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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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