I asked Dr. Wise Young, a top spinal cord injury researcher and owner of
the CareCure website, why there's so much interest in embryonic stem cells,
as adult stem cells don't invoke controversy and have already been used in
some treatments.  Here's his reply (posted with permission, and including
the site url sciwire.com.  The thread discussing this is in the Cure forum
of that site.)

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WHY ARE EMBRYONIC STEM CELLS IMPORTANT?

Wise Young, Ph.D., M.D.
W. M. Keck Center for Collaborative Neuroscience
http://sciwire.com

Opponents of embryonic stem cell research often argue that embryonic stem
cell research is not necessary because adult stem cells from bone marrow or
umbilical cord blood cells are already curing diseases, are more likely to
cure diseases, and that adult stem cells would not be rejected by the
immune system when transplanted. They are troubled by the cloning because
they oppose creating an embryo in order to harvest its stem cells. Finally,
they point out that current methods of collecting embryonic stem cells are
inefficient and cloning embryonic stem cells may not be a practical method
of producing cells for transplantation purposes. These arguments are partly
true but are misleading and do not justify the current restrictions of
human embryonic stem cell research. Let us consider some of the most
frequently cited arguments for or against adult and embryonic stem cells.

1. Are adult stem cells already curing diseases? Some opponents of
embryonic stem cell research say that embryonic stem cells have never cured
any condition while adult stem cells are already curing many diseases. Bone
marrow and umbilical cord blood stem cells have been used for over two
decades to treat blood-making (hematopoietic cells) disorders, such as
sickle cell anemia, thalassemia, radiation or chemotherapy induced bone
marrow damage, and autoimmune diseases. Bone marrow stem cell transplants
may accelerate and improve healing from heart attacks (myocardial infarcts)
or failing hearts (congestive heart failure). However, there is no credible
evidence yet that bone marrow stem cells are replacing heart cells. Bone
marrow cells may be releasing factors that help hearts heal faster. Neither
bone marrow nor umbilical cord blood stem cells, or other types of adult
stem cells, have cured neurological conditions, such as brain or spinal
cord injury, amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's
disease, Parkinson's disease, or non-neurological diseases such as
diabetes, liver damage from hepatitis, and other currently incurable
conditions. We do not know enough now to predict whether adult or embryonic
stem cells would be better or which would be more effective.. Many animal
studies have shown beneficial effects of embryonic stem cell treatments of
animal models of diseases. Human embryonic stem cells have not yet been
transplanted into humans. There are no restrictions on adult bone marrow or
umbilical cord blood research at all while U.S. federal government
signficantly restricts funding of human embryonic stem cell research.

2. Do adult stem cells circumvent the problem of immune rejection? It is
true that autologous grafts, i.e. adult stem cells taken from a person and
transplanted into the same person, would not be rejected by the immune
system. However, autologous grafts cannot be used for many conditions.
First, autologous grafts are not useful for most genetic diseases. For
example, there is no point in taking bone marrow from a person with a
genetic disease such as sickle cell anemia or thalassemia and then
transplanting these cells back into the same person. Heterologous grafts,
i.e. transplant from one person to another, are necessary. Second,
autologous bone marrow grafts cannot be used to treat many autoimmune
diseases, such as multiple sclerosis, diabetes, lupus erythematosus,
scleroderma, etc. The transplanted bone marrow will produce cells with the
same autoimmune tendencies. Third, bone marrow transplants often cause
graft-versus-host disease where the transplanted cells regard the host body
as "foreign" and attack it. As much as 50% of heterologous bone
marrow transplants cause graft-versus-host disease with a high mortality
rate. Fourth, even closely matched heterologous bone marrow transplants
require immunosuppression. Destroying the bone marrow usually does this.
Finally, umbilical cord blood does not produce as much immune response or
as severe graft-versus-host disease. Human embryonic stem cells, because
they are immature, are even less immunogenic and do not produce
graft-versus-host disease.

3. Are adult stem cells safer than embryonic stem cells for treatment of
neurological disorders? Autologous transplants are very attractive for
treating non-genetic diseases. For example, they would be an ideal
treatment for traumatic brain and spinal cord injury, Parkinson's disease,
stroke, and non-genetic conditions. However, heterologous transplants may
be required for genetic conditions such Alzheimer's disease or amyotrophic
lateral sclerosis. Even when bone marrow cells are carefully matched for
histocompatibility, prolonged immune suppression may be required for
engraftment. If selected stem cells are transplanted, particularly into
brain or spinal cord, graft-vs. host disease would be unlikely. On the
other hand, bone marrow transplants have a high complication rate, with a
20-50% incidence of graft-versus-host disease and 20-30% mortality rates.
For these reasons, heterologous bone marrow transplants are seldom carried
out except for life-threatening conditions. In contrast, embryonic stem
cell transplants may not require as much or as prolonged immunosuppression
because they do not express as many immunogenic proteins. Cloned embryonic
stem cells that are genetically matched for the person should pose even
less immunogenic risk.

4. Do embryonic stem cells cause tumors? There are some claims that
embryonic stem cells are more likely to cause tumors. All cells have the
potential to cause tumors, particularly if they are grown for long periods
in culture. Prolonged culturing increases the risk of malignant
transformation of the cells. Immaturity of cells also may increase the
likelihood of tumor formation. Some stem cells will not respond to all
tissue factors and may produce the wrong type of cells, too many cells, or
cells that transgress tissue boundaries. Cells with any of these three
behaviors would be called a tumor. Because embryonic stem cells are often
grown for long periods of time in culture, they have more of an opportunity
to undergo malignant transformation. Because embryonic stem cells are
immature compared to adult stem cells, they may not possess receptors to
all tissue factors. However, much evidence now suggests that embryonic stem
cells can be matured and pre-differentiated in culture before
transplantation and that such cells are unlikely to produce tumors. This is
a risk but not an insurmountable risk.

5. Are adult stem cells more likely to result in cures for diseases than
embryonic stem cells? Adult bone marrow or umbilical cord blood stem cells
have been used to treat hematopoietic disorders for over 20 years. As
pointed out above, heterologous bone marrow grafts may cause serious
complications and are currently only used under the most dire
circumstances. Umbilical cord blood transfusions show promise and presents
less risk but appear to be less effective. Although some scientists have
claimed that bone marrow mesenchymal stem cells can be induced to produce
neurons and other cells under certain circumstances, their ability to do so
is limited and substantial manipulation of the cells is required. In
contrast, embryonic stem cells readily produce all types of cells,
including neurons, insulin-producing cells, muscle, skin, and heart cells,
both in culture and after transplantation. Embryonic stem cells also grow
faster in culture and can be easily modified to produce specific cells with
specific functions. We are perhaps expecting too much from adult stem cells
when we transplant them into different tissues and expect them to fix
disparate problems such as replacing insulin-producing cells, promote
regeneration, remyelinate axons, repair heart tissues, and restore function
to diverse organs. From this perspective, embryonic stem cells possess a
significant advantage over adult stem cells. Because they grow indefinitely
in culture, they can be produced in large numbers and be optimized to have
more predictable and beneficial behavior after transplantation. At the
present, we don't know which is more likely to result in cures for
diseases.

6. Is cloning necessary for embryonic stem cells to be used for
transplantation? In biology, cloning means simply to produce cells with the
same genes. Cellular cloning unfortunately has been associated with
"reproductive cloning" or production of an individual with the
same genes. One method of cloning embryonic stem cells is to transfer a
nucleus into an egg and then trick the egg into producing stem cells.
Called somatic cell nuclear transfer (SCNT), this method produces stem
cells that have the same genes as the transferred nucleus. This method of
cloning stem cells is currently inefficient, requiring dozens of eggs. In
my opinion, cloning is not necessary for embryonic stem cells to be used
for therapy. Doctors have successfully transplanted cells (blood) and
organs (kidney, heart, liver, pancreas) for many decades without cloning.
If the stem cells are carefully matched for histocompatibility genes (HLA)
and immunosuppressive therapies are used, the cells will engraft. It is
also possible to develop embryonic stem cell lines that express a limited
set of histocompatibility antigens that would match 90% of people. Finally,
immune rejection is not necessarily bad. The immune system eliminates
cancer or excess cells. We may want the immune system to eliminate the
cells after they have finished their work. In any case, much work needs to
be done on cloning before it can be used clinically. For example, I don't
think that it would be harmful to have 3-year moratorium on cloning of
human embryonic stem cells but allow animal and human embryonic stem cell
research to go forward. On the other hand, a ban of SCNT would be a serious
mistake because it is a general technique that is important for many other
clinical applications.

7. Are embryonic stem cells practical? At the present, we do not have
enough cells from any source that can be used to treat millions of people.
For example, the world supply of umbilical cord blood is about 200,000
units. This is barely sufficient to satisfy the needs of 12,000 pediatric
patients who need umbilical cord stem cells every year to treat their
hematopoietic disorders. Although many laboratories have been trying for
decades to grow stem cells from bone marrow, umbilical cord, placental, and
other postnatal sources of stem cells, no reliable method is available to
produce sufficient diversity and amounts of bone marrow or umbilical cord
stem cells to treat millions of people. If any stem cell turned out to be
useful for any of the major diseases, we do not have enough cells to treat
even a tiny fraction of the people. Because they grow indefinitely in
culture, embryonic stem cells provide a possible inexhaustible supply of
stem cells that can treat millions of people. A cell bank with several
thousand lines of human embryonic stem cells, for example, would be very
helpful to satisfy current therapeutic and research needs.

8. What other ways can human embryonic stem cells help cure diseases?
Opponents of embryonic stem cell research seldom mention one important use
of human embryonic stem cells. An embryonic stem cell line derived from a
person with a genetic disease would be a very powerful tool to study that
genetic disease. For example, if we had an embryonic stem cell lines from
somebody with Alzheimer's, amyotrophic lateral sclerosis, Huntington's
disease, diabetes, rheumatic arthritis, lupus erythematosus, etc. the cells
can be used to assess mechanisms and treatments. At the present, we have to
use animal models or human cadaver materials. Availability of human
embryonic stem cell lines will allow large-scale screening of drugs and
other treatments. Finally, many parents who use in vitro fertilization
methods may have specific genetic conditions causing infertility. Eggs from
infertility clinics may provide insight into genetic causes of infertility.
Last but not least, the availability of such human disease-specific stem
cell lines should reduce use of animals for studying human disease.

In summary, adult bone marrow and umbilical cord blood stem cells have long
been used to treat hematopoietic disorders. Obtaining stem cells from one
part of the body and transplanting to another would circumvent immune
rejection but most genetic diseases cannot be treated with such
transplants. Bone marrow grafts tend to be immunogenic and cause
graft-versus-host disease where transplanted immune cells attack the host.
Human umbilical cord blood transplants are less immunogenic and cause less
serious graft-versus-host disease. Human embryonic stem cells are even less
immunogenic and do not cause graft-versus host disease. Stem cells may
produce tumors after transplantation if they do not respond to all tissue
factors, produce the wrong type or numbers of cells that do not respect
tissue boundaries. All cells have some potential for malignant
transformation. Differentiating stem cells in culture before
transplantation reduces the risk of tumors. Cloning should produce
genetically matched stem cells but we have much work to do before cloning
can be applied clinically. Doctors have been transplanting cells and organs
for many years without cloning. Embryonic stem cell research should go
forward, even without cloning. We do not now have an adequate stem cell
supply to treat even a small fraction of people who may benefit from stem
cell therapies. Embryonic stem cells can be grown indefinitely to treat
millions of people. Human embryonic stem cell lines obtained from people
with specific genetic diseases will greatly accelerate research on many
genetic diseases, including infertility, and will reduce the use of
animals. Thus, it is important not to close the door on human embryonic
stem cell research.

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