Last reviewed and revised on May 20, 2013
By Anthony L. Komaroff, M.D.
Brigham and Women's Hospital
Stem cells have generated great excitement because many human diseases involve the death of certain specialized cells, and stem cells have the potential to replace those dead cells. Stem cells can become specialized cells, if they are coaxed to do so by certain chemical signals. Unlike specialized cells, stem cells also can reproduce themselves indefinitely.
There are three types of stem cells that occur naturally:
- Embryonic stem cells are found in an early stage of the embryo. They can reproduce themselves, making more embryonic stem cells, or they can turn into specialized cell types.
- Umbilical cord stem cells are present in the umbilical cord blood, which is removed at birth. They are currently used to treat human blood cancers and related conditions. They can form blood cells and several other types of specialized cells. However, unlike embryonic stem cells, they probably cannot form all types of specialized cells.
- Adult (or somatic) stem cells are found in certain organs, such as the bone marrow, brain, muscles and skin. These cells also can reproduce themselves, and they can turn into different specialized cells of the organ where they are found. For example, brain stem cells can form into neurons — the cells responsible for the main activities of the brain, such as thinking, emotion, vision, hearing, and directing body movement. Blood stem cells, for example, can produce oxygen-carrying red blood cells, all the different types of white blood cells that are part of the immune system, and the cells that form blood-clotting cell fragments called platelets.
Because embryonic stem cells appear to be much more capable of turning into virtually any type of specialized cell, they have theoretical advantages over adult stem cells in cell therapy. However, because embryonic stem cells come from human embryos, they are the subject of ethical controversy. That's because of the way doctors would obtain embryonic stem cells:
- A fertilized egg grows into an early-stage embryo called a blastocyst. This process takes about five to seven days.
- As it grows, the blastocyst forms a ball, and there are a group of cells on the inside of the ball. These are the embryonic stem cells.
- The embryonic stem cells can be removed, which destroys the blastocyst. Scientists then place the stem cells into a laboratory dish that is full of nutrients to encourage them to grow. The nutrients and other chemical signals that are put in the dish either encourage the cells to keep reproducing or to begin to change into specialized cells.
- With the right chemical signals, the embryonic stem cells can be encouraged to grow into essentially all the types of cells in the body.
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Anthony L. Komaroff, M.D., is professor of medicine and editor-in-chief of Harvard Health Publications at Harvard Medical School. Dr. Komaroff also is senior physician and was formerly director of the Division of General Medicine at Brigham and Women's Hospital. Dr. Komaroff has served on various advisory committees to the federal government, and is an elected Fellow of the American Association for the Advancement of Science.