To determine the extent to which hippocam- pal synapses are typical of those found in other cortical regions, we have carried out a quantitative analysis of olfac- tory cortical excitatory synapses, reconstructed from serial electron micrograph sections of mouse brain, and have cont- pared these new observations with previously obtained data from hippocampus. Both superficial and deep layer I olfactory cortical synapses were studied. Although individual synapses in each of the areas-CAI hippocampus, olfactory cortical layer Ia, olfactory cortical area Ib-might plausibly havc been found in any of the other areas, the average characteristics of the three synapse populations are distinct. Olfactory cortical synapses in both layers are, on average, about 2.5 times larger than their hippocainpal counterparts. The layer Ia olfactory cortical synapses have fewer synaptic vesicles than do the layer Ib synapscs, but the absolute number of vesicles docked to the active zone in the layer ia olfactory cortical synapses is about equal to the docked vesicle number in the smaller hippocam- pal synapses. As would be predicted from studies on hip- pocampus that relate paired-pulse facilitation to the number of docked vesicles, the synapses in layer Ia exhibit facilitation, whcreas the ones in layer lb do not. Although hippocampal synapses provide as a good model system for central synapses in general, we conclude that significant differences in the average structure of synapses from one cortical region to another exist, and this means that generalizations based on a single synapse type must
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