The controversy over stem cell research in the US is not so much about the research or methodology itself, but more about federal funding of a program with significant moral and ethical issues attached to it. Private funding is available, and researchers are able to perform many of their experiments. Because federal funding can be substantial for projects of this scope, however, the uncertainty about funding can have a detrimental effect on the future of such research.
Broadly speaking, the controversy comes from the nature of the stem cells themselves. Stem cells are essentially like chemistry labs waiting for an order to create a specific type of cell. An eye cell has a different set of coding and chemicals than a muscle cell or a blood cell. Because these stem cells are such blank slates, they can be artificially manipulated into becoming almost any type of cell needed to replace defective or missing tissue. Research has shown that these unfinished cells might be useful for treating the underlying cause of many neuromuscular and systemic conditions such as Parkinson's disease, Alzheimer's disease, and muscular dystrophy.
One of the controversies over stem cell research arises from the preferred source of these malleable stem cells. A human embryo contains the highest level of viable stem cells, since many of them have not been "ordered" to form specific structures in the body yet. Harvesting enough embryonic stem cells for research and treatment purposes means the destruction of human embryos, although they are usually obtained through natural miscarriages or voluntary donations of aborted fetuses. Donors are not compensated financially, nor are they encouraged to become pregnant for the expressed purpose of research.
Stem cells can also be harvested from umbilical cords and other tissues expelled during the birthing process, although these tissues may not contain as many viable cells as embryos. Adults also have a small amount available for research purposes, but the lack of federal funding has hampered many of the researchers' studies in that area. When federal funding was cut off, researchers were only allowed to use embryonic stem cell lines which existed at the time. They cannot harvest new lines of stem cells from human embryos.
The controversy also centers around the moral and ethical issues of using human embryos as sources of medical research or treatment. Many religious groups believe that life begins at conception, so the killing of embryos would be the moral equivalent of murdering a child. Ethically, the possibility that embryos might be created or donated strictly for medicinal purposes is also a consideration. An embryo's stem cells might be harvested and stored in case the parent develops a debilitating disease later in life, for instance.
Because of these ethical and moral issues, the administration of President George W. Bush maintained a policy of no federal funding for stem cell research involving embryonic material. In 2009, President Barack Obama reversed this decision, revoking Bush's executive order, and allowing research on embryonic stem cells by scientists who receive federal funding, as long as such research is "scientifically worthy" and performed within current laws.
Research on stem cells has also provided some insight into the nature and potential treatment of diabetes, which affects millions of people around the world. Other sources for stem cells beside human embryos may be discovered, or the use of human embryos may be monitored more stringently in upcoming projects.