Neurogenesis - Occurrence in Adults

Occurrence in Adults

New neurons are continually born throughout adulthood in predominantly two regions of the brain:

• The subventricular zone (SVZ) lining the lateral ventricles, where neural stem cells and progenitor generate new neurons (Neuroblast) that migrate to the olfactory bulb via the rostral migratory stream
• The subgranular zone (SGZ), part of the dentate gyrus of hippocampus.

Many of the newborn cells die shortly after they are born, but a number of them become functionally integrated into the surrounding brain tissue.

Adult neurogenesis is an example of a long-held scientific theory being further studied. Early neuroanatomists, including Santiago Ramon y Cajal, considered the nervous system fixed and incapable of regeneration. The first evidence of adult mammalian neurogenesis in the cerebral cortex was presented by Joseph Altman in 1962, followed by a demonstration of adult neurogenesis in the dentate gyrus of the hippocampus in 1963. In 1969, Joseph Altman discovered and named the rostral migratory stream as the source of adult generated granule cell neurons in the olfactory bulb. Up until the 1980s, the scientific community ignored these findings despite use of the most direct method of demonstrating cell proliferation in the early studies, i. e. 3H-thymidine autoradiography. By that time, Shirley Bayer (and Michael Kaplan) again showed that adult neurogenesis exists in mammals (rats), and Nottebohm showed the same phenomenon in birds sparking renewed interest in the topic. Studies in the 1990s finally put research on adult neurogenesis into a mainstream pursuit. Also in the early 1990s hippocampal neurogenesis was demonstrated in non-human primates and humans. More recently, neurogenesis in the cerebellum of adult rabbits has also been characterized. Further, some authors (particularly Elizabeth Gould) have suggested that adult neurogenesis may also occur in regions within the brain not generally associated with neurogenesis including the neocortex. However, others have questioned the scientific evidence of these findings, arguing that the new cells may be of glial origin. Recent research has elucidated the regulatory effect of GABA on neural stem cells. GABA's well-known inhibitory effects across the brain also affect the local circuitry that triggers a stem cell to become dormant. They found that diazepam (Valium) has a similar effect.