Histology, cytology and other related scientific disciplines study the microscopic anatomy of tissues and cells. In order to achieve a good tissue and cellular structure, the samples need to be stained in a correct manner. Toluidine Blue O powder dye is used in various staining methods in microscopy. It is a thiazine dye, which makes it especially suitable for staining nuclei of histology materials. It is used to display metachromasia, an occurrence possible while using diluted solutions for staining that contain only stain monomers. Metachromatic staining is most commonly used for mucosubstances, especially sulfate mucopolysaccharides, cartilage, cylindrical cells and mastocyte granules. It is used for displaying amyloids, nuclei, plant DNA and for staining frozen sections and animal material.