Postnatal development and synaptic connections of hilar mossy cells in the hippocampal dentate gyrus of rhesus monkeys

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Abstract
Mossy cells of the hippocampal dentate gyrus were analyzed through postnatal development. At birth, a few thorny excrescences were found on the proximal dendrites of mossy cells, whereas distal dendrites displayed pedunculate spines. Thorny excrescences increased in number and complexity until the third month. After that age, the complexity of thorny excrescences is so great that an increase in spine density can be seen only in electron microscopic preparations. An increase in the number of pedunculate spines per unit length of distal dendrite was detected via light microscopy during the first 9 postnatal months. The somata and dendrites of mossy cells displayed adult‐like characteristics after the ninth postnatal month. Mossy fiber terminals at birth frequently displayed immature ultrastructural characteristies and formed synapses with dendritic shafts and spines. At later postnatal ages and in adults, axospinous synapses were found almost exclusively. This is consistent with the postnatal development of the complex spines of the mossy cells. Axons of mossy cells were generally confined to the hilus in our 150 ‐μm‐thick sections, where they gave rise to several collaterals. The axon terminals from these collaterals formed asymmetric synapses with dendrites and dendritic spines in the hilar region of the dentate gyrus. These data provide the first anatomical evidence that hilar mossy cells of the primate dentate gyrus have excitatory projections similar to their equivalent cell type in subprimates. The present study indicates that mossy cells of the dentate gyrus are in a more advanced stage of development at birth and mature faster than similar neurons of the human hippocampus. This may represent a faster maturation of hippocampal circuitry in nonhuman primates compared to that in the human.