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Definition of pluripotent stem cells from Pluripotent Stem Cells: A Primer, January 28, 1999.

Pluripotent Stem cells are a mass of cells; cells in this mass develop in different ways. The outer layer of cells will go on to form the placenta and other supporting tissues needed for fetal development in the uterus. The inner cell mass cells will go on to form all of the tissues of the human body.

Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form a human because they are unable to give rise to the placenta and supporting tissues necessary for development in the human uterus. These inner cell mass cells are pluripotent - they can give rise to many types of cells but not all types of cells. Because their potential is not total, they are not totipotent and they are not embryos. In fact, if an inner cell mass cell were placed into a woman's uterus, it would not develop into a human being.

The pluripotent stem cells undergo further specialization into stem cells that are committed to giving rise to cells with a particular function; for example, blood stem cells or skin stem cells. A blood stem cell will then give rise to the various types of blood cells - white blood cells, red blood cells and platelets. But once a pluripotent stem cell specializes into a blood stem cell, it does not change course and produce skin stem cells, liver cells or any cell other than a blood stem cell or a specific type of blood cell. In addition, pluripotent stem cells do not produce totipotent stem cells.

While stem cells are extraordinarily important in early human development, they are also found in children and adults. For example, consider one of the best understood stem cells, the blood stem cell. Blood stem cells reside in the bone marrow of every child and adult, and in fact, they can be found in very small numbers circulating in the blood stream. Blood stem cells perform the critical role of continually replenishing our supply of blood cells - red blood cells, white blood cells, and platelets - throughout life. A person cannot survive without blood stem cells.

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