Synaptogenesis and development of pyramidal neuron dendritic morphology in the chimpanzee neocortex resembles humans

Neocortical development in humans is characterized by an ex-tended period of synaptic proliferation that peaks in mid-child-hood, with subsequent pruning through early adulthood, as wellas relatively delayed maturation of neuronal arborization in theprefrontal cortex compared with sensorimotor areas. In macaquemonkeys, cortical synaptogenesis peaks during early infancy anddevelopmental changes in synapse density and dendritic spinesoccur synchronously across cortical regions. Thus, relatively pro-longed synapse and neuronal maturation in humans mightcontribute to enhancement of social learning during developmentand transmission of cultural practices, including language. How-ever, because macaques, which share a last common ancestorwith humans ∼25 million years ago, have served as the predomi-nant comparative primate model in neurodevelopmental research,the paucity of data from more closely related great apes leavesunresolved when these evolutionary changes in the timing of cor-tical development became established in the human lineage. Toaddress this question, we used immunohistochemistry, electronmicroscopy, and Golgi staining to characterize synaptic densityand dendritic morphology of pyramidal neurons in primary somato-sensory (area 3b), primary motor (area 4), prestriate visual (area 18),and prefrontal (area 10) cortices of developing chimpanzees ( Pantroglodytes). We found that synaptogenesis occurs synchronouslyacross cortical areas, with a peak of synapse density during the juve-nile period (3–5 y). Moreover, similar to findings in humans, dendritesof prefrontal pyramidal neurons developed later than sensorimotorareas. These results suggest that evolutionary changes to neocorticaldevelopment promoting greater neuronal plasticity early in postnatallife preceded the divergence of the human and chimpanzee lineages.