Cell therapies for neurological disease--from bench to clinic to bench.

The lack of any meaningful regeneration in the adult central nervous system (CNS) subsequent to damage or degeneration stimulated the concept of replacement of the deficient cells by transplantation. Thus, much time and effort has been spent on investigating the potential of cell replacement therapy for repair in a range of conditions of the CNS over the last 25 years. As promising proof of principle basic science results were slowly converted to success in clinical transplantation trials in Parkinson's disease (PD), the future seemed very encouraging for cell therapy. However, the recent randomised, double-blind, placebo-controlled studies of fetal neural transplantation in PD have produced more equivocal results, which has dampened enthusiasm for this approach. However, whilst the translation of cell therapies to the clinic is in limbo, the emergence of stem cells as a source of the replacement tissue has revitalised the laboratory-based studies. This paper attempts to reconcile these disparate views and put forward the authors' view on the future of this form of biological therapy and its implications for related therapies.