Embryonic stem cells arise from the inner cell mass of the blastocyst, the five-day-old preimplantation embryo. Embryonic stem cells are pluripotent. Embryonic stem cells from mouse, nonhuman primate, and humans (68) have been differentiated into dopaminergic neurons. In current research, human embryonic stem cells typically have been donated as excess material, following in vitro fertilization. When implanted into animal models, these stem cell-derived neurons reinnervate the stria-tum and ameliorate some parkinsonian symptoms (69,70). One concern has been the development of teratomas in the brains of some (71) but not all (72) animals. Recent studies have encouraged differentiation of stem cells into dopaminergic neurons through genetic manipulation and hormonal treatment, such as over-expression of the Nurr-1 transcription factor and exposure to fibroblast growth factor (FGF) and Sonic Hedgehog (73).
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