Q. What is the difference between embryonic and human stem cells?

A. Embryonic stem cells emerge in the first five to seven days of the embryo's development. They have the potential to become every cell of the body.

Adult stem cells, on the other hand, emerge later in the fetus's development, when the individual tissues of the body begin to form. Adult stem cells serve as a reservoir of continuous replenishment for the tissues in which they reside.

A liver stem cell, for instance, can turn into a liver cell. It can also divide to become another liver stem cell. In this sense, it is constantly replenishing itself.

Scientists are working to determine how they can harness the capabilities of adult stem cells to treat the tissues in which they reside. Liver stem cells could treat damaged liver tissue. Neural stem cells could prove useful in treating brain diseases, and so on.

Some adult stem cells could prove to be key players in disease formation, as well. Indeed, the latest evidence suggests that neural stem cells may be the cause of the most common form of primary brain tumor - malignant gliomas.

And in a novel twist, scientists are trying to turn fat cells into bone or muscle by controlling their shared adult stem cell "parent."

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