Tannic acid as an electron microscope tracer for permeable cell membranes

To recognize damaged cells in preparations for transmission electron microscopy, high molecular weight (1700 MW) tannic acid (1-4%) has been added to glutaraldehyde fixing solutions. During fixation, the tannic acid penetrates only those cells whose plasma membranes were previously damaged. It enhances the electron density of the injured cells, which become clearly distinguishable from the undamaged ones. As a tracer tannic acid shows great advantages over either lanthanum hydroxide, ruthenium red, or horseradish peroxidase. It diffuses evenly throughout the tissue block and is not removed by preparative steps. Furthermore, it is also a good tracer at the light microscope level.