arkinson's disease affects 5 million people worldwide over the age of 50. Unlike Alzheimer's disease, whose pathology is more widespread, Parkinson's disease is sufficiently localised to be receptive to cell-transplantation therapy, which involves substituting new for dead or dying dopamine-producing neurons in the affected region of the brain.

The first tissue-implantation procedure was reported in 1989. A group at Lund University, Sweden, implanted foetal ventralmesencephalic (midbrain) tissue into the striatum (a part of the brain that controls movement, balance, and walking) of a Parkinson's disease patient. The researchers included Olle Lindvall, professor of neurology and one of Europe's most respected experts in stem-cell research today, and Patrik Brundin, professor of neuroscience, who collaborated with Singapore doctors in the mid-1990s in the country's first tissue-implantation procedure.

To date, only about 150 patients worldwide have undergone tissue implantation. Brundin mentioned that the limitations preventing widespread practice of the procedure is the low survival rate (less than 10%) of neurons in the graft, resulting in a need in each patient for multiple donors of aborted foetal tissue. Furthermore, difficulties in predicting which patients will experience dramatic improvement have to do not only with the graft but also with the particular stage of the disease and the underlying disease condition of different patients.

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