Stem Cells are more Complecated for the brain
Your Stem cells specified to build specifie organs are not that difficult to deal with. Replacing neurons whose only task is to create dopamine is a waste of time if the cause of their death is not known in the first place. I have had this disease for 19 years. In the early 1990's a member of my support group had embryonic cells placed in his brain, to temporarily improve, then the cells died off and he was worse than before.
Investigators and researchers are beginning to lean toward the belief that the dopamine cells may not be committing suicide, but the delivery system or as others on this site feel toxins are getting past the blood brain barrier. I have a genetic form of Parkinsons. So does my sister. We grew up togeather and experienced the same environment, however; you would never in a million years be able to recognize us as sisters by our symptoms.
She is so thin she looks like she has an eating disorder. I am 100 lbs overweight. Then there is our third sister who shows no symptoms of PD.
If stem cells were the answer, them Japan, or an Europeon country without America's laws would have found a way. Or one of our own Pharmaceutical companies would were they willing to risk their own funds for the research.
The brain is the last frontier open to researchers for new exploration and believe me, they are exploring it with the curiosity of man as never before.
Understanding takes time. Look how far we have come since Madame Curie got her Nobel prize for discovering radium.
Peace to you,