Jonathan M.W. Slack

“There are also some potential safety concerns with grafts derived from pluripotent cells. Although Parkinson’s disease is a distressing condition that causes much suffering, it does not greatly reduce lifespan, so the period of time available for the development of complications is quite long, perhaps about 20 years on average. For any cell therapy derived from pluripotent stem cells there is always an issue about the possibility of persistence of a few pluripotent cells in the graft which might give rise to teratomas. Although the animal experiments indicate that the teratoma risk is very low, the long survival time of Parkinson’s patients does make even a very small cancer risk seem significant. Moreover, the differentiation protocols for pluripotent stem cells never produce 100 per cent of the desired cell type. Even if all the pluripotent cells are gone there will certainly be other types of neuron and glial cell present and these may generate unwanted effects. For example, the uncontrolled movement problems seen in some of the foetal midbrain graft recipients has been ascribed to the presence of other types of neuron which make inappropriate connections.”
― Jonathan Slack, Stem Cells: A Very Short Introduction