A range of neurodegenerative conditions that primarily involve cell loss, such as Parkinson's disease, might be treated with transplants of neural stem cells or more specialized differentiated cells. Replacing the cells doesn't address the underlying causes that led to their loss, the rising toll of molecular damage that accompanies aging, but it may be a far more effective patch treatment than those presently available. Perhaps more importantly, it is expected that any more general rejuvenation toolkit that does address underlying causes will still need some way of making up the numbers in various small populations of long-lived nerve cells of the brain and central nervous system that have diminished over time. Progress towards this goal is to be welcomed:
[Researchers] have grafted neurons reprogrammed from skin cells into the brains of mice for the first time with long-term stability. Six months after implantation, the neurons had become fully functionally integrated into the brain. This successful, because lastingly stable, implantation of neurons raises hope for future therapies that will replace sick neurons with healthy ones in the brains of Parkinson's disease patients, for example. "Successes in human therapy are still a long way off, but I am sure successful cell replacement therapies will exist in future. Our research results have taken us a step further in this direction."
The stem cell researchers' technique of producing neurons, or more specifically induced neuronal stem cells (iNSC), in a petri dish from the host's own skin cells considerably improves the compatibility of the implanted cells. The treated mice showed no adverse side effects even six months after implantation into the hippocampus and cortex regions of the brain. In fact it was quite the opposite - the implanted neurons were fully integrated into the complex network of the brain. The neurons exhibited normal activity and were connected to the original brain cells via newly formed synapses, the contact points between nerve cells. "Building upon the current insights, we will now be looking specifically at the type of neurons that die off in the brain of Parkinson's patients - namely the dopamine-producing neurons." In future, implanted neurons could produce the lacking dopamine directly in the patient's brain and transport it to the appropriate sites.