Abstract. The mammalian brain is a paradox of evolution. Although the advance in complexity of the human brain has exceeded the development of other organs, it has practically lost the ability to regenerate, and damage is repaired mainly by functional plasticity. This disparity is, however, not due to the lack of progenitor cells in the adult mammalian brain, but to their diminished or repressed capacity to replace neurons in most brain regions. Here, we discuss the current literature describing the processes of neurogenesis in the adult mammalian brain, and the recent advances in adult neural stem cells (aNSCs) with a focus on their identity, cell cycle and niche signals. Understanding these processes may hopefully lead to therapies in the future to reinstate self-repair of the brain from endogenous progenitors.Keywords. Neural stem cells, neurogenesis, stem cell niche, adult brain, subventricular zone.
The discovery of neurogenesis in the adult brainNSCs are the foundations of the brain during development, and despite poor regenerative capacity, they persist in the mammalian brain throughout postnatal development and into adulthood and continue to generate neurons. aNSCs self-renew and retain multipotency in almost all mammalian species analyzed, including humans. NSCs reside in specialized germinal layers where multiple cell-types interact with them and contribute to generate niches that control their fate choices and permit neurogenesis. Genetic and functional analyses have revealed roles for several genes and signaling pathways in controlling adult neurogenesis. However, the identities of the stem cells in the brain remain elusive, which presents an elementary problem for the interpretation of these results. In attempts to identify aNSCs, several groups have defined populations that contain cells with stem cell features, although unambiguous markers are still missing. Given their potential as putative tools for cell-based therapies, elucidating the molecular pathways that identify aNSCs and govern their cell fate is essential. Neurons in most regions of the adult mammalian brain are generated from NSCs and restricted progenitors during embryonic development before a switch to gliogenesis in peri-and postnatal development [1]. As early as the 1960s and 70s, newly generated neurons were identified by nucleotide analogue incorporation assays in adult rodents [2, 3]. These results, however, met with skepticism due to the poor regenerative capacity of the mammalian brain. Detailed studies went on to show extensive neurogenesis in the vocal control center in the brains of adult canaries, which correlates with seasonal song learning [4,5]. After a brief re-visitation to the topic in the mid-1980s, it was the use of retroviral lineage tracing assays that identified newborn neurons in the adult mammalian brain. These findings were quickly followed by the identification of zones with neurogenic capacity in