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Stem Cells differ from other kinds of cells in the body. All stem cells—regardless of their source—have three general properties: they are capable of dividing and renewing themselves for long periods; they are unspecialized; and they can give rise to specialized cell types.
Scientists are trying to understand two fundamental properties of stem cells that relate to their Long-Term Self-Renewal:
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why can Embryonic Stem Cells proliferate for a year or more in the
laboratory without differentiating, but most adult stem cells cannot;
and
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what are the factors in living organisms that normally regulate stem cell Proliferation and self-renewal?
Discovering the answers to these questions may make it possible to
understand how cell Proliferation is regulated during normal embryonic
development or during the abnormal Cell Division that leads to cancer.
Importantly, such information would enable scientists to grow embryonic
and adult stem cells more efficiently in the laboratory.
Stem cells are unspecialized. One of the fundamental properties of a
stem cell is that it does not have any tissue-specific structures that
allow it to perform specialized functions. A stem cell cannot work with
its neighbors to pump blood through the body (like a heart muscle
cell); it cannot carry molecules of oxygen through the bloodstream
(like a red blood cell); and it cannot fire electrochemical Signals to
other cells that allow the body to move or speak (like a nerve cell).
However, unspecialized stem cells can give rise to specialized cells,
including heart muscle cells, blood cells, or nerve cells.
Stem cells are capable of dividing and renewing themselves for long
periods. Unlike muscle cells, blood cells, or nerve cells—which do not
normally replicate themselves—stem cells may replicate many times. When
cells replicate themselves many times over it is called proliferation.
A starting population of stem cells that proliferates for many months
in the laboratory can yield millions of cells. If the resulting cells
continue to be unspecialized, like the parent stem cells, the cells are
said to be capable of Long-Term Self-Renewal.
The specific factors and conditions that allow stem cells to remain
unspecialized are of great interest to scientists. It has taken
scientists many years of trial and error to learn to grow stem cells in
the laboratory without them spontaneously differentiating into specific
cell types. For example, it took 20 years to learn how to grow human
embryonic stem cells in the laboratory following the development of
conditions for growing mouse stem cells. Therefore, an important area
of research is understanding the signals in a mature organism that
cause a stem cell population to proliferate and remain unspecialized
until the cells are needed for repair of a specific tissue. Such
information is critical for scientists to be able to grow large numbers
of unspecialized stem cells in the laboratory for further
experimentation.
Stem cells can give rise to specialized cells. When unspecialized stem
cells give rise to specialized cells, the process is called Differentiation.
Scientists are just beginning to understand the signals inside and
outside cells that trigger stem cell Differentiation. The internal
signals are controlled by a cell's genes, which are interspersed across
long strands of DNA, and carry coded instructions for all the
structures and functions of a cell. The external signals for cell
differentiation include chemicals secreted by other cells, physical
contact with neighboring cells, and certain molecules in the Microenvironment.
Therefore, many questions about stem cell differentiation remain. For
example, are the internal and external signals for cell differentiation
similar for all kinds of stem cells? Can specific sets of signals be
identified that promote differentiation into specific cell types?
Addressing these questions is critical because the answers may lead
scientists to find new ways of controlling stem cell differentiation in
the laboratory, thereby growing cells or tissues that can be used for
specific purposes including Cell-based Therapies.
Adult stem cells typically generate the cell types of the tissue in
which they reside. A blood-forming adult stem cell in the bone marrow,
for example, normally gives rise to the many types of blood cells such
as red blood cells, white blood cells and platelets. Until recently, it
had been thought that a blood-forming cell in the bone marrow—which is
called a hematopoietic stem cell—could not give rise to the cells of a
very different tissue, such as nerve cells in the brain. However, a
number of experiments over the last several years have raised the
possibility that stem cells from one tissue may be able to give rise to
cell types of a completely different tissue, a phenomenon known as Plasticity. Examples of such Plasticity include blood cells becoming Neurons, liver cells that can be made to produce insulin, and Hematopoietic Stem Cells
that can develop into heart muscle. Therefore, exploring the
possibility of using adult stem cells for cell-based therapies has
become a very active area of investigation by researchers.
* Page citation: Stem Cell Basics: What are the unique properties of
all stem cells? . In Stem Cell Information [World Wide Web site].
Bethesda, MD: National Institutes of Health, U.S. Department of Health
and Human Services, 2006
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