Two investigations published on Sunday call for a re-look at the pioneering treatment for Parkinson's disease that requires grafting foetal cells into the brain.
In several patients examined in the studies, the transplanted cells appear to have been attacked by the protein aggregates that cause this tragic degenerative disease, the papers said.
A third study, though, into five other individuals found no evidence of such a problem.
Parkinson's is an incurable disease of the central nervous system that causes uncontrollable shaking, along with impaired speech and movement. In approximately one third of cases it also results in dementia. The disease affects at least one percent of people over the age of 65.
Well-known people with the disease include Pope John Paul II and the movie actor Michael Fox.
The cause is a loss of dopamine, a chemical messenger that helps direct movement. The substance is provided in a part of the brain called the substantia nigra.
Treatment paths have focussed essentially on providing a pharmaceutical substitute for dopamine or on restoring or protecting dopamine-producing cells.
Another experimental route, launched in the 1990s, has been to inject dopamine-making brain cells from foetuses into the brain.
Post-mortems of three individuals with the grafts found evidence that so-called Lewy bodies -- the protein that accumulates in the substantia nigra and kills dopamine cells -- had spread to the implanted cells, according to two letters published in the journal Nature Medicine.
The patients, who did not die from effects of Parkinson's disease, had had the implants 11, 14 and 16 years earlier.
But a third study, in which five subjects who had had the grafts between nine and 14 years earlier were autopsied, found no evidence of any spread of Lewy bodies.
The mixed picture reflects the complexity of Parkinson's, a disease whose causes and propagation remain fuzzy.
Although the discovery of Lewy bodies in the grafted cells is troubling, "available data suggest that the majority of grafted cells are functionally unimpaired after a decade," said one of the studies.
"Recipients can still experience long-term symptomatic relief."