In Parkinson’s disease and in Alzheimer’s disease, the main issue is that the brain cells which are being destroyed are not being replaced. The brain does produce new cells, but it does so exceptionally slowly.
Stem cells are capable of tremendous division. Basically, they can keep on dividing forever. They can transform into any kind of cell, including brain cells. Therefore, some people have proposed that stem cells should be used to replace the dead cells in Parkinson’s disease, Alzheimer’s disease and other neurodegenerative diseases.
Stem cell research could fundamentally change medicine.
This is truly difficult. Implanting stem cells into the brain is one problem. Having them transform themselves into brain cells is another problem. But the truly difficult problem is this: How can you make the new cells make the right connections to the rest of the cells in the brain? The brain is a network. A cell that is not connected to other cells is useless.
Currently it is impossible to accurately guide the development and connections of stem cells.
The long term effects of stem cell therapy are not known. They may increase the risk of cancer.
Should you have stem cell therapy? Most people peddling stem cell therapy have no understanding of the complexities involved. So my answer (for now: Jun 2018) is a firm NO, with one caveat: If you can find a truly knowledgeable person in a very reputed institution who has a well defined research protocol, you can think of enrolling yourself in the research study.
As of June 2018, do not undergo any stem cell therapy outside of a research setting.