The subventricular zone responds dynamically to mechanical brain injuries

Brain injury is a common yet relatively untreatable problem (Thurman, 1999). Of the 500,000 cases of traumatic brain injury annually in the United States, greater than 90,000 result in disability (Valadka, 2000). Mechanical injuries often occur to external regions of the brain: the cerebral cortex and other parts of the telencephalon. They can produce a wide variety of longterm and devastating symptoms and pathologies due to neuronal death. The underlying biological predicament in recovery from brain injury is that the adult central nervous system is generally incapable of replacing dead neurons. This concept has been challenged by the discovery of neurogenic adult stem cells in the subventricular zone (SVZ) (Alvarez-Buylla et al., 2000). It has been estimated that the SVZ replaces tens of thousands of olfactory bulb (OB) interneurons per day in rodents. Behavioral studies suggest that the constant turnover of OB neurons allows olfactory discrimination (Gheusi et al., 2000). Adult humans and other primates have also been shown to possess neurogenic SVZ cells with many of the features delineated in rodents (Eriksson et al., 1998; Bernier et al., 2000; Weickert et al., 2000; Kornack and Rakic, 2001; Pencea et al., 2001b). Granted, the sense of smell is not of vital importance for humans, unless one is a sommelier. Yet, since the SVZ is in close proximity to the cerebral cortex and other functionally important forebrain nuclei, hope has risen that the great neurogenic and migratory potential of adult stem cells may be co-opted for repair. This optimism is bolstered by the fact that adult SVZ cells are descendants of the proliferative neuroepithelia which, during development, give rise to the myriad neural cells in the telencephalon.