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Certain neurological diseases involve the destruction of nerve cells. Such
diseases seem to be prime candidates for treatment by stem cell transplantation.
If multipotent stem cells could be successfully encouraged to migrate to
areas of damaged or dead nerve cells and replace them with new and healthy
cells, those neurological diseases could theoretically be improved or perhaps
cured. One area in which much research has been focused is Parkinson's disease.
In Parkinson's disease, the degeneration of a group of nerve cells that
produces the chemical transmitter dopamine leads to a movement disorder
featuring tremors.
Transplantation of fetal nerve (brain) tissue directly into the brains
of Parkinson's disease patients has been done in a few centers, with varying
results. Because the tissue is typically derived from the products of
legal abortions, the procedure has been controversial. Scientists at a
number of institutions are performing a variety of animal studies with
stem cells from different sources, but most of the human procedures performed
have used fetal tissue.
Experiments conducted at the National Institutes of Health have demonstrated
that embryonic rat nerve cells can be grown in culture, transplanted into
rats with a Parkinson-like disease, and then differentiate into healthy,
mature brain cells. The rats undergoing this procedure experienced some
neurologic recovery. Researchers in Britain have isolated human embryonic
nerve cells, grown them in cultures in the presence of substances known
as growth factors, and then transplanted those cultured cells into the
brains of rats with Parkinson-like diseases. At autopsies of the rats
20 weeks later, those human cells were located far from the original transplant
sites, and were further found to have differentiated into several types
of rat adult brain cells.
The successful transplantation of human fetal cells into animals gives
hope that fetal nerve cells from other species could be transplanted into
humans, with some recovery of neurologic function. Using fetal tissue
from other species would eliminate the controversy surrounding the use
of human fetal tissue. Researchers at Harvard Medical School have transplanted
fetal pig neural tissue into the brain of a human with Parkinson's disease,
which raises other ethical questions about mixing species. After the later
death of that patient, apparently of causes unrelated to the transplant,
an autopsy was performed that showed that the pig cells had matured into
cells that would produce dopamine, the missing chemical transmitter in
Parkinson's disease. The researchers suggest that further refinement of
that technique may one day offer promise for humans.
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