Unlike other structures in the adult brain, the hippocampus contains neural stem cells that give rise to new neurons throughout adult life. Under normal conditions, hippocampal neural stem cells (which have a radial glia-like morphology and vascular end-feet) are thought to give rise to both neuronal progenitor cells and astrocytes, which are a type of glia. Neuronal progenitor cells with a high proliferative capacity then give rise to granule cell neurons.(Bottom) During brain inflammation in response to infection or damage induced by, for example, irradiation, inflammatory cytokines—including tumor necrosis factor–α (TNF-α), IL-1β, and IL-6—are secreted by activated microglia and invading macrophages. IL-6 interferes with the production of new neurons in adult brain tissue, perhaps by perturbing the hippocampal stem cell microenvironment. IL-6 may induce bipotent neural stem cells to generate more astrocytes than neuronal progenitor cells. Alternatively, IL-6 may cause a reduction in proliferation of neuronal progenitor cells or trigger them to undergo programmed cell death. www. sciencemag. org SCIENCE VOL 302 5 DECEMBER 2003