Plots of the spine density, as calculated per 10 μm of dendrite from the cell body to the distal tips of the dendrites, of pyramidal cells in V1, area TE, and PFC at five different postnatal ages. Thick lines and shaded areas indicate the means and SDs, respectively (A). Total number of dendritic spines in the basal dendritic tree of the “average” cell in each cortical area/age group. Inset indicates the location of sampled areas. Error bars indicate SDs (B).