Astrocytes give rise to new neurons in the adult mammalian hippocampus

Neurogenesis in the dentate gyrus of the hippocampus persists throughout li fe in many vertebrates, including humans. The progenitors of these new neur ons reside in the subgranular layer (SGL) of the dentate gyrus. Although st em cells that can self-renew and generate new neurons and glia have been cu ltured from the adult mammalian hippocampus, the in vivo primary precursors for the formation of new neurons have not been identified. Here we show th at SGL cells, which express glial fibrillary acidic protein and have the ch aracteristics of astrocytes, divide and generate new neurons under normal c onditions or after the chemical removal of actively dividing cells. We also describe a population of small electron-dense SGL cells, which we call typ e D cells and are derived from the astrocytes and probably function as a tr ansient precursor in the formation of new neurons. These results reveal the origins of new neurons in the adult hippocampus.