The biochemistry of brain development and mental retardation.

Mental retardation may be associated with a number of environmental factors such as undernutrition, lead poisoning or exposure to neuroactive drugs during a critical period of brain development. Possible biochemical mechanisms operating in these various conditions and in animal models are reviewed in relation to the vulnerable period hypothesis. Small brains are common in the mentally retarded, and this may be related to a developmental abnormality particularly at the level of the synapse.