Lab-grown dopamine neurons derived from stem cells have survived and connected within the brains of Parkinson’s patients for as long as 14 years.

The successful long-term transplantation of dopamine neurons in the live brain may lead to new and improved therapies for Parkinson’s disease as well as other neurodegenerative conditions, according to Ole Isacson, a Harvard neurologist and director of the Neuroregeneration Research Institute.

"We have shown in this paper that the transplanted cells connect and live well and do all the required functions of nerve cells for a very long time," he said in a press release.

In the study, Isacson and his colleagues examined the brains of five deceased study participants who had received transplants over the course of 14 years, finding healthier levels of proteins known as dopamine transporters along with improved levels of mitochondria.

Post-mortem examinations also showed that the transplanted dopamine neurons resisted decay as the patients aged.

“The transplanted dopamine neurons showed a healthy and non-atrophied morphology at all time points,” the researchers wrote in a paper published Wednesday in the journal Cell Reports. "These findings are critically important for the rational development of stem cell-based dopamine neuronal replacement therapies for Parkinson's.”

The findings contradict earlier expectations that transplanted cells would become corrupted with time, given evidence of such deterioration in previous studies, Isacson and his colleagues said in the paper. As for the efficacy of the treatment on Parkinson’s patients, most of the two dozen or so patients who have received the infusions over the past two decades have experienced improvement of symptoms.

The neurological disease is the 14th leading cause of death in the United States with a 4.6 percent jump in prevalency in 2010 over the preceding year, according to an analysis from the Centers for Disease Control and Prevention.

Source: Hallett P, Cooper O, Sadi D, et al. Long-Term Health of Dopaminergic Neuron Transplants in Parkinson’s Disease Patients. Cell Reports. 2014.