Will embryonic stem cells be a useful source of dopamine neurons for transplant into patients with Parkinson's disease?

Human embryonic stem (ES) cells entered the popular lexicon during the political debate of summer 2001 and the decision by President George Bush to allow federal funding for research on the 64 human ES cell lines that had been derived by the day of his speech. Before the policy decision, the National Institutes of Health did a comprehensive review of the field (http://www.nih.gov/news/stemcell/scireport.htm). Human ES cells come from preimplantation embryos, most of which are generated at in vitro fertilization clinics (1). Within days after fertilization, the embryo consists of a hollow sphere, the blastocyst, which contains a cluster of a few hundred identical cells called the inner cell mass that can eventually develop into a fetus. When removed from the blastocyst, these cells can be propagated indefinitely in specialized media (2). When the media are changed to allow differentiation, cells continue to divide and aggregate, forming embroid bodies. Although these cell clusters lack the organization of an embryo, they contain all tissue types including skin, muscle, bone, and neurons. Because ES cells can become any cell in the body, there is hope that they will lead to treatment for diseases like diabetes, Parkinson's, Alzheimer's, and heart failure. The problem is controlling cell growth and differentiation. If large numbers of ES cells are transplanted into an organ like the brain, they grow into every cell type and form tumor-like masses called teratomas, eventually killing their host. How can ES cells be restricted to produce useful cells without overgrowing? In their article in this issue of PNAS, Bjorklund et al. (3) have transplanted small numbers of partially differentiated mouse ES cells from embroid bodies into a rat model of Parkinson's disease and have shown that at least some of the cells become the dopamine neurons that are needed to reverse the Parkinson …