Synaptogenesis, heterochrony and epigenesis in the mammalian neocortex

In the neocortex of the macaque monkey we have identified and described quantitatively five different phases of synaptogenesis and related them to its functional maturation. In all the cortical areas observed so far, the most rapid phase of synaptogenesis occurs within a time-window of only 40 days, centred on birth. The modifiability of phase 3 has been tested by bilaterally equilibrated stimulation or deprivation, using experimentally obtained preterm or blind monkeys. Our developmental and experimental observations, as well as phylogenetic comparisons, strongly suggest that the timing and rate of production of synapses during phase 3 are determined by mechanisms that are both intrinsic and common to the whole cortical mantle but can be modified epigenetically later on.