The properties and effects of osmium tetroxide as a tissue fixative with special reference to its use for electron microscopy

Abstract Osmium tetroxide has been demonstrated to produce excellent fixation of the cytoplasm of cells, particularly for those to be examined in the electron microscope. The action of osmium tetroxide on various tissue constituents differs in its rate and nature. The reaction with lipids is probably by way of an affinity of osmium tetroxide for double bond configurations, thus forming unstable osmium esters which decompose to deposit osmium oxides or hydroxides at the site of activity. Little is known of the reaction of osmium tetroxide with proteins, but in preliminary studies it has been shown to produce quickly an initial gelation of protein solutions which, in the case of albumin, show no well-defined structure in the electron microscope. Coagulation images are, however, evident after other fixing agents. It is postulated that by virtue of its tetrahedral configuration, OsO4 may form polymer-like structures with proteins establishing linkages at double bond containing groups. This initial gelation may then be followed by a further oxidation and the production of soluble end products, which after long fixation wash out of the cell.