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It has long been known that mature, differentiated neurons do not divide see Chapter It does not follow, however, that all the neurons that make up the adult brain are produced during embryonic development, even though this interpretation has generally been assumed. The merits of this assumption were questioned in the s, when Fernando Nottebohm and colleagues at Rockefeller University demonstrated the production of new neurons in the brains of adult songbirds.
They showed that labeled DNA precursors injected into adult birds could be found subsequently in fully differentiated neurons, indicating that the neurons had undergone their final round of cell division after the labeled precursor was injected. Moreover, the new neurons were able to extend dendrites and project long axons to establish appropriate connections with other brain nuclei.
Production of new neurons was apparent in many parts of the birds' brains, but was especially prominent in areas involved in song production see Box B in Chapter These observations showed that the adult brain can generate at least some new nerve cells and incorporate them into neural circuits see also Chapter The production of new neurons in the adult brain has now been examined in mice, rats, monkeys and, finally, humans.
In all cases, however, the new nerve cells in the mammalian CNS have been restricted to just two regions of the brain: Furthermore, the new nerve cells are primarily local circuit neurons or interneurons. New neurons with long distance projections have not been seen. Each of these populations in the olfactory bulb and hippocampus is apparently generated from nearby sites near the surface of the lateral ventricle.
As in bird brains, the newborn nerve cells extend axons and dendrites and become integrated into functional synaptic circuits. Evidently, a limited production of new neurons occurs continually in a few specific loci.