Contrary to widely held belief, a small number of new neurons are generated in the adult brain and even in the aging brain. Although this adult neurogenesis is minute compared with the vast number of neurons in our brains, and although adult neurogenesis does not lead to substantial regeneration in cases of neuronal loss, the new neurons may serve an important function in learning and memory processes. Adult neurogenesis is neuronal development in nucleo and is controlled by genetic and environmental factors. It exemplifies that, throughout life, brain development is activity and experience dependent, and, more important, that it never ends.
“Adult neurogenesis” is the generation of new nerve cells in the adult brain (Fig. 4.1), a process that was long believed to be impossible, although it occurs in both nonhuman primates (Gould et al., 1999) and humans (Eriksson et al., 1998). Today, adult neurogenesis has become a prime topic in biomedical research because of its implications for the treatment of neurodegenerative disorders and essentially all diseases that involve a loss of nerve cells (neurons). Because it is the stem cells residing in the adult brain from which new neurons originate in adult neurogenesis, many researchers believe that we might learn from adult neurogenesis how to “grow” stem cells into new neurons for transplantation – in cases of Parkinson's disease, for example (Bjorklund & Lindvall, 2000).