Parkinson's disease is a disease most often associated with aging. In it, neurons in the brain lose their ability to produce dopamine, an important neurotransmitter. Neurotransmitters are the chemicals that brain cells emit to signal one another. Patients with Parkinson's develop tremors and become progressively debilitated.

Scientists have long wondered if transplanting healthy neurons that still have the ability to produce dopamine into the brains of those with Parkinson's disease might reverse the damage done and reduce symptoms. Such transplants have been performed with varying degrees of success.

The source for the healthy neurons has often been human embryonic nerve cells, but this is not without controversy, as is any research that involves fetal tissue. Scientists at the University of Colorado School of Medicine have been working on creating cloned cow embryos and isolating their neurons. 42% of their clones survived at least 40 days of fetal development. The researchers collected neurons from the fetuses and transplanted them into rats bred to have both Parkinson's disease and weakened immune systems (so that their bodies would not reject the transplants). The rats that had received transplants of either cloned cow neuronal tissue or naturally produced cow neuronal tissue showed improvement in their neurological symptoms, in the form of improved motor performance. Though these results are preliminary, they show promise and provide the basis for similar studies.

Humans have undergone such surgery, having fetal nerve tissue transplanted into their brains. A recent (and somewhat controversial) study was published in the New England Journal of Medicine in March, 2001. In this study, 40 patients with Parkinson's disease received either nerve cell transplants or sham surgery, in which the surgeons drilled holes in their skulls, but no fetal cells were transplanted. The scientists found that there was some neurological improvement in the younger patients (under age 60) with actual transplants, and no improvement of note in the older patients (age 60-75), and neither age group benefited from sham surgery. Should cloning of human neuronal tissue be successful, overcoming the ethical objections of some of its opponents, this procedure might offer benefit to people with Parkinson's disease under the age of 60. However, at this time, it is not clear that transplants are any better than current pharmacologic therapies. And several significant technical challenges still remain. Putting cells into the brain is one step, getting these cells to make the right connections and then regulating those connections is even more complex and difficult.