Cerebral dysplasias as expressions of altered maturational processes.

This review identifies the fundamental anatomical and physiological processes that provide the substrates of maturation in the developing nervous system. Cerebral malformations may be viewed from the perspective of aberrations in one or more of these processes. The maturational processes are generally sequential but with considerable temporal precision and overlap: 1) neuronogenesis and gliogenesis, including neural induction by the notochord and primary segmentation of the nervous system; 2) programmed cell death of excess neuroblasts and glioblasts; 3) neuroblast migration; 4) formation and growth of neurites; 5) development of membrane polarity and excitability; 6) synaptogenesis; 7) biosynthesis of neurotransmitters and other secretory products; and 8) myelination of axons. The anatomical and synaptic organization of the fetal brain differs greatly from the mature state. An excess of axonal collaterals, dendritic branches, spines and synapses are formed and later selectively deleted Transitory neurons, specialized glial cells, synaptic circuits and transient neurotransmitter systems serve as functional elements for a limited time. The clinical expression of these transitory features of the fetal brain are incompletely understood, particularly in the context of embryonic cerebral plasticity.