Studies on the fine structural localization of zinc-iodide-osmium reaction in the brain. II. Some characteristics of localization in certain identified synapses.

Samples of the olfactory bulb and cerebellar cortex from rats were impregnated in zinc-iodide-osmium (ZIO) solution. The fine structural localization of ZIO impregnation product was studied and, in addition, the percentage distribution of ZIO-positive synaptic vesicles was ascertained in certain synapses of known function. Spherical vesicles, particularly in aldehyde-prefixed material, were found to be more resistant to ZIO impregnation than ovoid vesicles, both in dendro-dendritic and in axo-dendritic synapses. The different ZIO reactivity of synaptic vesicles in excitatory and inhibitory synapses, apart from reflecting an important difference in the chemical composition of synaptic vesicles, may offer a new method for characterizing synapses in relation to certain functions.