Electron Microscopical Autometallography: Immunogold-Silver Staining (IGSS) and Heavy-Metal Histochemistry

Immunogold-silver staining (IGSS) utilizes a histochemical method called autometallography (AMG) to amplify tiny gold particles to sizes easily visible both in light and electron microscopy. In both applications it is advisable to use the smallest possible gold diameters (1-6 nm) to obtain the highest sensitivity, thus, allowing minute amounts of the target substance to be demonstrated. Gold labels smaller than 10 nm in diameter have been clearly shown to give the highest labeling densities of antigen-antibody binding sites. AMG can be used for the detection of catalytic crystal lattices of metallic gold and silver, and sulfides or selenides of mercury, silver, copper, bismuth, and zinc. The method has its roots in "physical development" technique, transplanted from photography to histology by Liesegang at the beginning of this century. In 1981, a series of papers were published by one of us with the purpose of introducing a reliable and easy-to-handle technique for light microscopical and ultrastructural studies. AMG has a multitude of applications apart from its use in detecting tissue metals. These include the highly sensitive and efficient in situ colloidal gold tracing of peptides, proteins, and amines by immunocytochemistry using the IGSS method, of carbohydrates by lectin IGSS, and of nucleic acids by IGSS in situ hybridization, IGSS in situ polymerase chain reaction, and IGSS in situ self-sustained sequence replication-based amplification (in situ 3SR) techniques, the last two even performing with single-copy sensitivity. Applications of pre- and postembedding AMG for semithin and ultrathin tissue sections are described.