Several other stem cell types have also been considered. Genetic and hormonal manipulations of stem cells from bone marrow (77), skin (78), and umbilical cord blood (79) have produced cells with neuronal characteristics. Some bone marrow-derived cells express tyrosine hydroxylase (80), though evidence to prove that these cells produce dopamine is lacking. In addition, some of the apparent conversion of stem cells to neurons may be due to cell fusion not differentiation (81). In light of their increased complexity, the use of non-neuronal stem cells to generate neurons for the treatment of PD remains controversial.
Stem cells provide a potential solution to the problems associated with the acquisition and transplantation of fetal dopaminergic neurons for the treatment of PD. However, the use of stem cells also introduces additional complexity, as greater control of cellular growth and differentiation is required. The results, though preliminary, are encouraging, as stem cell transplantation has been demonstrated to produce functional improvements in animal models of PD. Human studies, however, must await the solution to the many technical and safety challenges.
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