A tale of two signals: Wnt and Hedgehog in dentate neurogenesis

It is now widely accepted that discrete regions of the adult brain contain stem cells that continue to generate new neurons. However, it remains unclear what molecular signals define the neurogenic niche and how such signals act on the heterogeneous cell populations within these regions. Here we discuss two recent studies that demonstrate the role of Wnt and Sonic Hedgehog signaling in neurogenic zones. Wnts act on neuronal precursors that mature and contribute to the dentate gyrus (DG), whereas Sonic Hedgehog affects the bona fide stem cells and transit amplifying cells (the partially committed progeny of stem cells). These studies further define how discrete populations of cells react to specific extracellular signals provided within the neurogenic niche to survive, proliferate, and form functional mature cell types.