Although there is currently no stem cellbased treatment for diseases of the 1) ___ system, the field has moved much closer to making this goal a reality. Progress has been driven by major jumps in our understanding of how neuronal subtypes and glia develop in vivo and in vitro. Further research will inform our ability to coax endogenous 2) ___ ___ in the adult nervous system to respond to injury and to transplant the correct type of cell to ameliorate different types of diseases. Just as important, the ability to differentiate subtypes of neurons and glia from embryonic stem cells (ESCs) will allow drug screens to identify neuroprotective drugs, which are currently unavailable. The types of nervous system 3) ___ that represent the best targets for stem cellbased therapies are those that would be improved by the transplant or induced replacement of a limited number of cell types. Parkinsons disease, sensory disorders, and glial diseases fall into this category and could potentially be cured by a cell-replacement therapy. Motor system disorders and spinal cord injuries are more complex, but given their severity and lack of current treatment options, it can be argued that any improvement in function would be of great benefit. There are nervous system disorders that do not currently make good targets for cell 4) ___ therapies because the associated neurodegeneration is too widespread and diffuse. Alzheimers disease is one example, and it is unlikely to be ameliorated by adding more cells to the system. However, Alzheimers research could benefit enormously from disease-specific 5) ___ lines that could be used to study the degeneration of neurons in vitro. The prospect of using stem cells to intervene in 6) ___ disease is promising, but given the complexities of the nervous system, progress will likely continue in measured steps. To move the research toward useful therapies, it is critical that the efficacy of any experiment involving human subjects, and even experiments involving animal procedures, be performed using a double-blind placebo-controlled method. Certainly this increases the cost and the labor of an experiment, but the cost of doing research that is merely tantalizing and not conclusive, or worse, research that raises false hopes, is much higher. When there is a lack of rigorous controls, it is easy to falsely attribute an observed improvement to an incorrect cause. Finally, it is important to have scientific meetings to communicate both positive and negative results, as well as successes and cautionary tales. Despite the 7) ___ differences between neurodegenerative diseases, their eventual stem cell therapies will likely share many features; Information gleaned in one field can drive forward progress in the others..

ANSWERS: 1) nervous; 2) stem cells; 3) diseases; 4) replacement; 5) ESC; 6) neurodegenerative; 7) molecular

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