Synaptic density changes during human, chimpanzee, and macaque PFC development. (A) Example of synapses viewed by electron microscopy (red arrows), in the PFC of a 32-d-old chimpanzee. (B) Mean synaptic density per 100 μm2 measured in the PFC of humans (red), chimpanzees (blue), and rhesus macaques (green) at different ages. (Error bars) 95% confidence intervals obtained by bootstrapping synaptic density values within samples 1000 times. Independent assessment of synaptic density by another investigator is plotted on Supplemental Figure S11. (C) Mean synaptic density in macaque per 100 μm2 and in human per 100 μm3 measured in previous studies (Rakic et al. 1986; Huttenlocher and Dabholkar 1997). (D) Statistical analysis of synaptic density in three age groups. The distribution of mean synaptic density from samples within each age group is shown in a boxplot. Within the age range of 0–2 mo, PFC synaptic density in humans is significantly lower than in both chimpanzees and macaques (one-sided Wilcoxon test, P = 0.016 in human–chimpanzee comparison and P = 0.018 in human–macaque comparison), while there was no significant synaptic density difference between chimpanzees and macaques (P > 0.1). Sample numbers were not sufficient to estimate statistical significance in the other two age intervals.
