For years, it was generally believed that the normal mammalian postnatal brain had little capacity to produce new neuronal cells or to repair itself after injury (1 ). However, recent progress in neurobiology has led to a new understanding of the development and cell lineages of mammalian brain cells. Studies have demonstrated that neurons and glial cells are derived from common precursor cells, called neural stem cells (NSCs) (2 ,3 ), and that NSCs are present in the periventricular zone of fetal, postnatal, and adult mammalian brains (2 –14 ).
