A second source of stem cells is the brain itself. Neuronal stem cells have been identified in the adult mammalian subventricular zone and the subgranular region of the hippocampal dentate gyrus. Adult neuronal stem cells possess the theoretical advantage that they may be obtained from individual adult patients, potentially circumventing issues of graft rejection. As an additional potential advantage to embryonic stem cells, neuronal stem cells are committed to a neuronal phenotype and, there fore, production of dopaminergic neurons from neuronal stem cells may be more straightforward than from other stem cell lines (74). Although dopaminergic neurons have been obtained from neuronal stem cell lines, particularly after treatment with FGF, GDNF, and other modulators of cell signaling (75), the dominant cell type produced is often not neuronal but astroglial. Nonetheless, some encouraging results have been obtained in animal models transplanted with neuronal stem cells (76).
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