A unifying hypothesis on mammalian neural stem cell properties in the adult hippocampus.

Continuously generated new neurons promote circuitry plasticity within specialized regions and contribute to specific functions of the adult mammalian brain. A number of recent studies have investigated the cellular origin of adult neurogenesis in the hippocampus, yielding divergent models of neural stem cell behavior. An essential question remains whether these models are overlapping or fundamentally discrete. We review evidence that primary neural precursors in the adult hippocampus exhibit significant heterogeneity in their properties of self-renewal, multi-lineage differentiation and regulation, representing a range from unipotential committed precursors to bona fide self-renewing multipotent neural stem cells. We further present a testable unifying hypothesis of adult neural stem cell behavior in vivo to outline a common framework for future studies of molecular and cellular mechanisms regulating adult neural stem cells and how these cells may contribute to hippocampal function and repair.