Plasticity of nonneuronal brain tissue: roles in developmental disorders.

Neuronal and nonneuronal plasticity are both affected by environmental and experiential factors. Remodeling of existing neurons induced by such factors has been observed throughout the brain, and includes alterations in dendritic field dimensions, synaptogenesis, and synaptic morphology. The brain loci affected by these plastic neuronal changes are dependent on the type of experience and learning. Increased neurogenesis in the hippocampal dentate gyrus is a well-documented response to environmental complexity ("enrichment") and learning. Exposure to challenging experiences and learning opportunities also alters existing glial cells (i.e., astrocytes and oligodendrocytes), and up-regulates gliogenesis, in the cerebral cortex and cerebellum. Such glial plasticity often parallels neuronal remodeling in both time and place, and this enhanced morphological synergism may be important for optimizing the functional interaction between glial cells and neurons. Aberrant structural plasticity of nonneuronal elements is a contributing factor, as is aberrant neuron plasticity, to neurological and developmental disorders such as epilepsy, autism, and mental retardation (i.e., fragile X syndrome). Some of these nonneuronal pathologies include abnormal cerebral and cerebellar white matter and myelin-related proteins in autism; abnormal myelin basic protein in fragile X syndrome (FXS); and abnormal astrocytes in autism, FXS, and epilepsy. A number of recent studies demonstrate the possibility of using environmental and experiential intervention to reduce or ameliorate some of the neuronal and nonneuronal abnormalities, as well as behavioral deficits, present in these neurological and developmental disorders.