Adult neural stem cell behavior underlying constitutive and restorative neurogenesis in zebrafish.

Adult Neural Stem Cells (aNSCs) generate new neurons that integrate into the pre-existing networks in specific locations of the Vertebrate brain. Moreover, aNSCs contribute with new neurons to brain regeneration in some non-mammalian Vertebrates. The similarities and the differences in the cellular and molecular processes governing neurogenesis in the intact and regenerating brain are still to be assessed. Toward this end, we recently established a protocol for non-invasive imaging of aNSC behavior in their niche in vivo in the adult intact and regenerating zebrafish telencephalon. We observed different modes of aNSC division in the intact brain and a novel mode of neurogenesis by direct conversion, which contributes to stem cell depletion with age. After injury, the generation of neurons is increased both by the activation of additional aNSCs and a shift in the division mode of aNSCs, thereby contributing to the successful neuronal regeneration. The cellular behavior we observed opens new questions regarding long-term aNSC maintenance in homeostasis and in regeneration. In this commentary we discuss our data and new questions arising in the context of aNSC behavior, not only in zebrafish but also in other species, including mammals.