Factors in manifestation of developmental abnormalities of the central nervous system by environmental agents.

The developing brain can be regarded as one the embryonic and fetal structures most susceptible to teratogenic insults, because of its longlasting sensitive period, a great vulnerability of the undifferentiated neural cells and no further reproductive capacity of neurons. Among the factors in manifestation of brain malformations, developmental stage-dependent susceptibility to teratogens and sensitive period for teratogenesis are the most important one which characterize developmental toxicity. The mammalian proliferating matrix cells in the developing brain are particularly sensitive to teratogenic insults at the time when the matrix cells start to differentiate into cortical neuron. As the factors determining the vulnerability of matrix cells, proliferative and metabolic state, restoration capacity and phase of chemical differentiation prior to actual differentiation were discussed.