Embryonic Stem Cells
Of the types of stem cell research, the use of cells derived from fetal tissue remains the most controversial, but also perhaps the most medically promising.

Embryonic stem cells hold promise because research in mice points to these cells' ability to develop into any of the cells in the body. These cells could potentially be used to replace cells and tissues destroyed by disease. Diabetes, heart disease and spinal cord injuries are just a few of the conditions that could potentially be treated using embryonic stem cells.

Despite its promise, the prospect for this research is not entirely free of pitfalls. The International Society for Stem Cell Research, which was founded to "encourage the general field of research involving stem cells," outlines several challenges scientists would face if they tried to treat people using tissues developed from stem cells.

Embryonic stem cells grow very quickly, which means there is the potential for tumors to develop if such cells are implanted into a person before the cells become specialized. The Society for Stem Cell Research also cautions that specialized cells would have to learn how to function as part of a person's body. For example, stem cells that developed into heart cells would need to beat in synch with the patient's heart cells once the new cells were implanted. Patients might also reject the new tissue just as organ transplant recipients can reject the donated organs.

As the name suggests, human embryonic stem cells are derived from early-stage embryos.

About five days after an egg is fertilized, it develops into a ball of about 150 cells called a blastocyst. The inner cells that make up that blastocyst are embryonic stem cells.

One method for extracting these stem cells uses a technique that could also be the first step toward human cloning. In that process, scientists remove the nucleus from an unfertilized human egg cell and implant the nucleus of a cell from another person. This process then yields a blastocyst from which stem cells can be extracted. Scientists may eventually be able to isolate stem cells at this stage and grow those cells into tissue that would be compatible with the person who needs treatment.

Another potential source for stem cells is to use embryos left over from fertility treatments. At four to five weeks of development, it is possible to extract cells that have similar properties to the stem cells taken from five-day old fertilized eggs. According to the International Society for Stem Cell Research, the cells recovered from four-to-five-week-old embryos may be of more limited use than cells harvested after just a few days.

In both these cases, the extraction of stem cells prevents the embryo from developing further. This has caused some to question the morality of the research, a debate that will be explored later.

Once stem cells are isolated in a laboratory, more and more copies of the cells can be created under the right conditions. When embryonic stem cells continue to create new cells, they become a stem cell line that can grow indefinitely in incubators under the right conditions. Cells in stem cell lines retain their ability to develop into different, specialized cell types.

Adult Stem Cells
Stem cells do not disappear as humans develop. These cells are also found in various parts of the adult body, including bone marrow, blood and the brain. They are also present in umbilical cords.

Researchers have tried to ascertain whether adult stem cells have the same abilities to develop into other types of cells, but results have been mixed.

Preliminary research published in a March 2004 New England Journal of Medicine suggested that stem cells in the blood are capable of developing into various organs, including liver and skin tissue.

However, a separate study published in the same month yielded different results. When researchers at Stanford tried to validate previous findings that stem cells from bone marrow could develop into heart muscle, their study contradicted those earlier results. In a study they published in March 2004 in Nature, the researchers reported that when they lodged the stem cells in damaged hearts, the cells retained their original function and did not become heart muscle cells.

Determining to what extent adult stem cells can be coaxed into transforming into different types of tissue is important in part because research suggests not all organs have corresponding stem cells.

A study published in May 2004 in Nature found that cells in the pancreas create copies of themselves without using adult stem cells. Some scientists had hoped that they would find adult stem cells in the pancreas and would then be able to look for ways to use those cells to develop treatments for Type-1 diabetes, a disease that develops when cells in the pancreas are destroyed.

Currently, the only routine treatment that uses stem cells is bone marrow transplants. These transplants are used to treat leukemia, lymphoma and several blood disorders.

The Ethical Debate
While researchers debate adult stem cells' ability to change from one type of cell into another, an entirely different debate continues regarding embryonic stem cells.

Advanced Cell Technology, a small Massachusetts company, in November 2001 reported that it created cloned human embryos for the purpose of extracting stem cells. That announcement led to a flurry of criticism from those ethically opposed to such research.

"The new human life has been created for the purposes of experimentation. It's not the first time, but it is -- it is that moral boundary. But I think even more important, this is the first step down the road toward cloning human beings even if that's not the intent of the people who have done this," Leon Kass, a bioethicist at the University of Chicago and chairman the President's Council on Bioethics, told the NewsHour, speaking for himself and not the council.

Some also oppose stem cell research conducted using embryos remaining after fertility treatments.

The National Conference of Catholic Bishops wrote in a letter to Congress in 2001 stating, "We believe it is more important than ever to stand for the principle that government must not treat any living human being as research material, as a mere means for benefit to others."

This group has also argued that research should instead focus on adult stem cells.

"Medical research is developing new and promising treatments for Parkinson's, diabetes, heart disease and other illnesses -- but these are from adult stem cell research and other approaches that pose no moral problem," the U.S. Conference of Catholic Bishops wrote in a 2003 letter to Congress.

Supporters of stem cell research maintain that its potential benefits outweigh the ethical concerns.

In a letter sent in 2001 to President Bush, 80 Nobel Prize winners wrote, "While we recognize the legitimate ethical issues raised by this research, it is important to understand that the cells being used in this research were destined to be discarded in any case. Under these circumstances, it would be tragic to waste this opportunity to pursue the work that could potentially alleviate human suffering."

In August 2001, President Bush weighed in on the debate, saying the federal government would only fund research on existing embryonic stem cell lines. Although some scientists argued the decision greatly inhibited potential medical treatments, Mr. Bush argued the ethical questions were too great to move forward on new embryonic stem cell development.

"At its core, this issue forces us to confront fundamental questions about the beginnings of life and the ends of science," President Bush said. "It lives at a difficult moral intersection, juxtaposing the need to protect life in all its phases with the prospect of saving and improving life in all its stages."

-- By Karyn Schwartz, Online NewsHour