Stem Cells: Medicine's New Frontier
By Mayo Clinic staff

October 13, 2004

Medical researchers are working hard to discover the secrets and healing potential
lurking inside your stem cells. These cells, unlike other cells in your body — such
as your skin cells or muscle cells — do more than perform a specific function. They
have the ability to produce the many different types of cells and tissues that make
up your body. This unique quality means that these cells, if properly controlled,
might someday treat a range of conditions — from baldness to spinal cord injuries
— by replacing damaged or diseased cells.

More importantly, as scientists learn more about stem cells, they also gain a better
understanding of how embryos develop all the cells, tissues and organs needed to
sustain life. And they learn more about how certain diseases and conditions
develop.

However, stem cell research is a very young field of study. Although stem cells
already have some practical applications, scientists must overcome a number of
obstacles before they learn how best to use these cells to improve your health.

What are stem cells?

Stem cells are your body's basic building blocks. Certain qualities make them
unique from other cells in your body. Stem cells are:
Undifferentiated. As undifferentiated cells, stem cells don't have a particular
function — the way cells in your arm muscles that help you lift a package or cells in
your blood that carry oxygen do. Instead, stem cells are like neutral observers
waiting to be pressed into duty. Although they're undifferentiated, they can turn into
specialized cells. Imagine that you're repairing a home and you have a substance
that you can transform into any material you need throughout the house — from
carpet to paint to shingles. This unique substance could be used to fix many
different problems. Stem cells may be useful in repairing various parts of your body
in this way.
Self-replicating. Unlike other cells in your body, which don't typically produce copies
of themselves (replicate), stem cells can divide and replicate over and over again.
In the laboratory, stem cells can replicate over long spans of time without becoming
specialized. Researchers can grow such unspecialized cells in a laboratory dish,
then separate groups of them away and start new batches of dividing cells until they
have millions of cells. This allows scientists to produce plenty of unspecialized cells
for study — and possibly for future use in medical treatment.

Stem cell replication
When a stem cell divides, it creates a stem cell that's specialized ...

Types of stem cells: From embryos to adults

Not all stem cells are created equal. The potential for stem cells to differentiate into
other specialized cells changes as an embryo develops.

Stem cells can be:
Totipotent. In the early stages after fertilization — immediately after a sperm and
egg join together and begin dividing — stem cells are considered totipotent. This
means that these cells can form any type of cell, including those cells necessary for
an embryo to develop into a human, such as placenta cells.
Pluripotent. Several days after conception, as an embryo begins to develop, its
stem cells become pluripotent. Such cells can form any kind of cell found in adults,
making them very versatile. Unlike totipotent stem cells, however, they can't
become cells necessary for an embryo to develop into a human. Cells with similar
properties are also found in fetal tissue.
Multipotent. Eventually, pluripotent stem cells become multipotent stem cells — the
type found in adults. Multipotent stem cells can no longer develop into all or most
cell types. Instead, they can develop into certain cell types within a specific tissue,
organ or system. For example, bone marrow stem cells can develop into all types of
blood cells, which are normally produced by your bone marrow, as well as bone,
cartilage and fat cells. Stem cells in your brain can turn into nerve cells and other
types of brain cells.

Multipotent stem cells exist in certain tissues and organs throughout your body,
including your brain, blood vessels, liver, muscles and bone marrow. Researchers
believe that multipotent cells remain — undivided — in these parts until they're
called into duty to create necessary new cells, such as after a disease or injury.

Some scientists suspect that stem cells in one body part may be able to form cells
used in other body parts, a quality called plasticity. For example, a stem cell in your
bone marrow that makes blood cells might also be able to make heart-muscle cells
or liver cells.

Stem cells in use: Current treatments
Although they're not a cure for any condition, stem cells can assist in the treatment
of certain conditions. Despite the fact that stem cell research is still in its relative
infancy, doctors have used stem cells in bone marrow transplants for more than 30
years. Bone marrow contains blood-forming stem cells. These cells continuously
produce oxygen-carrying red blood cells, white blood cells that help fight infection
and platelets that help stop bleeding. Bone marrow and stem cell transplants help
treat certain cancers, such as leukemia and lymphoma, and some noncancerous
conditions, such as aplastic anemia and some inherited immune disorders.
Researchers are also investigating their use in some breast and ovarian cancers.
Aplastic anemia
Leukemia
Non-Hodgkin's lymphoma

The chemotherapy and radiation used to treat these diseases can destroy bone
marrow, leaving your body unable to produce the new blood cells you need to carry
oxygen throughout your system and to fight infections. Stem cell transplants —
though not a cure for the cancer itself — provide you with new cells that can
regenerate the bone marrow.

The first bone marrow transplants gave bone marrow from a healthy relative to a
person affected by a bone marrow disease. These transplants, called allogeneic
transplants, are riskier than are transplants using the person's own bone marrow.
More recently, for some diseases, bone marrow transplants have used a person's
own marrow after it is removed from bone, treated and frozen. Chemotherapy and
radiation treatments destroy the cancer cells or defective material in the removed
marrow, and the remaining healthy marrow is transplanted back into the individual.
Today, the treatment commonly involves using blood — as some stem cells also
circulate in the bloodstream — rather than marrow. This is known as peripheral
blood stem cell transplantation.

What lies ahead: Possible uses
Potential uses for stem cells could someday extend far beyond bone marrow and
blood transplants. Stem cells may help to:
Replace diseased cells. If scientists can harness stem cells' ability to become
specialized into any type of cell, they may be able to use them to treat any number
of diseases and conditions. For example, Parkinson's disease — a condition
marked by tremors and loss of muscle control — is caused by the loss of certain
brain cells that create the chemical dopamine. Stem cells could potentially be used
to replace such cells. In fact, Parkinson's may be one of the first diseases to be
treated with stem cells, since experts have already prompted embryonic stem cells
to specialize into cells similar to the dopamine-creating cells.

Stem cells may also prove to be helpful in treating type 1 diabetes. With this
disease, islet cells in the pancreas, which produce a crucial hormone called insulin,
are damaged. Doctors could someday prompt stem cells to form new, healthy islet
cells, then inject them into the liver of a person who has diabetes. This may
eliminate the need for insulin injections, which are typically necessary to manage
type 1 diabetes.

Healthy cells provided by stem cell transplants may one day help treat a variety of
other conditions as well. These could include Alzheimer's disease, spinal cord
injuries, liver disease, arthritis and hair loss.
Form new tissues. Stem cells injected into a diseased organ — such as a failing
heart — may also one day help keep an organ functioning. They may even be used
to grow a new organ. This would give a second chance to people who are waiting
for organ transplants for which available organs are in short supply.
Develop new medications. If researchers could use stem cells to generate specific
types of cells found in the body, they might test the safety of new drugs on those
resulting cells.
Learn about cancer and birth defects. By learning more about how stem cells divide
and differentiate, researchers may increase our understanding of cancer and birth
defects, conditions marked by improper cell growth and differentiation.
Organ transplantation: Coping with the wait

Obstacles facing stem cell research
Reaching the point where society can reap the potential benefits of stem cells will
take time. Researchers face a number of major challenges as they study stem cells.

Technical hurdles
Controlling stem cells isn't easy. Both adult and embryonic stem cells present
challenges. Though stem cells exist in adult tissue, they're not present in great
numbers, so they can be hard to find and to extract for growth. They also may be
difficult to grow into large batches of unspecialized cells in the laboratory — a
necessary step if they're to serve as replacement cells in disease treatment. And
their ability to transform into different kinds of cells appears to be limited.

Working with embryonic stem cells has its challenges, too. Though they're easier to
grow into batches of unspecialized cells, scientists need to better understand how
these cells reproduce in the laboratory, and how to reliably trigger them to
differentiate into the specific types of cells needed. Concerns that transplanted stem
cells may not work in conjunction with the tissue of the person receiving them also
exist. Transplanted cells could "overgrow," as happens in many cancers. And a
person's immune system may reject the cells as foreign, because cells derived from
embryonic or fetal tissue are genetically different from mature tissue.

Ethical questions
Because of the technical limitations involved in using adult stem cells, embryonic
stem cells are generally more appealing to stem cell researchers. But the use of
embryonic stem cells gives rise to ethical questions.

Embryonic stem cells are derived from embryos created outside of a woman's body
(in vitro) — using donated eggs fertilized in a laboratory — not from eggs fertilized
inside a woman's body. Gathering embryonic stem cells requires destroying the
embryo. Some people believe this process represents a destruction of human life,
and feel that it shouldn't be done for research purposes. Others disagree,
suggesting that an embryo doesn't have the same rank as a fetus. Some believe it's
OK to use embryos for research if they're not being created specifically for
research, but instead exist as unused byproducts of an in vitro fertilization
procedure for infertile couples.

Infertility
Even more ethical issues are raised when cloning enters the discussion. Some
scientists have obtained embryonic stem cells using a process known as
therapeutic cloning. During this process, scientists replace the nucleus of an egg
cell with the nucleus from another cell in the body to grow an embryo from which to
harvest stem cells with specific properties, such as a new gene. This process differs
from reproductive cloning, during which an egg is given the new nucleus and
implanted into a woman's uterus so it will grow into a full-term fetus.

Legal limitations
Laws in the United States also place limitations on the use of embryonic stem cells
for research. In 2001, an executive order limited federal funding for research on
human embryonic stem cells.

This order means that if funded by federal money — as many research studies are
— a scientist can only work with a limited number of already existing human stem
cell lines that meet certain criteria. A stem cell line is a group of cells that can self-
replicate outside the body for an extended period of time. Stem cell lines are
appealing to scientists because the lines can be grown and shared indefinitely,
eliminating the need to go through the difficult process of isolating and obtaining the
stem cells again.

The allowable stem cell lines originated from embryos that already existed for
reproductive purposes and were no longer needed for those purposes. Those who
donated these embryos weren't paid for them and consented to their use in
research.

Working exclusively with these stem cell lines presents some challenges for
researchers. The number of available lines is limited. Some researchers question
the quality and genetic diversity of the lines. Many of these cell lines have been
exposed to animal-derived substances, which may increase the risk of certain
infections. And, because private parties developed the lines, they don't have to
share them and can charge researchers any amount they choose.

In March 2004, a group of scientists announced the existence of 17 new stem cell
lines developed with the use of private funds. They plan to share these lines with
other researchers. But research funded by government monies can't use these lines
because they aren't derived from embryos that existed prior to the 2001 executive
order.

The great unknown
Much remains to be learned about stem cells, including potential hazards. Real
applications, for the most part, are still years away. But if progress with blood and
bone marrow transplants is any indication, stem cell research may someday help
many people. These tiny cells may significantly advance disease treatment and
expand human knowledge of the body's basic processes.

Related Information

Alzheimer's Center
Cancer Center
Diabetes Center

Additional Resources

National Institutes of Health: Stem cell information
National Cancer Institute: Bone marrow and peripheral blood stem cell transplants

May 03, 2004 - updated June 03, 2004

GA00012

SOURCE: MayoClinic.com
http://tinyurl.com/3x6wc

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