Cytology of cobalt-filled neurons in flies: cobalt deposits at presynaptic and postsynaptic sites, mitochondria and the cytoskeleton

Combined light and electron microscopy of identified neurons requires an intracellular marker that is both photon opaque and has electron scattering properties. We describe results using cobalt chloride block intensified with silver as an intracellular label. The novelty of the method is its integration in tissue fixation, prior to dehydration, resulting in fine grain precipitates that resolve certain intracellular structures. Filled neurons are clearly distinguishable from unfilled profiles by cobalt-silver precipitates. Energy dispersive X-ray analysis confirms that silver is specifically deposited onto cobalt sulphide cores which are characteristically associated with microtubules, mitochondria, presynaptic and postsynaptic specializations and gap junction-like membrane appositions.