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Adult/Tissue Stem Cell Questions
Blood-forming stem cells from bone marrow or umbilical cord blood are the most common type of stem cell used to treat human diseases today. Adult/tissue stem cells are present in adults, children, infants, umbilical cords and developing fetuses. They are more limited and specialized than embryonic stem cells. They can make just one or two specific kinds of tissue, such as blood and immune system cells. Additionally, their capacity to replace themselves is more limited than with embryonic stem cells, so there are often fewer cells to work with. Although they are less versatile than embryonic stem cells, they may also be used to treat patients. For example, when cancer patients are given a bone marrow transplant, they receive blood-forming stem cells that rebuild their blood and immune systems. While blood-forming stem cells can make all types of blood and immune system cells, they cannot make cells from other tissue like the brain, skin or other tissues. In addition to bone marrow and umbilical cord blood, adult/tissue stem cells have been found in: the dental pulp of the tooth, liver, skin, nervous system, gastrointestinal tract and muscle. The primary roles of adult/tissue stem cells in the body are to maintain and repair the tissues in which they are found. An example would be muscle stem cells that replace damaged muscle cells. Blood from the placenta and umbilical cord, also considered adult/tissue stem cells, that is left over after birth is also considered to be a rich source of blood-forming stem cells. These so-called umbilical cord stem cells have been shown to be able to differentiate into bone cells as well as the cells lining the inside of blood vessels. For more than 30 years, bone marrow stem cells have been used to treat cancer patients with conditions like leukemia and lymphoma. During chemotherapy, while most of the leukemia cells are killed, so are the blood-forming stem cells in the bone marrow that patients need to make new blood cells. As a result, many patients die from the toxic effects of chemotherapy against normal blood cells. However, if stem cells are removed from the patient before chemotherapy and then re-injected after treatment is completed, the stem cells in the bone marrow are able to produce large amounts of red and white blood cells. This keeps the body healthy and helps fight infections. Adult blood-forming stem cells are already widely used clinically to restore the blood-forming system after chemotherapy or radiation therapy. Ongoing research on adult stem cells of various types will likely lead to additional new therapies. But while scientists are devoting considerable effort to studying adult stem cells, they believe that embryonic stem cells offer unique capabilities not offered by adult stem cells that make them advantageous for certain types of applications. Adult stem cells have a limited capacity to proliferate (duplicate) in contrast to embryonic stem cells which can proliferate indefinitely. This is important because the limited ability of adult stem cells to proliferate has been a limiting factor in the treatment of diseases such as Parkinson's disease. Moreover, adult stem cells are partially specialized and can generally only form mature cells from their tissue of origin, unlike embryonic stem cells which can form any cell in the body. As far as we know now, adult stem cells do not exist in some tissues, like the pancreas. As a result, it may only be possible to generate new insulin secreting cells for the treatment of juvenile diabetes patients from embryonic stem cells. For this reason, the Juvenile Diabetes Research Foundation is investing millions of dollars into embryonic stem cell research because they believe that this research represents the best hope for a cure for their children. The bottom line is that most scientists and physicians believe that research must proceed using both embryonic stem cells and adult/tissue stem cells in order to maximize our ability to develop new cures.
There are currently several limitations to using adult/tissue stem cells. Adult/tissue stem cells are often present in only very limited quantities. For example, the limited numbers of blood-forming stem cells that can be isolated from umbilical cords means that these cells can only be used to treat small children, not adults. Evidence also suggests that adult/tissue stem cells do not have the same capacity to proliferate (duplicate) as embryonic stem cells do. Finally, adult/tissue stem cells do not exist in certain important tissues, as far as we know currently. These weaknesses limit the usefulness of adult/tissue stem cells. |
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