The generation of neuronal and glial cell types found in the mature central nervous system (CNS) involves the massive proliferation of ventricular zone cells of the fetal brain and spinal cord (1 ) that stop dividing early in development and terminally differentiate. Most neurons in the adult CNS are terminally differentiated and last through the life span of the animal. However, recent studies indicate that cell proliferation in the adult CNS is ubiquitous, but is primarily confined to the production of glia, with the exception of discrete regions in the hippocampus and subventricular zone (2 –4 ) where neurogenesis persists. The discovery that undifferentiated proliferative cells could be isolated from the adult mammalian CNS under favorable cell culture conditions, and induced to differentiate along both glial and neuronal lineages, revolutionized our thinking about the limited ability of the CNS to replace neural cells lost to injury or disease.
