(MayoClinic.com) 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. You may wonder what stem cells are, how they're being used to treat disease and injury, and why they're the subject of such vigorous debate.

Here are some answers to frequently asked questions about stem cells.

Researchers hope stem cell studies can help to:

• Increase understanding of how diseases occur. By watching stem cells mature into cells that eventually become bones, heart muscle, nerve cells, and other organs and tissue, researchers and doctors may better understand how a variety of diseases and conditions develop.
• Generate healthy cells to replace diseased cells (regenerative medicine). Researchers hope they can train stem cells into becoming specific cells so that those specialized cells can be used to regenerate and repair diseased or damaged tissues in people. People who might benefit from stem cell therapies include those with spinal cord injuries, type 1 diabetes, Parkinson's disease, Alzheimer's disease, heart disease, stroke, rheumatoid arthritis, osteoarthritis and multiple sclerosis. Stem cells could also be grown to become new tissue for use in transplant medicine.
• Test new drugs for safety and effectiveness. Before using new drugs in people, researchers could use stem cells to test the safety and quality of investigational drugs. For instance, nerve cells could be generated in order to test a new drug for a nerve disease. Tests could show whether the new drug had any effect on the cells and whether the cells were harmed.

Stem cells are the body's raw materials — cells from which all other cells with specialized functions are generated. 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, heart muscle or bone. Stem cells are unique — no other cell in the body has the natural ability to generate new cell types.

Researchers have discovered several sources of stem cells:

• Embryonic stem cells. These stem cells come from embryos that are four to five days old. At this stage, an embryo is called a blastocyst and has about 150 cells. These are pluripotent (ploo-RIP-uh-tunt) stem cells, meaning they can divide into more stem cells or they can specialize and become any type of body cell. Because of this versatility, embryonic stem cells have the highest potential for use to regenerate or repair diseased tissue and organs in people.
• 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 is somatic stem cell, meaning "of the body." Until recently, it was believed that adult stem cells could only create similar types of cells. For instance, it was thought that stem cells residing in the bone marrow could give rise only to blood cells. However, emerging evidence 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 muscle cells. This research has led to early-stage clinical trials to test usefulness and safety in people.
• Adult cells altered to have properties of embryonic stem cells (induced pluripotent stem cells). Scientists have successfully transformed regular adult cells into stem cells using a technique called nuclear reprogramming. By altering the genes in the adult cells, researchers can reprogram the cells to act similarly to embryonic stem cells. This new technique may help researchers avoid the controversies that come with embryonic stem cells, and prevent immune system rejection of the new stem cells. But, it's not yet known if altering adult cells will cause adverse effects in humans. Researchers have been able to take regular connective tissue cells and reprogram them to become heart cells. The new heart cells were injected into mice with heart failure, where they improved heart function and survival time.
• Amniotic fluid stem cells. Researchers have also discovered stem cells in amniotic fluid. Amniotic fluid fills the sac that surrounds and protects a developing fetus in the uterus. Researchers have identified stem cells in samples of amniotic fluid drawn from pregnant women during a procedure called amniocentesis. During this test, a doctor inserts a long, thin needle into a pregnant woman's abdomen to collect amniotic fluid. The fluid can be tested for abnormalities, such as Down syndrome, and fetal maturity. The procedure is generally considered safe for the developing fetus and the mother. More study of amniotic fluid stem cells is needed to understand their potential.

Embryonic stem cells are obtained 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 embryos raises significant ethical questions.

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 can live and grow in special solutions in test tubes or petri dishes in laboratories.

While research into adult stem cells is promising and moving forward rapidly, adult stem cells may not be as versatile and durable as embryonic stem cells are. Adult stem cells 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 environmental hazards, such as toxins, or from errors acquired by the cells during replication. However, researchers have found that adult stem cells are more adaptable than was initially suspected. There have been significant advances in work with adult stem cells, and more studies are under way.

A stem cell line is a group of cells that all descend from a single original stem cell. Cells in a stem cell line keep growing 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 shared with other researchers. That way, researchers don't have to get stem cells from an embryo itself.

Stem cell therapy, also known as regenerative medicine, is the replacement of diseased, dysfunctional or injured cells with stem cells or their derivatives. 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 could then be implanted into a person. For example, if the person had heart disease, the cells could be injected into the heart muscle. The healthy, transplanted heart cells could then contribute to repairing defective heart muscle. In fact, researchers have already shown that adult bone marrow cells guided to become heart-like cells can repair heart tissue in mice, and much more research is ongoing.

Yes, stem cell transplants, also known as bone marrow transplants, have been performed in the United States since the late 1960s. These transplants use adult stem cells.

Adult stem cells are being tested in other applications, including a number of degenerative diseases, such as heart failure. Stem cells from umbilical cord blood have been successfully used in clinical trials to treat cancer and blood-related diseases.

The Food and Drug Administration has approved an initial safety study using an embryonic stem cell therapy for the treatment of serious spinal cord injuries.

To be useful in people, researchers must be certain that stem cells will differentiate into the specific cell types desired. 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. Researchers have found ways to direct stem cells to become specific types of cells, and research into this area has advanced significantly.

Embryonic stem cells could also grow irregularly — something that's been reported 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. Researchers are working on ways to avoid these possible complications.

Therapeutic cloning is a technique to create versatile stem cells independent of fertilized eggs. In this technique, the nucleus, which contains the genetic material, is removed from an unfertilized egg. The nucleus is also removed from a somatic cell of a donor. 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 stem cells that is genetically identical to the donor's — in essence, a clone. This technique is also called somatic cell nuclear transfer.

Some researchers believe that 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 may allow researchers to see exactly how a disease develops.

No. Researchers haven't been able to successfully perform therapeutic cloning with humans, despite success in a number of other species.

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 may be able to treat a wider variety of diseases. Studies using embryonic stem cells or embryonic-like stem cells to regenerate tissue and organs in people are just getting started. Researchers are enthusiastic about the potential for regenerative medicine treatments.

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