Environmental causes of central nervous system maldevelopment.

The central nervous system is the most vulnerable of all body systems to developmental injury. This review focuses on developmental processes by which the nervous system is formed and how those processes are known or suspected to be injured by toxic agents. The processes discussed are establishment of neuron numbers; migration of neurons; establishment of connections, neurotransmitter activity, and receptor numbers; deposition of myelin; and 2 processes that are prominent in postnatal development, trimming back of connections and postnatal neurogenesis. Our knowledge of the risks of exposure to environmental hazards in childhood and adolescence is minimal. Most of our information concerns the effects of neurotoxicants in prenatal and early postnatal life. More worrisome than our lack of data regarding later stages of development is the minimal effort that we have mounted to protect the public from known neurotoxic agents and that regulations for testing new drugs and chemicals still do not require any assessment of neuroteratologic effects.