Mapping of experimental axon degeneration by electron microscopy of golgi preparations

Summary

For the mapping of the terminal area of transected axons within the central nervous system, electron microscopy has recently been adopted. A greater accuracy is thereby obtained than with silver impregnation and light microscopy, since it becomes possible to determine the kinds of structure (soma, dendrites, spines) with which the degenerating boutons establish synaptic contact. In the present study this technique was extended by Golgi impregnation of such material with the aim of making possible classification of the postsynaptic neuron. A few days after transection of a pathway (commissural fibres to the hippocampus being used as a model in this study) the brain was fixed by perfusion with phosphate buffered formalin with sucrose. This was followed by immersion in an osmium tetroxide-potassium dichromate mixture (Dalton's fixative without sodium chloride) later replaced by a solution of silver nitrate. Satisfactory Golgi impregnation of nerve cells and processes was obtained. By careful trimming, and reembedding of selected areas, blocks for ultramicrotomy could be obtained which contained only one type of impregnated cell, e.g., hippocampal pyramidal cells.

The relation of basal dendrites of such cells to degenerating boutons of commissural fibres was studied. Numerous examples of contact between degenerating boutons and spines belonging to the basal dendrites were seen. Although the Golgi precipitate obscured the postsynaptic substance in the spines, a number of features at the sites of contact were considered strong indication that many of the contacts were synapses and not merely the result of random juxtaposition. This combined procedure is supposed to be applicable to other problems and to other parts of the nervous system as well.