The time course and rate of synaptogenesis were studied in the somatosensory cortex (Brodmann's areas 1 and 3b) of 27 rhesus monkeys ranging in age from embryonic day 41 to 20 years. Two to four vertical probes, each consisting of a series of overlapping electron micrographs and extending from the pial surface to the interface of the cortex with the white matter, were made from sections cut across the postcentral gyrus in the region of the upper limb representation. We found that the density of synapses per unit volume of cortex as well as per unit volume of neuropil increases steadily throughout the late fetal ages and early infancy. A density of 70/100 μm3 of neuropil was reached by the second postnatal month; thereafter, between 1 and years a slightly lower density of 50–60/100 μm3 was maintained. At around puberty, the decrease in concentration of synapses appears to be accelerated. Thus, the average synaptic density of a group of 10 adult animals composed of monkeys over 4 years of age was 30–40 synapses per 100 μm3 of neuropil. This value is significantly lower than that of the group of 11 infant and juvenile animals below 4 years of age. Since synaptic density per unit volume of neuropil is not affected by changes in other parameters of cortical growth, these numbers reflect an actual overproduction of synapses in infancy followed by their elimination during adolescence. The decline in the number of synapses is due primarily to elimination of asymmetrical junctions located on dendritic spines while symmetrical synapses on dendntic shafts and cell bodies remained relatively constant during postnatal life. The course of synapse formation recorded in the present study coincides with the course of overproduction and elimination of neurotransmitter receptors (Lidow et al., 1991) and the developmental schedule of synaptogenesis in other neocortical areas (Rakic et al., 1986). The timing of synaptogenesis and synaptic elimination in the postcentral gyrus may account for the maturation and plasticity of various aspects of somatosensory function during postnatal life.

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