Toluidine blue is a basic thiazine metachromatic dye that is used in histology to stain acidic components of cells and tissues.
Under the microscope, toluidine blue-stained cells and tissues appear purple or blue. The intensity and shade of the stain can vary depending on the concentration of the dye, the length of time the tissue is exposed to the dye, and the acidity of the components being stained.
Toluidine blue is commonly used in the diagnosis of certain diseases such as mastocytosis, which is a condition characterized by the abnormal proliferation of mast cells. The stain is also used to visualize the extracellular matrix in tissues such as cartilage and bone.