Critical periods in brain development.

The growth of the brain after organogenesis can be described as occurring in two somewhat overlapping phases: a phase of neuronal multiplication followed by one of glial proliferation, during and after which occur myelination and dendritic and axonal arborization. Within this gross chronology is a finer-grained chronology, with, for instance, different neuronal populations dividing at different times. The course of brain development can be affected by a variety of factors, the nature and extent of the perturbation dependent on the timing of the treatment with respect to stage of brain development. Growth processes completed before treatment are unaffected. Only those processes occurring at the time of the treatment are affected, plus some later-occurring processes, as a result of a cascade of effects. These concepts are examined briefly with reference to ionizing radiation, hormones and environmental stimulation and more fully with respect to nutrition. Undernutrition appears to depress the rate of all brain growth processes contemporaneous with it to the same extent. Whether the effects produced are likely to be permanent is discussed, together with the possibility that there may be mechanisms that attenuate or compensate for adverse effects.