2.What do the terms Totipotent, Pluripotent and
Multipotent mean?2
"Stem cells" is a term used to
describe all cells that can give rise to cells of multiple tissue
types. However, there are different types of stems cells. Totipotent
cells, like the cells of a fertilized egg in the first few days after
fertilization, can give rise to a fully functional organism. During
normal development, the Totipotent cells become more specialized and
are considered Pluripotent, meaning that they can give rise to every
cell type in the body, but will not give rise to the placenta or
supporting tissues necessary for fetal development. Because their
potential is not total, they are not Totipotent and they are not
embryos. Pluripotent stem cells undergo further specialization into
stem cells committed to giving rise to cells that are specialized for a
particular function. Multipotent cells can give rise to the cell types
found in the tissue from which they were derived, such as blood stem
cells that give rise only to red blood cells, white blood cells and
platelets, or skin stem cells that give rise only to the various types
of skin cells.
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3.
What is cell therapy?
Cell therapy can be defined as a
group of new techniques, or technologies, that rely on replacing
diseased or dysfunctional cells with healthy, functioning ones. These
new techniques are being applied to a wide range of human diseases,
including many types of cancer, neurological diseases such as
Parkinson's and Lou Gehrig's disease, spinal cord injuries, and
diabetes. Replacing dead cells in the retina with new ones may someday
cure even presently incurable eye diseases such as glaucoma and macular
degeneration. To understand how cell therapy works, it helps to
understand the role of cells in the body.
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4. How do cells function?
Cells are the basic building
blocks of the human body. These tiny structures compose the skin,
muscles, bones and all of the internal organs. They also hold many of
the keys to how our bodies function. Cells serve both a structural and
a functional role in the body, performing an almost endless variety of
actions to sustain the body's tissues and organs.
There are hundreds, perhaps
thousands, of different specialized cell types in the adult body. All
of these cells perform very specific functions for the tissue or organ
they compose. For example, specialized cells in the heart muscle "beat"
rhythmically through the conduction of electrical signals, while the
cells of the pancreas produce insulin to help the body convert food to
energy. These mature cells have been differentiated, or dedicated, to
performing their special tasks. Conventional wisdom has long maintained
that under normal conditions, once a cell has become specialized, it
cannot be changed into a different type of cell.
Like the body itself, cells have
a finite life span; they eventually die. Most of the body's cells
divide and duplicate throughout life, but some cells either don't
replenish themselves or do so in such small numbers that they cannot
replace themselves fast enough to combat disease.
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5. How does cell therapy work?
While cells are indispensable in
performing vital functions for the body, they can also exist outside
the body. They can live and divide in "cultures," special solutions in
test tubes or Petrie dishes. This ability of certain cell types to live
isolated from other cells under controlled conditions has allowed
scientists to study them independently of the organ or system they are
normally a part of. Through the isolation and targeted manipulation of
cells, scientists are finding ways to identify young, regenerating ones
that can be used to replace damaged or dead ones in diseased organs.
This therapy is similar to the process of organ transplant, only the
treatment consists of the transplantation of cells rather than organs.
The cells that have shown by far the most promise of supplying diseased
organs with healthy new ones are called stem cells.
Do adult stem cells have the
same capability as embryonic stem cells? For many years, scientists
have conducted studies to determine whether the stem cells in adult
tissue have the same developmental capability as embryonic stem cells.
The general consensus is that adult stem cells seem to be less
versatile. Scientists think that embryonic stem cells have a much
greater utility and potential than the adult stem cells, because
embryonic stem cells may develop into virtually every type of cell in
the human body. Adult stem cells, on the other hand, may only be able
to develop into a limited number of cell types. Embryonic Stem Cells
also continue to divide indefinitely when placed in culture, while this
may not be the case for adult stem cells, which would reduce their
capacity to form new cell types. Both adult and embryonic stem cell
research should continue simultaneously as they are both critical to
our understanding of the etiology, progression and treatment of disease.
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6. How
are cell therapies being used today?
Even though most of the work
done in this field has been experimental, most scientists find cell
therapy so promising that they believe it is only a matter of time
before its use becomes routine. And while many of the hoped-for uses of
cell therapy sound futuristic, there are a few forms of this technique
that have already been in use for years. Bone marrow transplants are an
example of cell therapy in which the stem cells in a donor's marrow are
used to replace the blood cells of the victims of leukemia and other
cancers. Cell therapy is also being used in experiments to graft new
skin cells to treat serious burn victims, and to grow new corneas for
the sight-impaired. In all of these uses, the goal is for the healthy
cells to become integrated into the body and begin to function like the
patient's own cells.
So far, the results of such
experiments have exceeded expectations. In a recent advance, pancreatic
cells grown from stem cells were implanted into the body of a diabetic
and began to produce insulin. Even though cell therapy is a new
science, early results like the above have caused great optimism in the
scientific community. However, there are several scientific challenges
that must be overcome before we can truly harness the power of stem
cells.
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7.
What are some of the challenges?
One of the first challenges that
must be overcome for stem cell therapies to become more commonplace is
the difficulty of identifying stem cells in tissue cultures, which
contain numerous types of cells. While scientists are discovering new
cell types almost every day, they estimate that there could literally
be thousands of human cell types. The process of identifying any
desired type of stem cell will involve painstaking research. Second,
once stem cells are identified and isolated, the right biochemical
solution must be developed to cause these progenitor cells to
differentiate into the desired cell type. This too will require a great
deal of experimentation.
Assuming that the above
obstacles have been overcome, new issues arise when the cells are
implanted into a person. The cells must be integrated into the
patient's own tissues and organs and "learn" to function in concert
with the body's natural cells. Cardiac cells that beat in a cell
culture, for example, may not beat in rhythm with a patient's own heart
cells. And neurons injected into a damaged brain must become "wired
into" the brain's intricate network of cells and their connections in
order to work properly.
Another challenge is the
phenomenon of tissue rejection. Just as in organ transplants, the
body's immune cells will recognize transplanted cells as "foreign,"
setting off an immune reaction that could cause the transplant to fail
and possibly endanger the patient. Cell recipients would have to take
drugs to temporarily suppress their immune systems, which in itself
could be dangerous.
Yet another concern is the
possible risk of cancer. Cancer results when cells lose their internal
"brakes" and keep dividing when further proliferation is no longer
desirable. Researchers must find a delicate balance between fostering
the growth of new cells to replenish damaged tissues and making sure
that cells don't overgrow and become cancerous. However, most
scientists believe that, with the appropriate research, these obstacles
can be overcome and the power of stem cells can be harnessed.
There are also ethical, social,
financial and political issues affecting this new industry. One of the
hardest issues for this industry to overcome is that patients are
generally offered stem cell therapy after all other treatments have
been exhausted, therefore limiting the chance for success. One must
keep in mind that the success of stem cell therapy poses a serious
threat to many other conventional treatments. Therefore, there are
powerful forces that go out of their way to minimize this industry's
successes and magnify its failures.
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8. What is the future of cell therapy?
Despite the many challenges
before us, most scientists believe that cell therapy will revolutionize
medicine. With the use of cell therapies, we may soon have dramatic
cures for cancer, Parkinson's, diabetes, kidney disease, multiple
sclerosis, muscular degeneration and a host of other diseases. Cell
therapies have also shown great promise in helping to repair
catastrophic spinal injuries, and helping victims of paralysis regain
movement. It is even possible that the human life span could be greatly
extended due to the replenishment of tissues in aging organs. We may
even have the ability one day to grow our own organs for
transplantation from our own stem cells, eliminating the danger of
organ rejection. While we will undoubtedly encounter the limits of cell
therapy one day, there is every reason to hope that this revolutionary
new approach will result in radically improved ways to treat diseases.
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