Electron Microscopic Study of the Hippocampal Formation of the Monkey with Special Reference to the Pyramidal Cells of Area CA3 and CA4 and the Mossy Fiber Endings

Abstract

The hippocampal formation of the monkey (Macaca fuscata) was investigated by electron microscopy. Our attention was directed to the fine structure of the pyramidal cells in CA 3, CA 4 and of the mossy fiber endings. In addition to ordinary cell organelles, coated vesicles and subsurface cisterns were frequently observed in the pyramidal cells. We could find the initial segment of the axon of the pyremidal cells in CA 3 and moreover on the surface of it we could detect mossy fiber endings as well as Gray's type II synapses. The structure of the mossy fiber endings was fundamentaly similar to that of other mammalia. Pyramidal cells were frequently covered with mossy fiber endings, some of which terminated on neuronal somata as axo-somatic synapses. We observed dark mossy fiber endings which contained many dark synaptic vesicles and mitochondria. We also demonstrated a feature of protrusion of dendritic spines into the mossy fiber endings by serial sections. Small axo-somatic, axo-dendritic and axo-spinodendritic synapses were observed in the hilus fascia dentatae, CA 4, and in the strata radiata and pyramidale of CA 3. We classified these into two types, S and F, according to the shape of the synaptic vesicles they contained. By using Timm's sulfide-silver method under electron microscopic level dark precipitations due to the presence of zinc were found in preterminal regions of both ordinary and dark mossy fiber endings. Their distribution seemed to be restricted to synaptic vesicles as they were not seen in mitochondria. We also found dark precipitations in the small preterminal boutons distinct from mossy fiber endings.