Techniques for converting Golgi precipitate in CNS neurons into stable electron microscopic markers

Direct electron microscopy of nervous tissue stained with the Golgi impregnation method is unsatisfactory because the cytoplasm of the cell bodies and processes of the impregnated neurons are completely filled with a compact precipitate of electron dense silver chromate. This precipitate entirely obscures the cytological details of the impregnated neurons. Because of its solidity and instability in aqueous solutions, the silver chromate is also a source of inconvenience during the preparation of the ultrathin sections. This review summarizes methods that have been developed with the aim of replacing the Golgi precipitate in CNS neurons with a more convenient electron dense material--for example, heavy metal salts or metallic particles. Conversion of the precipitate into a stable electron dense marker is done before the material is embedded for electron microscopy. The methods include lead, gold, and bromide substitution, treatment with ammonia, direct chemical reduction into metallic silver, and photoreduction of the silver chromate into silver through irradiation with ultraviolet light.