BASIC HUMAN BRAIN DEVELOPMENT.

Human brain development is a protracted process that begins in the third gestational week (GW) with the differentiation of the neural progenitor cells and extends at least through late adolescence, arguably throughout the lifespan. The processes that contribute to brain development range from the molecular events of gene expression to environmental input. Critically, these very different levels and kinds of processes interact to support the ongoing series of events that define brain development. Both gene expression and environmental input are essential for normal brain development, and disruption of either can fundamentally alter neural outcomes. But neither genes nor input is prescriptive or determinative of outcome. Rather brain development is aptly characterized as a complex series of dynamic and adaptive processes that operate throughout the course of development to promote the emergence and differentiation of new neural structures and functions. These processes operate within highly constrained and genetically organized, but constantly changing contexts that, over time, support the emergence of the complex and dynamic structure of the human brain.

This paper will review some of the major events that contribute to the development of the human brain from its early embryonic state through adolescence. It begins by examining the foundational changes that occur during the embryonic period, which in humans extends through the eighth week post conception (gestational week eight, or GW8). By the end of the embryonic period the rudimentary structures of the brain and central nervous system are established and the major compartments of the central and peripheral nervous systems are defined (see Fig. 1). The ensuing period of fetal development extends through the end of gestation. During this time there is rapid growth and elaboration of both cortical and subcortical structures, including the rudiments of the major fiber pathways . Changes in the gross morphology of the prenatal neural system are underpinned by changes occurring at the cellular level. Neuron production in humans begins on embryonic day 42. E42, i.e. 42 days post conception and is largely complete by midgestation. As they are produced neurons migrate to different brain areas where they begin to make connections with other neurons establishing rudimentary neural networks. By the end of the prenatal period major fiber pathways, including the thalamocortical pathway, are complete.