue stain) and sometimes clinically.
Toluidine blue solution is used in testing for lignin, a complex organic molecule that bonds to cellulose fibres and strengthens and hardens the cell wallsinplants. A positive toluidine blue test causes the solution to turn from blue to blue-green.[1] A similar test can be performed with phloroglucinol-HCl solution, which turns red.
Toluidine blue is a basic thiazinemetachromatic dye with high affinity for acidic tissue components.[2] It stains nucleic acids blue and polysaccharides purple and also increases the sharpness of histology slide images. It is especially useful today for staining chromosomes in plant or animal tissues, as a replacement for Aceto-orcein stain.
Toluidine blue is often used to identify mast cells, by virtue of the heparin in their cytoplasmic granules.[3] It is also used to stain proteoglycans and glycosaminoglycans in tissues such as cartilage. The strongly acidic macromolecular carbohydrates of mast cells and cartilage are coloured red by the blue dye, a phenomenon called metachromasia.
Alkaline solutions of toluidine blue are commonly used for staining semi-thin (0.5 to 1 μm) sections of resin-embedded tissue. At high pH (about 10) the dye binds to nucleic acids and all proteins. Although everything in the tissue is stained, structural details are clearly visible because of the thinness of the sections. Semi-thin sections are used in conjunction with ultra-thin sections examined by electron microscopy.
Toluidine blue is also commonly used to stain frozen sections (rapid microscopic analysis of a specimen). Because time is of the essence for a frozen section, toluidine blue allows for the frozen section to be stained and reviewed in 10-20 seconds.[4] The other staining method for frozen sections (rapid H&E) takes approximately 60 to 90 seconds.
The dye is sometimes used by surgeons to help highlight areas of mucosaldysplasia (which preferentially take up the dye compared to normal tissue) in premalignantlesions (e.g. leukoplakia).[8] This can be used to choose the best site of the lesion to biopsy, or during surgery to remove the lesion to decide whether to remove more tissue from the margins of the excision defect or leave it behind.
Carson FL (1997) Histotechnology. A Self-Instructional Text. 2nd ed. American Society of Clinical Pathologists, Chicago.
Green FJ (1990) The Sigma-Aldrich Handbook of Stains, Dyes and Indicators. Aldrich Chemical Company, Milwaukee, Wisconsin.
Horobin RW, Kiernan JA, Eds (2002) Conn's Biological Stains. A Handbook of Dyes, Stains and Fluorochromes for Use in Biology and Medicine. 10th ed. BIOS, Oxford.
Kiernan JA (2008) Histological and Histochemical Methods: Theory and Practice. 4th ed. Scion, Bloxham, UK.
