Fine structure of neurons and synapses in the feline hippocampus during postnatal ontogenesis

Abstract

Electron microscope studies were carried out to examine the maturational features of neuronal elements and the general pattern of postnatal synaptogenesis in the kitten hippocampus. Axonic and dendritic elements of the hippocampal neuropil are well developed at birth in the kitten as indicated by the presence of numerous small processes and abundant axodendritic synapses including many dendritic spine synapses. Complex axon terminals resembling typical mossy fiber endings described in adult animals are also well developed in neonatal kittens. Hippocampal neurons exhibit elaborate intracellular organelles suggestive of a high degree of metabolic activity of these neurons in neonatal kittens. Axosomatic synapses are rarely encountered in the immediate neonatal period. Large areas of somaglial and soma-to-soma membrane appositions are commonly observed in neonatal kittens but are less frequently found in older kittens. A marked increase in the number of axosomatic synapses occurs in the late postnatal period (2 to 3-week-old kittens) along with a further increase in the number of axodendritic synapses of various types. It is concluded that virtually all of the synaptic input to hippocampal pyramidal neurons is distributed to dendrites of these cells in the immediate neonatal period and that elaboration of axosomatic synapses is essentially a postnatal event.