Overall, the brain grows rapidly during fetal life through childhood, and matures more slowly during adolescence and young adulthood until its development plateaus and/or follows a declining trajectory with aging. After birth, brain development primarily depends on individual experiences (“self-organization”) (Lewis 2005), along with many other variables (Andersen 2003). However, of the whole human genome, 10,000 genes (approximately one-third) are only expressed during brain development (Johnson et al. 2009; Zhang et al. 2011). Change in gene expression is fastest in human brain tissue during fetal development, slows through childhood and adolescence, stabilizes through adulthood, then speeds up again after age 50, with distinct redirection of expression changes prior to birth and in early adulthood (Colantuoni et al. 2011). Genes with high expression during fetal development have the greatest decreased expression in the aging cortex, whereas genes with low expression during fetal development show increased expression in aging and neurodegeneration (Colantuoni et al. 2011). Recently, an increasing number of studies have attempted to examine the complex interactions between genes, environment and the developing brain. The cumulative data clearly indicates that brain development is under tight genetic control across the lifespan.
