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What Are Stem Cells?

Stem cells are special human cells that have the ability to develop into many different cell types, from muscle cells to brain cells. In some cases, they also have the ability to repair damaged tissues. Researchers believe that stem cell-based therapies may one day be used to treat devastating ailments like paralysis and Alzheimer's disease.

Types of stem cells

Stem cells are divided into two main forms: embryonic stem cells and adult stem cells. The embryonic stem cells used in research today come from unused embryos resulting from an in vitro fertilization procedure and that are donated to science. These embryonic stem cells are pluripotent, meaning that they can turn into more than one type of cell.

There are two types of adult stem cells. One type comes from fully developed tissues, like the brain, skin, and bone marrow. There are only small numbers of stem cells in these tissues, and they are more likely to generate only certain types of cells. For example, a stem cell derived from the liver will only generate more liver cells.

The second type is induced pluripotent stem cells. These are adult stem cells that have been manipulated in a laboratory to take on the pluripotent characteristics of embryonic stem cells. Scientists first reported that human stem cells could be reprogrammed this way in 2006. Although induced pluripotent stem cells don't appear to be clinically different from embryonic stem cells, scientists have not yet found one that can develop every kind of cell and tissue.

Stem cells in medicine

The only stem cells currently used to treat disease are hematopoietic stem cells—the blood cell-forming adult stem cells found in bone marrow. Every type of blood cell in the bone marrow begins as a stem cell. Stem cells are immature cells that are able to produce other blood cells that mature and function as needed.

Anatomy of  a  bone,  showing  blood cells
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These cells are used in procedures like bone marrow transplants that help cancer patients produce new blood cells after their own hematopoietic stem cells have been killed by radiation therapy and chemotherapy. They may also be used to treat people with conditions like Fanconi anemia, a blood disorder that causes the body's bone marrow to fail.

Stem cells may benefit our health in the future in many ways and through many new therapies. Researchers believe that stem cells will eventually be used to help regenerate damaged tissue. One day, for example, doctors may be able to treat people with chronic heart disease by growing healthy heart muscle cells in a laboratory and transplanting them into damaged hearts. Other therapies could target illnesses as diverse as type 1 diabetes, spinal cord injuries, Alzheimer's disease, and rheumatoid arthritis. New medications could also be tested on cells generated from pluripotent stem cells.  

Challenges in stem cell research

Stem cells require much more study before their use can be expanded. Scientists must first learn more about how embryonic stem cells develop so that they can understand how to control the type of cells created from them. Another challenge is that the embryonic stem cells available today have a high likelihood of being rejected by the body. Finally, some people find it morally troubling to use stem cells derived from embryos.

Scientists also face challenges with the use of adult pluripotent stem cells. These cells are difficult to grow in a laboratory, so researchers are investigating ways to improve the process. These cells are also found in relatively tiny quantities in the body, and there is a greater chance that they could contain DNA abnormalities.

A number of clinical trials that use stem cell therapies are currently being conducted in the U.S. If you are interested in trying this therapy to treat a specific condition, ask your health care provider how to find out about trials available in your area.