Studies with the high voltage electron microscope of normal, degenerating, and Golgi impregnated neuronal processes

Synaptic relationships between neuronal processes have been studied in thick sections (up to 1.0 mum) with high voltage electron microscopy. Some of the problems that arise in the study of such thick sections have been considered. (1) Where two parts of a single process are in continuity through a narrow stalk, as a dendritic stem and one of its spines, the continuity can generally not be seen if the stalk is included within the section or is crossed in the section by other processes. (2) Membranes that are viewed face-on cannot be seen in the thick sections and, therefore, processes belonging to two separate nerve cells may appear as though they are in cytoplasmic continuity. (3) Synaptic junctions that are viewed face-on cannot be seen in the thick sections, and junctions lying close to the same angle are also invisible; the relative number of invisible junctions depends upon a number of factors, including the section thickness and the size and contrast properties of the junctional specializations. Two methods of marking neuronal processes have been explored: (1) degenerating axons that are undergoing the dark degenerative change can be recognized readily within the thick sections. However, they stand out less clearly than in thin sections. (2) Neuronal processes that have been impregnated by an extra-rapid modification of the Golgi method can be easily traced in thick sections. It is possible to study some of the synaptic contacts made by these impregnated processes, especially if the processes have been only lightly impregnated or if the silver dichromate deposits are secondarily removed.