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What is a Stem Cell?
Stem cells are unspecialized cells that renew themselves for long periods through cell division, and can develop into many different types of cells with special functions, such as a nerve cell or a beating heart muscle cell.
They come in two main forms:
Stem cells isolated from an embryo at the blastocyst stage are called embryonic stem cells.
In a 3- to 5-day-old embryo, there is a small group of about 30 unspecialized cells that comprise the inner cell mass. These "embryonic stem cells" give rise to the hundreds of highly specialized cells needed to make up an adult organism.
Adult stem cells are undifferentiated cells found in tissue other than the egg, sperm or early embryos. These cells also can turn into specialized cell types as needed by the tissue where they're found.
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Stem cells can be derived from either an embryo or a fetus.
Graphic courtesy of the National Institutes of Health
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Scientists have successfully isolated human embryonic stem cells, in two ways:
1. Scientists remove the inner layer of cells from a blastocyst, an early stage embryo. These cells can be grown in a lab to become most of the cells in the body.
2. Specific cells destined to become egg and sperm cells are removed from the developing fetus. These cells can also be grown in a lab to become most of the cells in the body.
For a complete primer on stem-cell research, visit the National Institutes of Health stem cell Web site.
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