A more direct human application is transplantation, which has been tried with dopamine-producing cells in Parkinson's patients who suffer from dopamine deficiency. In the early work, human fetal cells that produced dopamine were transplanted, because such cells are more likely to retain the capacity to reproduce than adult cells. Later, the abortion controversy led to a U.S. ban on the use of fetal tissue in medical research or procedures. This political detour submerged the revolutionary impact of the finding that cells from outside the body can actually survive and reproduce after being placed inside the brain. A Mexican neurosurgeon reported the initial successful transplants in Parkinson's disease, but Scandinavian and American doctors could not replicate the results, and the jury is still out on this issue. But note that long-term follow-up of these transplanted Parkinson's patients has revealed a disturbing side effect: involuntary jerks and movements caused by the transplanted dopamine cells continuing to reproduce, because the normal regulatory mechanisms that suppress their action within the brain don't work well on transplanted cells.
Memory loss involves the hippocampus and surrounding areas, which are relatively small regions, but also the frontal cortex, which occupies a huge portion of the brain's surface. This wide representation of memory in the brain makes transplantation an unlikely candidate for the next memory "cure." Nonetheless, if a method can be developed to transplant cells that reproduce and differentiate into hippocampal nerve cells, preferably cholinergic nerve cells, the field would truly be revolutionized. My prediction, however, is that highly effective promemory medications will be developed long before implantation of cells into the brain can be used to solve the problem of memory loss.
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