Stem Cells

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In the battle over stem cell science, progress is at war with process. The research holds out hope for treatment of everything from Parkinson's disease to diabetes to cancer to spinal cord injuries, if scientists can unlock the secrets of how stem cells develop into their specialized functions. But the promise depends in part on the source of the cells, as does the controversy that swirls around the research.

Stem cells are the body's master cells. They have the ability to generate all of the different cell types needed to keep the human body functioning. Scientists hope that by understanding how these cells divide and develop, or differentiate, they will be able to understand how diseases develop, to create cell-based therapies, and to find new means of screening drugs.

Adult stem cells (ASC) are found in certain tissues of the body, including the heart, blood, skin, and even brain. They are capable of dividing and renewing themselves to replace dying cells, but often can regenerate only specific types of cells. Blood stem cells found in the bone marrow, for example, are best at producing more blood cells and immune cells. However, recent research shows that some ASC, including those found in bone marrow and umbilical cord blood, can be coaxed into becoming other types of cells; it's not yet clear how reliable these types of manipulations will be when it comes to devising stem-cell based treatments.

Embryonic stem cells (ESC) were first isolated and grown in a laboratory in 1998; they are "pluripotent," meaning they are able to turn into any of the 200 different cells types in the body. Because these cells are more elastic than ASCs, many researchers believe they might prove to be a more efficient source of new treatments. But harvesting them requires the destruction of embryos, which opponents equate with taking one human life in order to save another.

In 2001, President Bush determined that federal funds could be used for research on existing embryonic cell lines (or generations of colonies derived from a single embryo), but not for the destruction of further embryos. That decision dramatically limited research in the U.S., as the few available lines — by most counts fewer than 20 — were old and dependent on outdated culture techniques.

In the summer of 2006 Congress voted to expand funding for ESC research, triggering the first veto of Bush's presidency. California voted to devote $3 billion to stem cell research and other states have followed, in part out of fear that the U.S. will lose its edge in biotechnology to countries such as South Korea, Singapore and Japan, which impose fewer restrictions.

Much of the debate in the U.S. has centered on the use of embryos left over from fertility treatments. These embryos would otherwise be discarded, so proponents argue that if they are to be destroyed anyway, why not use them to develop treatments that could potentially save millions of lives. It's true, as opponents contend, that some of these embryos were not implanted because they might be less healthy; but researchers stress that they would still be a valuable source of stem cells for study.

Even more medically valuable, however, would be embryonic stem cells created through a technique called somatic cell nuclear transfer (SCNT, or "therapeutic cloning"). In this procedure, a patient's own cell is inserted into a denucleated egg, in hopes of creating an embryo whose stem cells would be a perfect match for the patient.

But this method is also the most ethically problematic, since it entails the intentional creation and destruction of embryos through the same process that could be used to create human clones. There are currently no U.S. laws prohibiting human cloning, because Congress has been unable to agree on where to draw the line and whether therapeutic cloning should be permitted and only "reproductive cloning" banned.

A maxim of medical politics holds that the greater the hope, the greater the risk of hype. In the stem cell debate, each side swears that "its" type of cell holds the most promise. To date no treatments have been derived from embryonic stem cell research, but many researchers believe that understanding these cells could provide the key to a revolution in regenerative medicine.

Meanwhile, it's becoming clear that in some cases, adult stem cells might be just as useful as embryonic ones as a source for new therapies. For example, scientists have identified a master heart stem cell that appears to generate all three heart cell types, making it a promising candidate for future treatments. Such developments highlight the surprises and possibilities of where this research can take us.

—Nancy Gibbs and Alice Park