Electron microscopical identification of 3,3',5,5'-tetramethylbenzidine-reacted horseradish peroxidase after retrograde axoplasmic transport.

Tetramethylbenzidine (TMB) histochemistry for retrogradely transported horseradish peroxidase (HRP) has been successfully applied in electron microscopical preparations in the guinea pig visual cortex. In ultrathin sections, the TMB--HRP reaction product appears as an accumulation of typical electron-dense crystals. These crystals were identified in the cytoplasm of labeled cells in the proximal portions of the dendrites, and in myelinated axons. It is demonstrated that the TMB--HRP method is well suitable to analyze the ultrastructure of labeled cells and their synaptology.