written by: Robyn Broyles•edited by: DaniellaNicole•updated: 2/26/2009
Get the facts about stem cells: what they are, where they come from, and why some stem cells are controversial. Find out what different types of stem cells exist.
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The Basic Facts about Stem Cells
In the body, each organ and tissue is made of cells that are highly specialized to a particular function. The process of a cell becoming specialized is called differentiation. Once a cell has become differentiated, it cannot change further into another type of cell.
Stem cells are cells that have not yet fully differentiated. They are capable of developing into any of several types of cell, such as blood cells, bone cells, nerve cells, skin cells, etc. The definition of a stem cell is a cell with ability to reproduce itself indefinitely. Fully differentiated cells (non-stem cells) either cannot reproduce or can only reproduce a certain number of times.
Stem cells develop into other types of cells through the process of cell division (mitosis). In mitosis, a parent cell replicates its DNA, and then divides into two daughter cells. Usually the daughter cells are identical to the parent cell, but in the case of a stem cell undergoing differentiation, the daughter cells are more specialized than their parent cell because of genetic signals.
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Types of Stem Cells
Stem cells are classified according to potency, that is, what types of cells they can become.
Totipotent cells, found only in the embryo within a few days of fertilization, can become any kind of cell in the body.
Pluripotent cells, found in an embryo a few days to a few weeks after fertilization, can become almost any kind of cell.
Multipotent cells can become any of a "family" of cells, but not cells of another "family." An example of a cell family is blood cells. There are many kinds of blood cell, and a multipotent blood stem cell (hematopoietic cell) can become any kind of blood cell, but not another kind of cell such as a bone cell.
Oligopotent cells can become one of only a few types of cells.
Unipotent cells are differentiated cells that can reproduce indefinitely. They are considered stem cells only because of their capacity to multiply an unlimited number of times, but they cannot become any other type of cell.
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Facts about Stem Cell Origins
Stem cells may be described according to their origin. Stem cells that are derived by removing an embryo's inner cell mass are called embryonic stem cells. Because this procedure kills the embryo, embryonic stem cells are ethically controversial. Stem cells that are derived from a developed tissue, such as the umbilical cord, bone marrow, or skin, are called adult stem cells. Adult stem cells do not necessarily come from adults. Because they can be derived without harming the donor, adult stem cells are generally not controversial.
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Horst Ibelgauft. Cytokines and Cells Online Pathfinder Encyclopedia (COPE). Available at http://www.copewithcytokines.de/