Brain Implants and Transplants

Abstract

Transplantation to the brain offers hope of treatment for common and devastating diseases of the brain. Parkinson's disease (PD) became the first target for trials of neural transplantation. Experiments in rodent and primate models of PD demonstrated that fetal mesencephalic tissue was the optimal source of cells for transplantation. The grafts survived, connected with the host neurons, secreted dopamine (the critical neurotransmitter lacking in PD), and resulted in behavioral recovery. Clinical trials that were based on the fetal transplantation paradigm established that neural transplantation could be a valid therapeutic option for human neuro-degenerative disease. Many of the transplanted patients had good graft survival and a significant improvement in disease symptoms. Several controlled randomized trials are going on to determine the efficacy of transplantation to treat PD. The clinical application of neural transplantation is limited mainly by the availability of tissue—fetal tissue is scarce and ethically problematic. Intensive research is directed at finding alternative sources of cells. Among the more promising are embryonic stem cells, immortalized cell lines, and porcine xenografts. Neural transplants may integrate into the host brain circuitry and help repair brain damaged by injury or disease. However, implants can also function as a way to deliver desired substances locally into the brain, or supply trophic signals and metabolic support for existing neurons, thus providing stimuli for neuronal survival and regeneration. These implants do not have to include neurons. Polymer-encapsulated grafts and gene therapy delivered through viral vectors are some of the alternative methods studied for applications such as neuroprotection, in which the implant does not need to integrate into the brain. Animal studies and initial clinical experiments are underway utilizing neural transplants and other brain implants for many common and devastating diseases including not only PD, but also Huntington's disease, brain trauma, stroke, Alzheimer's disease, and even chronic pain.