The transplants at the centre of the controversy are tiny clumps of brain
tissue from aborted human fetuses several weeks old. Isolating the correct fetal
tissue is tricky, and each operation requires up to six fetuses, severely
limiting how many patients can be treated.
Fetal cells from pigs
This is controversial, because of the threat of animal viruses spreading to
people. But Diacrin, a company in Charlestown, Massachusetts, has treated 42
Parkinson鈥檚 and stroke patients with neural cells from pig fetuses. The
Parkinson鈥檚 patients have so far suffered no ill effects, but results from the
largest trial suggest that the implants don鈥檛 work. The stroke trial was halted
last year when one patient had seizures and another suffered brain swelling.
Human teratoma cells
Another controversial approach. A team at the University of Pittsburgh
Medical Center has treated 12 stroke patients with neurons from bizarre human
cancers called teratomas. The tumours are ferocious, made of cells that can turn
into a range of tissues including teeth and hair. The scientists claim the
neurons they derive from the cells are safe to implant. It鈥檚 unclear yet whether
the implants work.
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Embryonic stem cells
Unlike fetal brain tissue, which can
easily be contaminated with unwanted cells, stem cells are removed in a pure
state from embryos just a few days old鈥攂efore specialised cells start to
form. The cells multiply well in the lab and scientists hope to discover how to
turn them into specific types of brain cell. In theory, stem cell implants
should be easier to control and make in large numbers. The goal with Parkinson鈥檚
is to turn them into cells that release dopamine, the brain chemical lost in the
disease. One day it may even be possible to add a gene to stem cell implants
that allows patients to turn the dopamine production up or down with drugs. This
is the type of control needed to eliminate the risk of brain transplants going
haywire.