Tannic acid in histology: an historical perspective.

The development of tannic acid as a reagent in histological methods is traced against a background of widespread use in science and technology from times of antiquity. Numerous light microscopic methods involving tannic acid, particularly in conjunction with iron and silver, have been described for a variety of tissue components. In most applications, tannic acid functions as a mordant. Current use is generally restricted to methods based on its affinity for collagen. The most significant histological use of tannic acid in contemporary times is as an adjunct to conventional glutaraldehyde-osmium-heavy metal fixation and staining for ultrastructural studies of tissue structures not normally clearly demonstrated. Tannic acid reacts with various components by mechanisms which are often not fully understood.