Adult neurogenesis has been found in many vertebrates although different groups of animals exhibit different degrees of limitations in terms of where in the CNS adult neurogenesis takes place, how many new neurons in normal unperturbed conditions are made and whether or not this neurogenesis can be triggered or enhanced somehow after any injury or in neurodegenerative diseases. Thus, for example, fish, amphibians and reptiles show a persistent neurogenesis throughout adulthood and can respond to brain injuries by inducing a variable regenerative response. In mammals and birds, adult neurogenesis appears mainly restricted to the olfactory bulb and hippocampus and the vocal center, respectively; however, a regenerative or repair response in the mammalian CNS is quite limited.
Among vertebrates, amphibians are the real champions of regeneration and that includes the capacity to regenerate neuronal populations in the CNS. Thus, works from the laboratory of András Simon in newts have shown that a regenerative response can occur from the activation of cells located in normally quiescent brain regions (http://www.ncbi.nlm.nih.gov/pubmed/21068061), and this way dopaminergic neurons from the midbrain can be completely regenerated after ablation, being this process regulated by dopamine itself (http://www.ncbi.nlm.nih.gov/pubmed/21474106). Now, a paper by Malcolm Maden and collaborators report on the regeneration of the axolotl brain (http://www.ncbi.nlm.nih.gov/pubmed/23327114). The authors describe the presence of proliferating endogenous neural progenitors throughout the ventricular zone of the adult brain, especially in the telencephalon and cerebellum and more reduced in other brain regions. Upon removal of telencephalon segments there is an upregulation of the proliferation of progenitor cells from the ventricular zone of the damaged region. The telencephalon is then effectively regenerated after several weeks. Interestingly, the results presented here appear to confirm the idea that the olfactory nerve may provide an important cue to stimulate brain regeneration. Thus, if the anterior third of the telencephalon is removed severing the olfactory nerve, the telencephalon does not regenerate until the olfactory nerve re-grows and contacts the remaining telencephalon. What is the nature of such stimulating cue is something that still needs to be addressed.