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Monitoring Stem Cell Research 
also causes differentiation to cardiomyocytes [341, and 5-aza-2’- 
deoxycytidine or density gradient separation allows some 
enrichment of cardiomyocyte populations [321. Human ES cell- 
derived cardiomyocytes display many of the functional properties of 
native cardiomyocytes, including the generation of synchronized 
action potentials and response to cardioactive drugs [30-33]. Heart 
cells exhibiting action potentials characteristic of nodal, atrial, and 
ventricular cardiomyocytes are all present, and the human-specific 
mechanisms regulating QT interval are functional [30]. Thus, human 
ES cell-derived heart cells are already useful for drug screening, and 
their use should make the drug development process quicker, 
cheaper, and safer. 
There is also a great interest in using human ES cell-derived 
heart cells for transplantation, but this will likely be challenging. 
Studies in animal models demonstrate that cell transplantation is 
effective in increasing the myocyte population in damaged or 
diseased cardiac tissue [55]. However, when heart cells die in a 
heart attack, it is not because the heart cells themselves are 
defective, but because the blood supply is cut off. Thus, to be 
successful, transplanted heart cells would have to integrate 
functionally with the surrounding heart cells, obtain a new blood 
supply, and avoid immune rejection. Each of these problems has 
potential solutions, but will require significant time and effort to 
solve. Precursors of vascular tissue can also be derived from human 
ES cells [35], and such cells may be useful in supporting co- 
transplanted heart cells. 
Neural Differentiation 
Because of the country’s aging population, neural 
degenerative disorders such as Parkinson's disease and Alzheimer's 
disease are becoming increasingly prevedent in the United States. 
Historically, one of the difficulties in studying the pathogenesis of 
neural disease has been the very limited access to the specific neural 
cells involved in these diseases. Neural precursor (or stem) cells 
cultured from fetal and adult brains have been extensively studied, 
but appear to have limited developmental potential. For example, 
the sustainable differentiation of neural stem cells to dopaminergic 
neurons, the cell defective in Parkinson's disease, has not yet been 
achieved. Mouse ES cells, for example, differentiate efficiently to 
dopaminergic neurons, and several groups are beginning to apply 
approaches used with mouse ES cells to human ES cells. Human ES 
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