There's a lot of fiction surrounding stem-cell facts. To separate one from the other, WebMD has consulted experts including Mahendra Rao, MD, PhD, director of the Center for Regenerative Medicine at the National Institutes of Health; Todd McDevitt, PhD, director of the Stem Cell Engineering Center at Georgia Tech; Mary Laughlin, MD, past president of the International Society for Cellular Therapy; and Joshua Hare, MD, director of the Interdisciplinary Stem Cell Institute at the University of Miami.

Here are the questions they answered:

A: The term "stem cells" includes many different kinds of cells.

What they have in common is that they have the ability to make other types of cells. No other cell in the body can do that.

Some stem cells can renew themselves and become virtually any cell in the body. Those are called pluripotent stem cells. They include embryonic stem cells.

Other stem cells don't have as much potential for self-renewal and can't make as many types of cells.

The most basic kind of stem cells are the cells that make up an embryo soon after an egg is fertilized. These stem cells divide over and over, eventually making almost all the different cells in the body.

Adult stem cells, in contrast, are "fully differentiated." That means they are what they are and do what they do. They can't choose another career.

In many organs, however, adult stem cells linger throughout life. They are part of the body's internal repair system. Researchers are still working to discover what adult stem cells from various parts of the body can and can't do. Normally, these relatively rare cells act only on the organ or tissue type in which they are found.

Recently, researchers have learned to reprogram adult cells to become pluripotent cells. These cells, called induced pluripotent cells or iPSCs, have many of the same properties as embryonic stem cells. It's not yet clear whether these cells might carry subtle DNA damage that limits their usefulness.

A: Early in development, a fertilized egg becomes an embryo. The embryo is made up of stem cells that divide over and over again, until these stem cells develop into the cells and tissues that become a fetus.

During in-vitro fertilization, eggs taken from a woman's body are fertilized with sperm cells. If not implanted in a woman's womb, these embryos are discarded.

Researchers have learned to take embryonic stem cells from unused in-vitro fertilizations and, in laboratory culture, to get them to make more embryonic stem cells. Embryonic stem cells are not taken from fertilized eggs or embryos that have been in a woman's womb.

While embryonic stem cells can become any kind of cell in the body, it's unlikely they would be used directly as treatments. Because they have the ability to divide over and over again, they can become rapidly growing tumors. And because they are in such an early stage of development, they take a long time to become functional adult cells.

However, researchers are learning to coax embryonic stem cells to become more mature stem cells. One clinical trial, for example, matures embryonic stem cells into nerve stem cells. These nerve stem cells are being explored as a treatment for Lou Gehrig's disease.

A: Adult stem cells have some advantages. When they come from your own body, your immune system will probably not try to reject them. And adult stem cells aren't controversial.

But there are several main disadvantages to using adult stem cells:

A: A relatively small number of stem cells taken from the body can be grown in the laboratory until they have created millions and millions of new stem cells. This makes it possible for researchers to explore cell-based therapies.

Cell-based therapies, collectively known as regenerative medicine, hold the promise of repairing or even replacing damaged or diseased organs.

Depending on which tissues they come from, stem cells have very different properties. Those from umbilical cord blood are quite different from those from fat, for example.

A: Yes. Stem cells from bone marrow have long been used to treat certain types of leukemia.

The bone marrow is a rich source of blood stem cells. These cells replace the white blood cells crucial to the immune system.

When used for leukemia, the goal is to to wipe out all of a person's white blood cells with radiation and/or chemotherapy -- and then to replace them with a bone marrow transplant from a matched donor. Stem cells from the donor marrow replace the diseased blood cells with healthy blood cells.

A stem cell product designed to avoid the need for a matched donor recently received limited approval in Canada. The product, Prochymal, appears to rescue bone marrow transplant patients who are rejecting their transplant.

In the U.S., the FDA has approved a product called Hemacord, which contains blood stem cells derived from cord blood. The product is approved for patients with diseases that affect their ability to make new blood cells, such as certain blood cancers and immune disorders.

A: That remains to be seen. Potential dangers include:

There is also risk in some of the procedures used to get stem cells out of the body (such as from liposuction or spinal tap) or to deliver stem cells to the body (such as implanting them in the heart, brain, spinal cord, or other organs). That's not so much about the stem cells, but because of the procedures themselves.

Researchers are studying all of that. Without carefully controlled clinical trials, there's no way to know what might happen in the long term, or even in the short term. That's why the FDA discourages the use of stem cells except in clinical trials or approved therapies.

If you are thinking about pursuing stem cell therapy, talk to your doctor first. In the U.S. and abroad, many clinics offer unproven stem cell treatments that have never been tested for safety or effectiveness.

SOURCES:

Mahendra Rao, MD, PhD, director, Center for Regenerative Medicine, National Institutes of Health, Bethesda, Md.

Todd McDevitt, PhD, director, Stem Cell Engineering Center, Georgia Institute of Technology, Atlanta.

Mary Laughlin, MD, past president of the International Society for Cellular Therapy.

Joshua Hare, MD, director, Interdisciplinary Stem Cell Institute, University of Miami.

National Institutes of Health web site.

FDA web site.

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Your Stem Cell Questions Answered - webmd.com