Japanese scientists report that they have taken an important step toward the use of embryonic stem cells to treat Parkinson's disease.

The researchers successfully coaxed the immature cells taken from monkey embryos to form cells that produce dopamine, a brain chemical involved in movement that is depleted by the neurological disease.

Now a breakthrough has been made for making dopaminergic neurons that are necessary for transplantation therapy (in Parkinson's disease patients) using embryonic stem cell technology.

Although the experiments were carried out in monkeys the same technique should work in humans, since the characteristics of monkey and human stem cells are basically the same.

Investigators used a technique called SDIA, or stromal cell-derived inducing activity, to get the stem cells to turn into dopamine-producing neurons. They previously used SDIA to form dopamine-producing cells in mice.

Of course, monkeys are a lot closer to people than mice, so the researchers expect that the advance "should greatly facilitate research in numerous areas of Parkinson's disease therapeutics."

The SDIA method is a promising approach that brings stem cell therapy for Parkinson's disease toward the practical level.

Parkinson's disease causes tremor, muscle rigidity and movement problems. Treatment with L-dopa, a precursor of dopamine that the brain can use to produce dopamine, can alleviate Parkinson's symptoms, but it does not slow the progression of the disease.

The investigators point out that one experimental treatment for Parkinson's involves taking brain cells from aborted fetuses and transplanting them into the brains of Parkinson's patients.

The double "bottleneck" for this approach, according to the researchers, has been the insufficient supply of neurons and the ethical questions surrounding the use of fetal tissue.

Although the use of stem cells is controversial, too, the SDIA technique offers a practical alternative to fetal tissue. The researchers soon plan to transplant SDIA-derived neurons into the brains of monkeys.

The benefits of the research may not be limited to Parkinson's patients. An unexpected outcome of the work was that the researchers were able to make a type of eye tissue called pigmented epithelium, which is the outermost layer of the retina. This tissue can be used to treat retinitis pigmentosa, a degenerative disease of the retina that leads to blindness.

Proceedings of the National Academy of Sciences February 5, 2002;99:1580-1585