Stem cells are unspecialized cells that have the potential to become any of the 220 specialized cell types in our bodies. Most stem cells also have the ability to divide indefinitely, providing a limitless supply of new stem cells. Many medical experts think that stem cells will be key to treating and even curing dozens of diseases, including cancer, diabetes, and neurological disorders.

Perhaps the best-known stem cell therapy is bone marrow transplantation, a treatment used since the 1970s to treat patients with leukemia, a blood cancer. Leukemia occurs when leukocytes grow and function abnormally and are unable to fight infection. Like other blood cells, leukocytes are made in the bone marrow. During chemotherapy, a common form of cancer treatment, toxic agents kill not only diseased cells but also healthy stem cells located in the bone marrow. In bone marrow transplantation, all of the patient's existing bone marrow and abnormal leukocytes must be killed using a combination of chemotherapy and radiation. Next, donor bone marrow containing healthy stem cells is introduced into the patient's bloodstream. If the transplant is successful, the stem cells will replenish the supply of healthy leukocytes.

Another stem cell therapy targets type 1 diabetes, a disease that occurs in children and young adults when the body's immune system malfunctions and destroys pancreatic beta cells. These cells naturally produce insulin, the hormone that controls blood-sugar levels. Often, the condition is not diagnosed until 60 to 80 percent of all beta cells have been lost. While stem cell treatment for diabetes is still in the experimental stages, results are encouraging. In one trial, 15 young patients with type 1 diabetes were given drugs to suppress their immune systems followed by transfusions of stem cells that had previously been drawn from their own blood. In response to the treatment, the beta cells that remained in the patients' bodies multiplied, with restored function. After their bodies began producing insulin, 11 of the patients were able to stop their insulin shots.

Before stem cells can be more widely used for medical purposes, scientists face several challenges. Apart from the heated debate over the ethics of acquiring stem cells from human embryos, scientists cannot control how stem cells differentiate into a specific tissue or cell type once the cells are injected into a person. They recognize that turning genes on and off plays a key role in differentiation, but they are still searching for the mechanism that controls the switch. There is also the problem of rejection. The immune system of a patient receiving embryonic cell treatment may regard these donor cells as foreign invaders and launch an attack against them. Using adult stem cells drawn from the patient's own blood helps insure that the cells will not be rejected by his or her immune system. But adult stem cells are less adaptable than embryonic stem cells and found in limited numbers in certain tissues.

Explore in this NOVA scienceNOW classroom activity how stem cells become specialized cells.

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