Right now, only a few diseases
are treatable with stem cell therapies because scientists
can only regenerate a few types of tissues. However,
the success of the most established stem cell-based
therapies—blood and skin transplants—gives hope
that someday stem cells will allow scientists to
develop therapies for a variety of diseases previously
thought to be incurable.
Blood Stem Cells [top]
After scraping a knee or donating blood, the body
replenishes the blood cells that are lost by drawing on
a small number of semi-specialized
hematopoietic
(heem-AT-oh-poh-EH-tik) stem cells contained in the
blood and bone marrow. For decades, scientists have
been using this type of adult stem cell to treat patients
with diseases such as leukemia, sickle cell anemia,
bone marrow damage, and some metabolic disorders
and immunodeficiencies where the body has lost its
ability to replenish its own set of healthy blood cells.
Hematopoietic stem cells give rise to all the blood cell
types, from infection-fighting white blood cells to
blood-clotting platelets. Preliminary results have suggested
that they may also be able to produce other cell
types not found in blood, but this is not yet proven.
In the past, the only way to use hematopoietic stem
cells for therapies was through bone marrow transplants.
Extracting bone marrow is an uncomfortable
and invasive procedure, and in order for a transplant
to work, the donor and recipient must be genetically
similar. If they are too genetically different, the
blood cells produced from the transplanted marrow
may recognize the patient's body as foreign and
fight against the patient's own cells and organs.
Additionally, the patient's immune system may
reject the transplant, causing a dangerous "war"
within the patient's body.
More recently, scientists have developed ways to
derive hematopoietic stem cells from the blood contained
in the umbilical cord and placenta at birth.
The stem cells isolated from a person's own umbilical
cord blood and placenta, if used for therapies
later in life, would be less likely to cause an "internal
war" within the recipient's body. They are also
more accessible than the stem cells in bone marrow
because the extraction of this blood poses no risk to
the mother or infant.
Stem Cells Found in Umbilical Cord Blood
In 2005, the National Academies issued a report,
Cord Blood: Establishing a National Hematopoietic
Stem Cell Bank Program, which recommended that a national cord blood "bank" be established to
harness the medical potential of this source of stem cells. Such a bank would not only benefit the
people from whom the blood was collected but anyone in need of blood transplants. As with blood
banks for blood transfusions, scientists could screen the bank to find the best match for each
patient, providing a safer, more personalized living-cell therapy.
The Changed Face of Skin Grafts [top]
For many years, scientists have been harnessing the
regenerative capabilities of human skin to treat victims
of severe burns using skin transplants. Skin
transplants are possible because of the existence of
stem cells located just under the top layer of skin.
Every day, thousands of new skin cells are produced
to replace those that have been shed. When someone
suffers severe burns that destroy the source of these
stem cells, their skin can no longer regenerate on its
own.
Traditionally, doctors treated severe burns by
transplanting sections of skin from undamaged
areas of the body onto the burned areas, but if doctors
could not find enough unharmed skin to cover
the burned areas, the patient could die. Now, scientists
can grow vast sheets of new skin by culturing
the stem cells from small pieces of healthy skin.
This practice, which is a type of tissue engineering,
has become routine for treating burn victims over
the past 20 years.
Recently, scientists have identified
other types of stem cells in hair follicles and
deeper layers of the skin. The inclusion of these new
stem cells into engineered skin should help create
more natural-looking skin transplants in the future.
Possible Future Treatment for Parkinson's Disease? [top]
When most people reach for a pen, their body acts in
one smooth and controlled movement. This is
because the instant a person thinks of grabbing the
pen, a series of nerve cells fire in an orchestrated symphony
from the brain to the muscles responsible for
that action. For the movement to be precise and
smooth, all the nerve cells in the "grabbing-the-pen
network" must function properly, including cells that
tell unneeded muscles to stay still.
In Parkinson's disease,
the brain cells responsible for keeping unneeded
muscles from moving degenerate and die. This
results in progressively more dramatic and uncontrolled
movements, tremors, and spasms. To date,
there is no cure for Parkinson's disease because no
one has figured out a way to bring back the specialized
nerve cells that have died.
Because Parkinson's disease results from the loss of
one specific type of nerve cell, stem cells offer a very
tangible possibility for treatment. Researchers have
recently learned how to differentiate embryonic
stem cells into the specific type of brain cell that is
lost in Parkinson's disease. They have also successfully
transplanted adult nerve stem cells into rat
brains. When this technique is proven to be effective
and safe, transplantation of stem cells into the
brains of patients may one day allow doctors to
reverse the burden of Parkinson's disease and
restore control of movement.
Another strategy currently
under study is the addition of chemicals or
growth factors that aim to induce the patient's own
stem cells to repair the damaged nerves without
needing to grow and transplant stem cells.
Possible Fix for Diabetes? [top]
In people who suffer from type I diabetes, the beta
cells of the pancreas that normally produce insulin are
destroyed by the patient's overactive immune system.
Without insulin, the cells of the body cannot take up
glucose and they starve.
Patients with type I diabetes
require insulin injections several times a day for their
entire lives. The only current cure is a pancreatic
transplant from a recently deceased donor, but the
demand for transplants far outweighs the supply.
While adult stem cells have not yet been found in
the pancreas, scientists have made progress transforming
embryonic stem cells into insulin-producing
cells. Combining beta-cell transplants with methods
to "fix" the patient's immune system-including
chemotherapy to destroy malfunctioning immunesystem
cells and blood transplants to replenish healthy
white blood cells-could offer great hope for the
many Americans suffering with type I diabetes.
Cancer: Getting to the Root of the Problem [top]
Why are some cancers so hard to eliminate, even after
many rounds of chemotherapy? The answer may lie in
a few abnormal stem cells.
Cancerous stem cells were
first identified in 1997 when a research group from
the University of Toronto transferred a few blood
stem cells from human leukemia patients into mice
and watched leukemia develop in the mice. Stem cell-like
cells have also recently been found in breast and
brain tumors. Like normal stem cells, tumor stem
cells exist in very low numbers, but they can replicate
and give rise to a multitude of cells. Unlike normal
stem cells, however, cancerous stem cells lack the
controls that tell them when to stop dividing.
Traditional chemotherapy kills off the majority of
the tumor cells, but if any of the cancerous stem
cells survive the treatment, the cancer may return.
Research into the differences in gene expression
between normal and tumor stem cells may lead to
treatments where the root of the problem—the cancer
stem cell—is targeted.
Are the Promises of Stem Cell Therapies Realistic? [top]
The list of medical achievements stem cells could
offer seems to be expanding at an incredible pace.
The role of stem cells in medicine is already very
real, but there is a danger of exaggerating the
promise of new medical developments.
What tend
to be "over-promised" are not only the potential outcomes
of both embryonic and adult stem cell
research, but also the time scales that are involved.
The basic research needed to develop viable therapeutic
options is a lengthy process that may extend
over many years and decades. Even after science
has moved from basic research to developing medical
applications, it still takes many years to thoroughly
test those applications and demonstrate that
they are safe to prescribe for patients. This is true for
all medical treatments, including the development
of new drugs, procedures, and medical equipment,
and is not specific to the living cell therapies made
possible by stem cell research.
There are also many legal and social questions that
must be addressed before stem cell-based therapies
become clinically available. Legal issues that will
affect stem cell applications include how to address
intellectual property concerns and how to apply
and enforce diverse and sometimes conflicting state
and national laws. Social issues include concerns
about the destruction of embryos, the distribution of
the benefits of the research, and the protection of
both physical and privacy interests of egg and
sperm donors and clinical research subjects.
The Role of Stem Cells in Basic Research [top]
Stem cells offer opportunities for scientific advances
that go far beyond regenerative medicine. They
offer a window for addressing many of biology's
most fundamental questions.
Watching embryonic
stem cells give rise to specialized cells is like peeking
into the earliest development of the many tissues
and organs of the human body. Stem cell research
may help clarify the role genes play in human
development and how genetic mutations affect normal
processes. They can be used to study how infectious
agents invade and attack human cells, to
investigate the genetic and environmental factors
that are involved in cancer and other diseases, and
to decipher what happens during aging.
Stem cells may also revolutionize traditional
chemical medicine. Because
embryonic stem cells can continue to
divide for long periods of time and produce
a variety of cell types, they could
provide a valuable source of human
cells for testing drugs or measuring the
effects of toxins on normal tissues without
risking the health of a single human
volunteer. In the future, thousands of
compounds could be quickly tested on
a wide assortment of cell types derived
from stem cells, making drug discovery
more efficient and cost effective.
Using nuclear transfer to produce stem cells could
be particularly useful for testing drugs for disorders
that are of genetic origin. For example, it is difficult
to study the progression of Alzheimer's and
Parkinson's diseases in the brains of live patients-
but by using the cells of an Alzheimer's patient to
create stem cell lines with nuclear transfer, scientists
could trace the development of the disease in a
culture dish and test drugs that regenerate lost
nerve cells with no danger to the patient.
Stem cells may also help scientists calculate the
effects of toxic substances in drugs, food, and the
environment.
This Web page is based on
Understanding Stem Cells: An Overview of the Science and Issues from the National Academies.
