Further observations on the enlargement of synaptic vesicles in degenerating optic nerve terminals of the avian tectum.

Abstract The terminal degeneration of retino-tectal fibers was investigated electron microscopically in the pigeon. Continuing the research of Cuenod and coworkers, the effects of various technical procedures upon the configuration of synaptic vesicles during the initial phases of Wallerian degeneration were studied. The presence of large vesicles in the degenerating nerve terminals was shown to be independent of the method of fixation: paraformaldehyde, glutaraldehyde and acrolein with subseunt osmication gave essentially similar results to those obtained with osmic acid fixation alone. Larger vesicles were also observed in the freeze-etched preparations of the degenerating optic nerve terminals: there was no essential difference between aldehyde-fixed and unfixed specimens with respect to the enlarged size of vesicles. Enlarged vesicles of degenerating endings turned out to be ZIO (zinc iodide-osmium tetroxide)-positive. In 2 cases, we succeeded to obtain selective vesicle staining in degenerated boutons by slightly modifying the ZIO technique.

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