The synaptic spinule in the dendritic spine: electron microscopic study of the hippocampal dentate gyrus

The present report calls attention to a component of certain synaptic junctions which has received little attention since its description in 1962 by Westrum and Blackstad. This component, which we term the synaptic spinule, is found in dendritic spine synapses in rat telencephalon (e.g., hippocampus, caudate nucleus, temporal and frontal cortex). Its major feature is an invagination of the presynaptic terminal by the presynaptic membrane, with the postsynaptic membrane protruding into this invagination. The synaptic spinule occurs in close association with the post-synaptic density, often occurring between breaks in this specialization. Serial sections reveal that when a synaptic spinule is present, a spine apparatus is observed near the dendritic protrusion. Coated vesicles are sometimes observed associated with the synaptic spinule; in all such instances they are associated with the presynaptic membrane. We studied the distribution of synaptic spinules in the dentate gyrus of the hippocampus. In contrast to its presence in the molecular layer, the synaptic spinule has not been observed in terminals of the subgranular layer of the dentate hilus. It is speculated that the synaptic spinule may play a role in exchange of material at dendritic spine synaptic junctions.