Many scientists think stem cells hold the promise of dramatic new treatments for disease, offering hope to millions of patients.
But some groups oppose using stem cells taken from embryos because this results in the death of the embryo. Instead, they favor research limited to stem cells taken from mature tissue.
President Bush has ordered federal funding of embryonic stem cell research delayed while the policy is reviewed. A decision by the administration is expected later this month.
Here are questions and answers on this issue:
Q: What are stem cells?
A: Stem cells are the fundamental building blocks for all the tissues in the body. They can develop into bone, brain, muscle, skin and all the other organs.
Q: What kinds of stem cells are there?
A: There are three types _ totipotent, pluripotent and multipotent, each representing a stage in development.
Q: What are totipotent stem cells?
A: Totipotent stem cells form when a fertilized egg first divides. Totipotent stem cells can develop into a complete individual.
Q: What are pluripotent stem cells?
A: After a few days, totipotent stem cells form a blastocyst, a ball of cells. The inner layer contains pluripotent stem cells capable of developing into any tissue in the body. Pluripotent stem cells, however, cannot become a complete individual. Pluripotent stem cells are also called embryonic stem cells.
Q: What are multipotent stem cells?
A: Multipotent stem cells are found in mature tissue and are formed by the body to replace worn out cells in tissues and organs. Stem cells from the bone marrow, for instance, form the various kinds of blood cells. Neural stem cells can form nerve and brain cells. Multipotent stem cells are sometimes called somatic or adult stem cells.
Q: How are stem cells useful in medicine?
A: Researchers think stem cells have great promise in the treatment of many illnesses _ from brain disease to diabetes to heart failure. Experts think they can learn to direct the development of stem cells into various types of new cells that can rejuvenate or even replace ailing organs. For instance, some think it may be possible to grow insulin-producing cells to cure some forms of diabetes, or nerve cells to restore function for patients paralyzed by spinal injury.
Q: Which type of stem cell is best?
A: That is unknown. Some researchers think embryonic stem cells are best because they are the most versatile. Also, embryonic stem cells can grow vigorously, forming colonies that will expand virtually forever. But research has shown that adult stem cells also are capable of forming many types of cells. For instance, some mouse experiments have shown that neural stem cells from the brain can be coaxed into growing muscle, liver and heart cells. Adult stem cells, though, are more difficult to grow and do not survive in the lab as long as embryonic stem cells. Many researchers think both embryonic and adult stem cells should be studied because it is unclear now which will ultimately be the most useful in medicine.
Q: Why is embryonic stem cell research controversial?
A: An embryo is killed when the pluripotent stem cells are extracted. Many people are ethically opposed to killing human embryos for any purpose. A 1995 law specifically forbids federal funding of research in which a human embryo would be destroyed, injured or placed at risk.
Q: Does that mean federal money cannot be used for embryonic stem cell research?
A: Not necessarily. In 1999, the National Institutes of Health established regulations that would permit federal funding provided the embryonic stem cells were harvested by privately funded laboratories. Federal money would be used to study the stem cells, but not to harm the embryos from which cells were taken.
Q: Have any embryonic stem cell studies been funded by the federal government?
A: No. Two research projects have been proposed, but NIH consideration has been halted on orders of President Bush, who asked that the 1999 regulations be reviewed.
Q: Who opposes embryonic stem cell research?
A: Some members of Congress have proposed legislation that would forbid federal funding of such research on moral grounds. Some anti-abortion groups oppose the research because extracting the stem cells requires the death of a human embryo. Pope John Paul II has said injury or death of a human embryo "is not morally acceptable" even though it may advance research that would cure or treat disease.
Q: Who favors federal funding for embryonic stem cell research?
A: The majority of research organizations in the United States. Eighty Nobel Prize winners have signed a petition endorsing the research. Scores of scientific societies have announced their support. Many members of Congress also support the research, citing potential benefits for millions of patients.
_ Associated Press