Transneuronal labeling with horseradish peroxidase in the visual system of the house fly.

Horseradish peroxidase (HRP) is taken up by lesioned neurons in the fly CNS and passes from these into certain sets of adjoining neurons which are in synaptic contact. This transneuronal labeling resolves neurons in fine detail. Electron microscopy shows reaction product in secondary neurons associated with plasmalemma and microtubules. In some cases also vesicles were found containing reaction product. It is suggested that the transneuronal passage takes place in vivo.