Toluidine Blue O indicator solution is widely used in biological staining and histology. It selectively stains acidic tissue components, such as glycosaminoglycans and nucleic acids, making it valuable in identifying mast cell granules and evaluating cartilage and bone development. The solution is commonly applied in microscopic examination of tissue sections to differentiate between various cellular structures based on their metachromatic properties. It is also used in cytology for detecting abnormal cells, particularly in early cancer screening, such as oral lesions. In addition, Toluidine Blue O serves as a redox indicator in analytical chemistry, where it helps monitor oxidation-reduction reactions by changing color depending on the electron transfer state. Its ability to bind to polyanions makes it useful in studies of proteoglycans and mucins in tissues.