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- Stem cells: What they are and what they do
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Stem cells: What they are and what they do
Researchers believe stem cells offer great promise for new medical treatments. Learn about stem cell types, current and possible uses, ethical issues and the state of research.
By Mayo Clinic staffYou'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 struggle with understanding 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.
Why is there such an interest in 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. 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. 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.
What are stem cells?
Stem cells are master cells of the body — cells 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, heart muscle or bone. Stem cells are unique — no other cell in the body has the ability to self-renew or to differentiate.
Where do stem cells come from?
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 felt 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. Researchers have reported being able to transform regular adult cells into stem cells in laboratory studies. By altering the genes in the adult cells, researchers were able to reprogram the cells to act similarly to embryonic stem cells. While this new technique may help researchers avoid the controversies that come with embryonic stem cells, more research is needed. The technique of altering adult cells involves processes that may not be safe for use in people, though researchers are studying safer ways of altering these cells. And whether this new type of stem cells can be as useful as embryonic stem cells remains to be seen.
- 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. Researchers have used amniotic fluid to identify stem cells that could develop into several other types of cells. More study of amniotic fluid stem cells is needed to understand their potential.
Why is there a controversy about using embryonic stem cells?
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 the embryos destroys the embryos. This raises significant ethical questions.
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 can live and grow in special solutions in test tubes or petri dishes in laboratories.
Why can't researchers use adult stem cells instead?
Studies using adult stem cells are under way. The adaptability of adult cells and their application to treatments remains to be determined. Researchers believe that 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.
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