A study of silver degeneration methods in the central nervous system.

A number of reduced silver methods have been used for the demonstration of degenerating axons and their terminations within the central nervous system. The present study is an attempt to evaluate the relative potentialities of the two methods which the authors have found most useful, namely the Gless and Nauta techniques. Much of the literature dealing with silver impregnation methods, as applied to the termination of central nervous axons, is concerned with investigations into the structure of the synaptic region. Studies on the appearances of boutons terminaux, both normal and degenerating, have been reviewed by Gibson (1937), and recently by Glees & Nauta (1955). Work by Glees & Le Gros Clark (1941) and Glees (1941, 1942, 1944) has indicated that reduced silver techniques are especially useful for the investigation of neuro-anatomical connexions in experimental material. It is with this aspect that the present paper is concerned. Following the use of several modifications of the Bielschowsky-Gros technique, Glees developed a reduced silver method that is particularly suitable for the investigation of degenerating material (Glees, 1946). However, this technique also stains normal fibres, with the result that degenerating axons that are scattered through a matrix of normal fibres are not easily seen. This disadvantage has been overcome by a method designed to stain the degenerating axons while suppressing the normal fibre pattern (Nauta & Ryan, 1952), and this technique was later modified to give greater consistency in staining (Nauta & Gygax, 1954). Although several authors have investigated the degeneration cycle of boutons terminaux in their earlier stages (Hoff, 1932; Foerster, Gagel & Sheehan, 1933; Gibson, 1937), there appears to be little data on the time of persistence of the degeneration material derived from the axonal terminal aborizations or from degenerating axons of passage within the central nervous system. Furthermore, the appearance of degeneration produced by the Nauta technique differs from that given by the Glees method, and in some situations has a different time course. In the present work the two methods have been used at different sites within the central nervous system, with a view to determining their relative advantages, and to comparing the appearances produced by each. The time course of the degeneration process has been studied with particular reference to the time of persistence of the degeneration material. It was found that degeneration products stained with the Nauta method persisted for a much longer period than those stained by the Glees method. Additional experiments were therefore carried out in an attempt to investigate the nature of