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The term stem cell by itself can be misleading. There are many different types of stem cells, each with very different potential to treat disease. The so-called adult stem cells come from any organ, from the fetus through the adult. These are also called tissue stem cells. The so-called pluripotent cells, which have the ability to form all cells in the body, can be either embryonic or induced pluripotent stem (iPS) cells.

All stem cells, whether they are tissue stem cells or pluripotent cells, have the ability to divide and create an identical copy of themselves. This process is called self-renewal. The cells can also divide to form cells that go on to develop into mature tissue types such as liver, lungs, brain, or skin.

Embryonic stem cells exist only at the earliest stages of embryonic development and go on to form all the cells of the adult body. In humans, these cells no longer exist after about five days of development.

When removed and grown in a lab dish these stem cells can continue dividing indefinitely, retaining the ability to form the more than 200 adult cell types. Because the cells have the potential to form so many different adult tissues they are also called pluripotent ("pluri" = many, "potent" = potentials) stem cells.

James Thomson, a professor of Anatomy at the University of Wisconsin, isolated the first human embryonic stem cells in 1998. He now shares a joint appointment at the University of California, Santa Barbara.

Irv Weissman talks about the difference between adult and embryonic stem cells (3:29)

Pluripotent means many (pluri) potentials (potent). In other words, these cells have the potential of taking on many fates in the body, including all of the more than 200 different cell types. Embryonic stem cells are pluripotent, as are iPS cells that are reprogrammed from adult tissues. When scientists talk about pluripotent stem cells they mostly mean either embryonic or iPS cells.

What people commonly call adult stem cells are more accurately called tissue-specific stem cells. These are specialized cells found in tissues of adults, children and fetuses. They are thought to exist in most of the bodys tissues such as the blood, brain, liver, intestine or skin. These cells are committed to becoming a cell from their tissue of origin, but they still have the broad ability to become any one of these cells. Stem cells of the bone marrow, for example, can give rise to any of the red or white cells of the blood system. Stem cells in the brain can form all the neurons and support cells of the brain, but cant form non-brain tissues. Unlike embryonic stem cells, researchers have not been able to grow adult stem cells indefinitely in the lab.

In recent years, scientists have found stem cells in the placenta and in the umbilical cord of newborn infants. Although these cells come from a newborn they are like adult stem cells in that they are already committed to becoming a particular type of cell and cant go on to form all tissues of the body. The cord blood cells that some people bank after the birth of a child are a form of adult blood-forming stem cells.

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Stem Cell Definitions | California's Stem Cell Agency