Intensification of Cobalt-Filled Neurons in Sections (Light and Electron Microscopy)

Abstract

Cobalt-filled neurons in whole mounts of cleared ganglia are dramatic and extremely useful for giving a total picture of the extent and complexity of a neuron (Chap. 19). The distribution of branches in three dimensions is, however, difficult to determine, and one can get virtually no information about the relationship of the filled neuron to other known features in the neuropil. Ganglia must be sectioned to obtain a complete picture of a neuron and details of its connections. Our first attempts to examine cobalt-filled neurons in 10-μm wax sections were very disappointing: The cobalt sulfide deposits were pale brown and very difficult to see, especially if any background stain was used. Small profiles were impossible to identify (Fig. 21–1A). The early electron micrographs of cobalt material were also unrewarding, for only profiles containing large concentrations of cobalt sulfide could be recognized with certainty (Pitman et al., 1973; Mason and Nishioka, 1974). It was clear that some method of amplification was needed if cobalt sulfide were to become a really useful tool for the analysis of connections between neurons.