By Peter Aldhous THOSE developing advanced therapies based on neurons grown from stem cells face a perplexing new question: can cells transplanted into the brains of people with Parkinson’s disease acquire the disorder from the surrounding tissue? The possibility is raised by post-mortems of patients who received grafts of brain tissue from aborted fetuses during the 1990s. These grafts provided a fresh supply of cells that release the neurotransmitter dopamine,