Osmiophilic polymer generation: catalysis by transition metal compounds in ultrastructural cytochemistry.

A transition metal compound that is bound in tissues by any appropriate cytochemical reaction may catalyze the generation of an insoluble osmiophilic polymer from organic monomers such as 3,3'-diaminobenzidine. When the polymers are treated with osmium tetroxide, electron-opaque, insoluble osmium blacks (coordination polymers of osmium) are formed at the sites of the particular macromolecule or enzyme permitting its light, and electron, microscopic localization. This approach represents a distinct advantage over earlier cytochemical methods because the shorter incubation time needed here results in less artifactual deposition of metal ions, and less tendency to crystallize the reaction product. In addition, the shorter incubation times permit longer fixation of tissues and hence less artifact due to enzyme diffusion.