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| Do stem cells offer a viable strategy for confronting
the aging process? |
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What are stem cells? |
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Stem cells are cells that, in cell cultures at least, have the ability to
divide forever. They also have the capacity to develop into specialized
populations of cells. There are stem cells in developing embryos and in
recent years, scientists have confirmed the existence of stem cells in adult
humans. Recent data suggests that stem cells are not only active in embryos,
but act throughout our lives, replacing worn and damaged mature cells. Before
1998, when the first stem cells were actually identified, they were merely
assumed to exist. As evidence for their existence, researchers cited bone
marrow transplants. In the treatment of certain cancers, the chemotherapy
given destroyed all of the cells of the bone marrow. That treatment was
followed by transplants of bone marrow from healthy donors. The small volume
of transplanted bone marrow eventually gave rise to enough cells to repopulate
the body with red blood cells, white blood cells and platelets. The source
of all of those cells was presumed to be stem cells, and that presumption
has proved correct.
Stem cells are best understood in terms of how committed they are to
becoming any particular type of cell. The categories into which they fall
include:
 Totipotent stem cells
Pluripotent stem cells
Multipotent stem cells
Adult stem cells
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Totipotent stem cells |
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Human cells can be divided into sex or germ cells, eggs and sperm, and
somatic cells, all of the rest of our cells. When a sperm cell and an
egg cell unite, they form a one-celled fertilized egg. This cell is totipotent,
which means that it has the potential to give rise to any and all human
cells, such as brain, liver, blood or heart cells. The first few cell
divisions in embryonic development produce more totipotent cells. After
four days of embryonic cell division, the cells begin to specialize.
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Pluripotent stem cells |
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On the fourth day of embryonic development, the ball of cells forms itself
into an outer layer, which will become the placenta, and an inner mass,
which will form the tissues of the developing human body. These inner
cells, though they can form nearly any human tissue, cannot do so without
the outer layer, and so are not totipotent, but pluripotent. As these
pluripotent stem cells continue to divide, they begin to specialize further.
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Multipotent stem cells |
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The offspring of the pluripotent cells become the progenitors of such cell
lines as blood cells, skin cells and nerve cells. At this stage, they are
multipotent, in that they can become one of several types of cells within
a given organ (e.g., multipotent blood stem cells can develop into red blood
cells, white blood cells or platelets).
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Adult stem cells |
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In recent years, scientists have identified multipotent stem cells in adult
humans that are used to replace cells that have died or lost function. Stem
cells have been identified in adults for blood cells and nerve cells. Researchers
speculate that stem cells exist in other cell lines, but most have not yet
been found.
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