Fixation and staining of F-actin and microfilaments using tannic acid

A method employing tannic acid additives in glutaraldehyde fixatives followed by heavy metal postfixation has been used to negatively stain F-actin of muscle and actin-like microfilaments of nonmuscle cells in situ. This technique is potentially useful for ultrastructure research on microfilaments, especially in cases where these structures may be closely associated with, and thus obscured by, other elements of the cell. Analysis of the diameters of different kinds of filaments after fixation with tannic acid has shown that isolated actin filaments and microfilaments of stress fibers in BALB/3T3 fibroblasts appear to have about the same diameter, but they have smaller diameters than thin filaments of muscle. These results have been interpreted as a reflection of the molecular composition of these different kinds of filaments.