Toward cell replacement therapy: promises and caveats.

Studies in animal models have suggested a role for stem cells in repair and regeneration of the nervous system. Human equivalents of stem and precursor cells have been isolated and their efficacy is being evaluated in rodent and primate models. Difficulties exist in translating results of these preclinical models to therapy in humans. Evolutionary differences among rodents, primates, and humans; fundamental differences in the anatomy and physiology; differences in immune responses in xenotransplant models; the paucity of good transplant models of chronic disease; and allelic variability in the cells themselves make any study evaluating the efficacy of cells in transplant models difficult to interpret. As no better alternatives to testing in animals exist, we suggest that at this early stage a considered step-by-step approach to testing and comparison of different transplant strategies in isolation will prepare us better for clinical trials than simple evaluation of functional outcomes in various models of disease. We emphasize that we do not recommend delaying or abandoning clinical trials; rather, we suggest that one anticipate failures and design experiments and data collection such that we learn from these failures to ensure future success in as rapid a time frame as possible.