Pig cell transplants make a comeback.
A paper in Nature as long ago as 1997 explained how pig cells could be used to treat PD.
Clinical trials were carried out. Some good results were obtained. (See the Jim Finn recent thread.) But, a double blinded phase 2 trial showed both the control and treated groups improving. This raised the possibility that the placebo effect was paramount.
Fears of retroviral risks appear to have led to a halt in the procedure. Until now:
"The company behind the experimental diabetes treatment Diabecell is seeking Medsafe approval to transplant pig cells into the brains of Parkinson's patients."