Toluidine blue


Toluidine blue, also known as TBO or tolonium chloride is a blue cationic dye used in histology and sometimes clinically.

Test for lignin

Toluidine blue solution is used in testing for lignin, a complex organic molecule that bonds to cellulose fibres and strengthens and hardens the cell walls in plants. A positive toluidine blue test causes the solution to turn from blue to pink. A similar test can be performed with phloroglucinol-HCl solution, which turns red.

Other histological uses

Toluidine blue is often used to identify mast cells, by virtue of the heparin in their cytoplasmic granules. 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 sections of resin-embedded tissue. At high pH 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. 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. The other staining method for frozen sections takes approximately 60 to 90 seconds.

Clinical uses

The dye is sometimes used by surgeons to help highlight areas of mucosal dysplasia in premalignant lesions. 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.