A simplified model of adult neural stem cells (aNSCs) lineages. (A): In the traditional model, aNSCs are viewed as quiescent and directly give rise to transit amplifying progenitors (TAPs). (B): In the newly proposed coexistence model, active NSCs act as an intermediate between quiescent NSCs and TAPs. Active and quiescent NSCs are maintained by distinct niche signals. Abbreviations: BMP, bone morphogenetic protein; NSCs, neural stem cells; Shh, Sonic hedgehog; TAP, transit amplifying progenitors.