Cell Therapy
What is cell therapy?
How does cell therapy work?
What are stem cells?
Adult stem cells vs. embryonic—aren't they the same?
How are cell therapies being used today?
What are some of the challenges?
What is the future of cell therapy?
What is therapeutic cloning?
What do the terms totipotent, pluripotent and multipotent mean?
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.
The Function of Cells
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.
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.
What are stem cells?
Simply put, stem cells are primitive cells that
give rise to other types of cells. Also called progenitor cells, there are several
kinds of stem cells. Totipotent cells are considered the "master"
cells of the body because they contain all the genetic information needed to
create all the cells of the body plus the placenta, which nourishes the human
embryo. Human cells have this capacity only during the first few divisions of
a fertilized egg. After 3 - 4 divisions of totipotent cells, there follows a
series of stages in which the cells become increasingly specialized. The next
stage of division results in pluripotent cells, which are highly versatile and
can give rise to any cell type except the cells of the placenta. At the next
stage, cells become multipotent, meaning they can give rise to several other
cell types, but those types are limited in number. An example of multipotent
cells is hematopoietic cells—blood stem cells that can develop into several
types of blood cells, but cannot develop into brain cells. At the end of the
long chain of cell divisions that make up the embryo are "terminally differentiated"
cells—cells that are considered to be permanently committed to a specific function.
Scientists have long held the opinion that differentiated
cells cannot be altered or caused to behave in any way other than the way in
which they have been naturally committed. New research, however, has even called
that assumption into question. In recent stem cell experiments, scientists have
been able to persuade blood stem cells to behave like neurons, or brain cells.
Scientists now believe that stem cell research could reveal far more vital information
about our bodies than was previously known.
In addition, it was recently discovered that
some stem cells also occur in the bodies of adults, rather than exclusively
in embryos. Many kinds of multipotent stem cells have been discovered in adults,
and scientists believe that many more will be discovered. Research is now being
conducted on both adult and embryonic stem cells to determine the characteristics
and potential of both to cure disease.
Adult stem cells vs. embryonic—aren't they the same?
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 and this 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.
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.
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.
Yet 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.
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,
macular 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 disease.
What is therapeutic cloning?
Therapeutic cloning has recently been the subject
of intense debate. We believe the controversy surrounding this technique is
based, to a great extent, on misunderstanding. Therapeutic cloning is not the
same is as reproductive cloning—the type of cloning that is intended to genetically
duplicate a person.
Therapeutic cloning is based on a technology
called somatic cell nuclear transfer. A normal animal egg cell is treated to
remove the nucleus, the part of the cell that contains the genetic material.
Then the nucleus from a somatic cell (that is, any body cell other than an egg
or sperm) is fused to the egg from which the nucleus has been removed. This
somatic cell is taken from a patient who needs an infusion of new cells to treat
a disease or injury. The egg, which now contains the patient's DNA, is allowed
to divide and soon forms a hollow sphere of cells called a blastocyst. The blastocyst
has an outer layer of cells and an inner cluster called the inner cell mass.
Cells from the inner cell mass are isolated and used to develop new stem cell
lines. These cells are pluripotent, meaning that they can give rise to many
types of specialized cells in the body and can be used to replace cells or tissue
that have been damaged or destroyed. The reason therapeutic cloning is being
used to obtain stem cells is to address the vital issue of tissue incompatibility
and possible rejection. In therapeutic cloning, the somatic cell is removed
from the patient expected to receive the transplant and fused to the donor egg.
Because the majority of genetic information is contained in the nucleus, the
stem cells that are derived from this procedure would be genetically compatible
with the patient and would overcome the issue of rejection.
Therapeutic cloning is very different from the effort to clone
a human being. It is for this reason that many scientists, patient advocates
and other groups now prefer to use a less misleading term, such as "nuclear
transplantation."
What do the terms totipotent, pluripotent and multipotent
mean?
"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.
Source: http://stemcells.nih.gov
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