What are embryonic stem cells?

Embryonic stem cells are derived from human embryos that are less than a week old. They have the potential to grow into muscle, nerve and other types of cells. The embryos used in research are taken from excess embryos produced during the in vitro fertilization process and are donated for research purposes by infertile couples.

How are they different from adult stem cells?

Adult stem cells are taken from non-embryonic human tissue, such as the brain or bone marrow. They are only found in specific tissues inside the human body. Early research indicates that some may be able to form types of tissue other than their origin tissue, but their capacity to do so is expected to be more limited than that of embryonic stem cells.

Why are embryonic stem cells so highly valued in research?

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Scientists hope research on both embryonic and adult stem cells will lead to therapies for degenerative diseases such as Lou Gehrig's disease, Parkinson's disease, Duchenne's muscular dystrophy, diabetes and spinal-cord injuries. But because embryonic stem cells can grow into any type of human cell, they are more useful to researchers. They also are more plentiful and more easily obtained than adult stem cells, which are relatively rare in human tissue.

What has been the controversy over using embryonic stem cells in research?

Objections largely have been on religious grounds, since a human embryo is destroyed in the process of obtaining embryonic stem cells. Supporters of using embryonic stem cells argue that the embryo likely would have been destroyed anyway, and that it is a better use of the embryo to further therapies or cures for disease.

Source: National Institutes of Health, University of Michigan, Family Research Council