Stem cells: Sorting through the hype and hope

You've heard about stem cells in the news and perhaps you've wondered if they might help you or a loved one with a serious disease. Like many others, you may struggle with understanding what exactly stem cells are, how they're actually being used now to treat disease and injury, and why they're the subject of such passionate debate.

Here, you can sort through the hype and the hope and get answers to frequently asked questions about stem cells.

Why is there high interest in stem cells?

Researchers are interested in stem cells for two main reasons:

• Knowledge. By studying how stem cells mature into cells that eventually become bones, organs and other tissue, researchers hope to learn more about the function of stem cells and what can go wrong in development. This knowledge may shed new light on how a variety of diseases and conditions develop, such as cancer and birth defects.
• Therapy. Researchers hope they can manipulate stem cells into becoming specific types of cells. If this is done successfully, stem cells could be used to treat diseases and conditions such as diabetes, Parkinson's disease, inherited genetic diseases or spinal cord injuries. Stem cells could also be grown to become organs to use in transplants, since donor organs are notoriously in short supply. Stem cells may also one day be used to test experimental medications before human clinical trials.

What exactly are stem cells?

Stem cells are master cells of the body, from which all other cells with specialized functions are created. Under the right conditions in the body or a laboratory, stem cells divide to form more cells, called daughter cells. These daughter cells either become new stem cells (self-renewal) or become specialized cells (differentiation) with a more specific function, such as blood cells, brain cells or bone cells. Besides the stem cell, no other cell in the body has the ability to self-renew or to differentiate.

Stem cells: The body's master cells

Stem cells: The body's master cells

Stem cells are the body's master cells. All other cells arise from stem cells, including blood cells, nerve cells and others.

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If stem cells are master cells, where do they come from?

There are three sources of stem cells:

• Embryonic stem cells. These stem cells come from human embryos that are four to five days old. At this stage, an embryo is called a blastocyst and has about 50 to 150 cells. Some of these cells are pluripotent (ploo-RIP-o-tunt) stem cells, meaning they can divide into more stem cells or they can specialize and become any type of body cell. These are the stem cells that researchers have focused on and that have become a matter of controversy.
• Adult stem cells. These stem cells are found in small numbers in most adult tissues, such as bone marrow. Adult stem cells are also found in children and in placentas and umbilical cords. Because of that, a more precise term for these cells is somatic stem cell, meaning "of the body." Until recently, it appeared that adult stem cells could only create similar types of cells. For instance, it was thought that stem cells in the bone marrow could give rise only to blood cells. However, a new — but controversial — theory suggests that adult stem cells may be more versatile than previously thought and able to create unrelated types of cells after all. For instance, bone marrow stem cells may be able to create kidney cells. But that research is only in very early stages.
• Embryonic germ cells. These are stem cells that come from areas within an embryo or fetus that are destined to become either the testicles or ovaries. Like embryonic stem cells, embryonic germ cells can become any type of cell. Less research has been done on embryonic germ cells because the embryos used to obtain them must be deliberately aborted. In addition, these cells tend to differentiate spontaneously, so they may be more difficult to use in a controlled manner.

Why is there a controversy about using embryonic stem cells?

Embryonic stem cells are derived from early-stage embryos — a group of cells that forms when a woman's egg is fertilized with a man's sperm. Extracting stem cells from the embryos destroys the embryos. Some people view this as taking a human life, which raises moral and ethical considerations.

Where do these embryos come from?

The embryos being used in embryonic stem cell research come from eggs that were fertilized at in vitro fertilization clinics but never implanted in a woman's uterus because they were no longer wanted or needed. The excess embryos were frozen and later voluntarily donated for research purposes. The stem cells live and grow in cultures, or special solutions in test tubes or petri dishes in laboratories.

Why can't researchers use adult stem cells instead?

Researchers believe that adult stem cells may not be as versatile, durable or healthy as embryonic stem cells are. The problems with adult stem cells are that they may not be able to be manipulated to produce all cell types, which limits how they can be used to treat diseases, and they don't seem to have the same ability to multiply that embryonic stem cells do. They're also more likely to contain abnormalities due to exposure to environmental hazards, such as toxins, or from errors introduced into cells during replication.

What is a stem cell line and why do researchers want to use them?

A stem cell line is a group of cells that all descend from an original stem cell. Cells in a stem cell line keep dividing but don't differentiate into specialized cells. Ideally, they remain free of genetic defects and continue to create more stem cells. Clusters of cells can be taken from a stem cell line and frozen for storage or distributed to other researchers. This way, researchers don't have to get stem cells from an embryo itself.

Why do researchers want to create more embryonic stem cell lines?

Researchers who receive federal funding to support their experiments — as most academic researchers do — are limited by law to working with about 20 stem cell lines. These stem cell lines, sometimes called the presidential lines, date back to the late 1990s, and some researchers contend that they pose several problems:

• The limited number of lines limits the genetic diversity available, so cells may be useful only for certain diseases or people.
• The lines are old, so cells don't grow as well as new ones.
• The lines may have been contaminated by nonhuman cells in the growth cultures, compromising their safety.
• The DNA in some of the cells may subtly change over time, causing genetic flaws that could be passed along to daughter cells or to humans.

How can additional stem cell lines be made available more quickly to U.S. researchers?

It will take a presidential order or an act of congress signed by the president to make federal funding available for research on more stem cell lines, not just the presidential lines. This would speed the development of embryonic stem cell lines in the United States.

Until public funding is available, some researchers have turned to private funding to finance their embryonic stem cell studies and have created their own stem cell lines. Also, individual states can pass their own laws allowing funding of embryonic stem cell research with state money.

What is stem cell therapy and how does it work?

Stem cell therapy is the replacement of diseased, dysfunctional or injured cells with either adult or embryonic stem cells. It's somewhat similar to the organ transplant process but uses cells instead of organs.

Researchers grow stem cells in the lab. These stem cells are manipulated to make them specialize into specific types of cells, such as heart muscle cells, blood cells or nerve cells. This manipulation may involve changing the material in which the stem cells are grown or even injecting genes into the cells. The specialized cells are then implanted into a person. If the person has heart disease, the cells would be injected into the heart muscle. The normally functioning implanted heart cells, in theory, would replace the defective heart cells.

Have stem cells already been used to treat diseases?

Yes, stem cell transplants, also known as bone marrow transplants, have been performed in the United States since the late 1960s. These transplants have proved highly successful in treating a number of cancerous diseases, such as leukemia, and noncancerous diseases, such as aplastic anemia.

Stem cell transplants use cells harvested from a donor's or person's own bone marrow, circulating blood or umbilical cord blood. These are all adult stem cells — not stem cells derived from embryos. In addition, adult stem cells have also been used in human experiments involving the treatment of diabetes, heart disease and other conditions.

Embryonic stem cell treatment, on the other hand, has been purely experimental, involving animal studies only.

What are the problems with using embryonic stem cells in humans?

To be useful in people, researchers must be certain that embryonic stem cells will differentiate into the specific cell types desired. Because research on embryonic stem cells is still in early stages, that hasn't happened reliably enough in animal experiments to try in people. Researchers, for instance, don't want to transplant a stem cell into a person hoping it'll become a heart cell only to learn that it's become a bone cell, with potentially dangerous consequences.

Embryonic stem cells may also have other unpleasant surprises if used in people before the science is ready. They could become tumor cells — something that's happened in animal experiments — or travel to a part of the body where they're not intended to go. They also might trigger an immune response in which the recipient's body attacks the stem cells as foreign invaders, or simply fail to function normally, with unknown consequences.

What is therapeutic cloning and what benefits might it offer?

Therapeutic cloning is a technique to create embryonic stem cells without using fertilized eggs. In this technique, the nucleus is removed from a cell in a woman's egg. The nucleus is also removed from a somatic cell of a donor — a person with a disease or injury, such as type 1 diabetes. This donor nucleus is then injected into the egg, replacing the nucleus that was removed, a process called nuclear transfer. The egg is allowed to divide and soon forms a blastocyst. This creates a line of embryonic stem cells that is genetically identical to the donor — in essence, a clone. This technique is also called somatic cell nuclear transfer.

Some researchers believe that embryonic stem cells derived from therapeutic cloning may offer benefits over those from fertilized eggs because they're less likely to be rejected once transplanted back into the donor, and they allow researchers to see exactly how a disease develops from the beginning.

In addition, some researchers consider therapeutic cloning a good alternative to creating embryonic stem cell lines from fertility treatments, since they come from cells that were never fertilized. However, this technique is not without opponents. Critics contend therapeutic cloning can be perceived as destruction of a human life or potential human life.

Has therapeutic cloning in people been successful?

So far, human therapeutic cloning has worked only in theory. Researchers haven't been able to successfully perform therapeutic cloning of people. In 2005, South Korean researchers reported creation of human embryonic stem cells this way, but their claims were ultimately revealed to be untrue.

What does the future hold for stem cell therapy?

Researchers say the field has promise. Stem cell transplants using adult stem cells continue to be refined and improved. And researchers are discovering that adult stem cells may be somewhat more versatile than originally thought, which means they eventually may be able to treat a wider variety of diseases.

However, advances in embryonic stem cell research have been hindered by the limits of scientific knowledge, funding restrictions, politics and ethical debates. So for now, research on embryonic stem cells remains confined to laboratory animals.

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