Stem Cells FAQ

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:

• What are stem cells?
• What are embryonic stem cells?
• Why not just study adult stem cells?
• Why all the excitement about stem cells?
• Are there current stem cell treatments?
• Are stem cells safe?

Q: What are stem cells?

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.

Q: What are embryonic stem cells?

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.