Embryonic stem (ES) cells are expected as promising donor cells for cell transplantation therapy. For example, mouse or monkey ES cell-derived dopamine (DA) neurons can survive in the brain and relieve Parkinson's disease (PD) symptoms in rat or monkey models. In 2001 and 2003, the results of a double-blind trial of the transplantation of human embryonic DA neurons into patients with PD were reported. These results teach us two things. First, cell transplantation has been clinically proven to be effective as a treatment for PD, although the effects are still far from optimal. Second, several problems remain to be solved, including patient selection, optimal donor cell volume, targeting of injection, immunosuppression, and control of dyskinesia.
DA neurons have also been generated from several human ES cell lines. Furthermore, functional recovery of rat PD models after transplantation was observed. One of the major problems in ES cell transplantation is tumor formation, which is caused by a small fraction of undifferentiated ES cells in the graft. So, it is essential for undifferentiated ES cells to be eliminated from the graft for clinical application. These efforts will lead to clinical application of ES cell transplantation to the patients with PD.