Does silver impregnate normal and degenerating boutons? A study based on light and electron microscopical observations of the inferior olive

The problem of argyrophilia in normal and degenerating axons and boutons has been studied in experimental material from the inferior olive. The conclusions are based on single and serial electron micrographs of Glees, Nauta-Laidlaw and Fink-Heimer sections. The rings seen when the Glees method is used are not filament-filled boutons of the type described by Gray and Guillery. They are cross-sectioned small myelinated normal or degenerating axons, in which the axoplasm, but not the centrally located mitochondria, are stained. Compact ‘boutons’ in the same sections are cross-sectioned normal fibers without mitochondria or degenerating fibers of the same type, where, in addition to axoplasm, degenerating mitochondria are stained. In Nauta-Laidlaw sections of the present study, degenerating boutons have very little precipitate, and are not visible in the light microscope. The fine dust present in the Fink-Heimer sections is degenerating boutons and the coarser argyrophilic particles are fragments of degenerating fibers. The observations made in the inferior olive show that one should be extremely careful with statements concerning the nature of open rings and clubs (‘boutons’) seen when silver stains are used. Studies from various regions have so far shown that it is impossible to reach general conclusions. The capriciousness of the silver methods should always be considered when such techniques are used.