Brain development and the ontogeny of developmental disabilities.

Brain development is an active process that continues throughout life. During the past few decades, increased knowledge of this process has led to an improved understanding of the causes of its disruption. Complex genetic and molecular processes have been identified as components of the sequence of events leading to mature brain formation. However, the limitations in our knowledge about brain development are still significant. For example, 10% to 20% of persons with severe mental retardation of unknown origin appear entirely normal by standard neuropathologic studies. In this chapter, we review normal brain development and discuss how alterations during this process can lead to developmental disabilities. An overview of the most important steps in brain development, the basic mechanisms involved, and possible etiologies for the clinical manifestations of developmental disabilities is provided. General concepts about brain development, timing, and environmental influences are also considered. As we learn more about the genetic mechanism and molecular processes underlying brain development, we will better understand the ontogeny of various developmental disabilities and hopefully improve approaches to prevention and treatment.