ing new nerve and glial cells throughout life, is a phylogenetically highly conserved feature that challenges the dogma of the nervous system as a static, non-renewable tissue (Gage, 2000). Yet, unlike invertebrates and non-mammalian vertebrates, in which neurogenesis persists in wide regions of the central nervous system (CNS), in adult mammals it is mainly restricted to two specific brain sites: the forebrain subventricular zone and the hippocampal dentate gyrus (Gage, 2000; Kriegstein and AlvarezBuylla, 2009). It is now clear that the persistence of neurogenesis in these zones depends on stem cells which reside in niches as vestigial remnants of embryonic germinal layers (Kriegstein and AlvarezBuylla, 2009). This fact leaves the remainder of the CNS as a non-renewable tissue substantially incapable to repair damage coming from neurodegenerative diseases, ischemic/traumatic lesions. In the last few years, the persistence of neurogenesis has been observed also in other CNS sites and in sensory organs, in addition to the neurogenic zones quoted above. The most striking example, known for many years, is that of olfactory receptor neurons which are continuously renewed in the olfactory mucosa (see article by Mackay-Sim, in this issue). Other examples have subsequently emerged and this special issue deals specifically with these less studied stem/progenitor cells responsible for adult neurogenesis in niches outside the forebrain and hippocampus. The review by Locker et al. summarizes a decade of research on quiescent stem cells in the mammalian retina, also taking into account the advances made in animal models of stem cell-mediated retinal regeneration. The article by Metzger gives an overview of the development, structure and function of the enteric nervous system, focusing on the persistence of neural stem cells in the gut. Pardal et al. report on stem cells existing in the carotid body, a chemoreceptor organ of the peripheral nervous system that supports neurogenesis in response to hypoxemia. Finally, Miura and Barlow address the issue of the continuous renewal of taste receptor cells, a type of electrochemically excitable cells which are epithelial in origin but capable of releasing neurotransmitter onto afferent nerve fibers. Proliferating/quiescent progenitor cells have been demonstrated to occur even in diverse regions of the adult CNS out of the forebrain and hippocampus (Nishiyama et al., 2009; Ohira et al., 2009). These additional neurogenic sites also raise the issue of comparative studies in mammalian species and in other animal groups. The occurrence of neurogenesis within the mature brain parenchyma, not related with persistent germinal layers, can be frequently found in non-vertebrate species which harbour disperse, highly active niche-like neurogenic sites in widespread CNS regions (Kaslin et al., 2008) accounting for remarkable regenerative capacity (Zupanc, 2009). By contrast, most local Exploring neurogenesis outside the niche: atypical locations of mammalian neural stem/progenitor cells