Brain volume follows an inverted U shape trajectory, which peaks at approximately age 10.5 in girls and 14.5 in boys (see Fig. 2). Tissue classified as gray matter (GM) by MRI consists mostly of cell bodies, dendrites, and dendritic processes, with contributions from axons, glia, blood vessels, and extracellular space (Braitenberg 2001). White matter (WM) is composed primarily of myelinated axons of neurons with associated vasculature and glia. Although GM and WM are bound by lifelong reciprocal relationships they have different developmental trajectories. GM developmental trajectories follow an inverted U shaped curve with peak sizes occurring at different times in different regions. For instance, in the frontal lobes peak cortical GM volume occurs at 9.5 in girls and 10.5 years in boys; in the temporal lobes at 10.0 in girls and 11.0 years in boys; and in the parietal lobes at 7.5 in girls and 9 years in boys.
