The neurobiology of cell transplantation in Parkinson's disease

Over the past decade, neural grafting has emerged as a new treatment option for Parkinson's disease. When performed successfully, grafts of human embryonic neural tissue can give rise to major symptomatic relief in patients. However, a recent report on a double-blind placebo control study, which received worldwide attention, described less pronounced beneficial effects of the grafts, and found them to be significant only in patients younger than 60 years of age. Moreover, a subgroup of patients developed disabling dyskinesias as a result of the surgery. These findings, and great logistical problems in coordinating the harvesting of sufficient amounts of suitable human embryonic donor tissue with the transplantation surgery, have led the scientific community to question whether cell transplantation really has a future as a therapy for Parkinson's disease. In this review, we argue that the future of neural transplantation for Parkinson's disease is still bright. We relate clinical findings to observations made in experimental animals grafted with embryonic neural tissue and seek explanations for the variability in outcome seen in the clinical trials. We also briefly discuss alternative sources of donor tissue that may be applied in future clinical trials, and mention what features of cells may be crucial for them to be suitable as donor tissue for transplantation in Parkinson's disease.