It is likely that progressive events (e.g., myelination, axon terminal arborization, synapse formation and elaboration) and regressive events (e.g., axon collateral pruning, removal of axon terminal branching, synaptic pruning) in neurogenesis (Cowan et al. 1984; Luo and O'Leary 2005), play some role in the functional connectivity changes observed here. It is possible that developmental segregation of regions in local networks may be partly related to synaptic pruning. Synaptic density increases early in development, but by the age range studied in the papers presented here, the major effect is synaptic pruning (Huttenlocher 1979). Synaptic pruning is also thought to result in decreased gray matter, which is seen throughout this age range in structural MRI scans (Sowell et al. 2004). In contrast, integration of anatomically disparate regions into an adult network may be assisted by myelination of long distance cortical axon tracts. Structural MRI measurements of white matter (Giedd et al. 1999), diffusion tensor imaging (DTI, Snook et al. 2005) and post-mortem myelin staining (Yakovlev and Lecours 1967) have demonstrated increased cortical myelination in the age ranges found in the studies described
