At A Glance: Stem Cell Debate

What Are Stem Cells?

Stem cells are "blank" cells that are capable of developing into many different types of cells found in the adult human body. They are also capable of dividing and renewing themselves for long periods.

Researchers believe that stem cells hold a lot of promise for medical therapies such as growing replacement cells or whole replacement organs, as well as "patching" organs that don't work properly -- like helping a diabetic person's pancreas produce insulin.

Embryonic Vs. Adult Stem Cells

Researchers are focusing on two types of human stem cells: embryonic stem cells and adult stem cells. Research has been conducted on adult stem cells for more than 40 years, while embryonic stem cells were first extracted in 1998.

Embryonic stem cells are taken from a human embryo a few days after fertilization in a process that destroys the embryo. Adult stem cells are taken from adult tissue, or newborn babies' umbilical cords.

Embryonic stem cells can divide for a year or more in a laboratory without becoming specialized cells, while most adult stem cells can't. One stem cell can produce hundreds of genetically identical stem cells. Stem cell lines consist of cells that have been copied from one original stem cell.

Adult stem cells are also less flexible in giving rise to other types of cells. In most cases, an adult stem cell can only create cells that are found in the same part of the body from which the stem cell was taken. So an adult stem cell in the bone marrow can develop into a red blood cell, a blood platelet or a white blood cell, but generally not a skin cell or brain cell. However, research is discovering that adult stem cells may be more flexible than previously thought.

Where Embryonic Stem Cells Come From

Embryonic stem cells come from human embryos created in a laboratory, usually through in-vitro fertilization. First, scientists inject sperm into the egg. A day after the egg is fertilized, it splits into a two-celled embryo. The next day, it splits into four cells, then into eight cells and so on.

By the fourth or fifth day after fertilization, the embryo has become a multi-celled ball called a blastocyst. At this point, the embryo is still tiny -- about the size of the dot on an "i." At this point, it can either be implanted into a woman's uterus, frozen for later use or used for research.

If the embryo is used for research, scientists remove the stem cells from the inner cell mass of a blastocyst, a process that destroys the embryo. The stem cells are cultivated in a lab and can multiply indefinitely.

Potential Uses Of Stem Cells

Although stem cell research is in early stages, scientists say stem cells may someday treat or cure a variety of ailments, including:

Researchers also hope that embryonic stem cells may someday be used to produce cells or tissues to grow entire hearts, livers, kidneys or other organs, thus solving the problem of a shortage of organ donations.

Ethical Issues

The ethical debate surrounding stem cell research is focused on embryonic stem cells. Adult stem cell research is typically considered ethically sound.

Most critics of embryonic stem cell research argue that the research is unethical because extracting these stem cells destroys a human embryo. Thus, the debate centers on the status of human embryos. Are they clusters of cells or human individuals? Most people tend to gravitate to a position between these two extremes, but the discussion is unresolved in policy and legal debates.

Research on existing stem cell lines doesn't destroy embryos, but uses replicated cells from stem cells that have already been extracted from an embryo. That's the basis for current U.S. policy regarding funding for embryonic stem cell research. But others argue that even using existing lines is unethical because, at one point, an embryo was destroyed.

Proponents of the research argue that in the natural reproductive process, human embryos are often fertilized but don't implant in the uterus. Thus, they say a fertilized egg can't be considered a human being at least until it has been implanted in a woman's uterus.

Embryonic stem cells created through therapeutic cloning raise ethical issues surrounding cloning. Some fear the procedure may lead to cloning for reproductive purposes. It also raises the issue of creating embryos for research purposes only because of the potential for exploitation and abuse -- which is also an issue for embryonic stem cells taken from human embryos left over from in-vitro fertilization procedures.

However, some believe that embryos left over from in-vitro fertilization procedures can ethically be used for embryonic stem-cell research, since they are going to be discarded anyway. Excess embryos are created during in-vitro fertilization because harvested eggs can't be frozen for later use -- but embryos can. Thus, women get their eggs harvested once, and several embryos are made in case the couple decides to undergo further reproductive treatments in the future. The couple then has the option of having the embryos stored or donating their embryos to research.

Current U.S. Policy

On Aug. 9, 2001, President George W. Bush announced his decision on federal funding for stem cell research in an address to the nation. He said only research on stem cell lines already in existence by the time of his speech would be eligible for federal funding.

He said research on stem cell lines already in existence was permissible because an embryo had already been destroyed. But he refused to allow funding for research on any stem cell line created after Aug. 9, 2001.

At the time of the address, the National Institutes of Health determined that there were 64 stem cell lines in existence. However, researchers have expressed doubts about how many lines are actually available for use, whether the cells provided enough genetic diversity, and whether the lines are contaminated with animal cells.

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