Synaptogenesis in layer I of the human cerebral cortex in the first half of gestation

The formation of synapses is among the most important steps in neuronal differentiation and the establishment of neuronal circuits. To establish baseline data about the time of onset, density and the course of synaptic formation in different regions of the human cerebral cortex before birth, synaptogenesis in layer I was examined by electron microscopy in fetuses ranging in age from 6 to 24 gestational weeks. Synapses were first observed in the primordial plexiform layer (marginal zone) in both the lateral and medial cerebral walls between the 6th and 7th gestational week, before the formation of the cortical plate. The density of synapses increased rapidly after the formation of the cortical plate, increasing by 37% between 12 and 14 weeks. Synaptogenesis proceeded at the same rate in the lateral and occipital cortex during this period. Further, with one exception, the insular region, synaptic density was comparable in prospective areas of prefrontal, motor, visual, temporal and cingulate cortex in a group of fetuses at midgestation (20 weeks). The results are consistent with a synchronous course of synaptogenesis of the neocortex.