Fine-structural study of the pineal body of the monkey (Macaca fuscata) with special reference to synaptic formations.

Various types of synaptic formations on pinealocytes and pineal neurons were found in the pineal body of Macaca fuscata. Axo-somatic synapses of the Gray type-II category were detected on the pinealocyte cell body. Gap junctions and ribbon synapses were observed between adjacent pinealocytes. About 70 nerve-cell bodies were detected in one half of the whole pineal body bisected midsagittally. They were localized exclusively deep in the central part. When examined electron-microscopically, they were found to receive ribbon-synapse-like contacts from pinealocytic processes. They also received synaptic contacts of the Gray type-I category on their dendrites, and those of the Gray type-II category on their cell bodies from nerve terminals of unknown origin. All these synapse-forming axon terminals contained small clear vesicles. Thus, the pineal neurons of the monkey, at least in part, are suggested to be derived from the pineal ganglion cells in the lower vertebrates and not from the postganglionic parasympathetic neurons. The functional significance of these observations is discussed in relation to the innervation of the pineal body of the monkey.