Synaptogenesis in the Neocortex of the Newborn: The Ultimate Frontier for Individuation?

Abstract

We have identified five distinct phases of synaptogenesis in the cerebral cortex of primates from conception to death. The first two phases, with a low density of synapses, occur during early embryonic life. They are followed by a third phase of very rapid accumulation of synapses around birth. During the fourth phase the density of synapses is maintained at a very high level from the early postnatal period through puberty. This is followed by a true loss of synapses during puberty. The fifth phase begins after sexual maturity and extends into old age. It is characterized by a slow decline in the density of synapses. Experimental observations indicate that the early phases of synaptogenesis are robust neurodevelopmental processes determined by mechanisms intrinsic and common to the whole cortical mantle. The various postnatal phases coincide with final adjustments of many aspects of synaptoarchitectony, depending on normal functional inputs. They overlap with critical periods for diverse cortical functions until puberty. Inside each of these phases, distinct classes of synapses appear in successive waves of synaptogenesis, providing the first examples of a long list of discrete synaptogenetic events we are just beginning to explore.