Stem Cell Topic: Human Adult Stem Cells

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Human Adult Stem Cells

From post-embryonic development through the normal life of any organism, certain tissues of the body require stem cells for normal turnover and repair. Stem cells that are found in developed tissue, regardless of the age of the organism at the time, are referred to as adult stem cells. The most well-known example of this are the hematopoietic stem cells of blood. More recently, mesenchymal stem cells (MSC) required for the maintenance of bone, muscle, and other tissues have been discovered. Adult stem cells are multipotent; the number of tissues that they can regenerate compares poorly with the pluripotency of embryonic stem cells and embryonic germ cells. However, the MSC is in fact an excellent example of the potential for use of stem cells in human therapeutic procedures. MSCs are capable of differentiating into bone, cartilage, muscle, fat, and a few other tissue types. Their use for bone and cartilage replacement is undergoing FDA-approved clinical trials at the present time.

Adult-derived stem cell therapies will complement, but cannot replace, therapies that may be eventually obtained from ES cells. They do have some advantages. For example, adult stem cells offer the opportunity to utilize small samples of adult tissues to obtain an initial culture of a patient’s own cells for expansion and subsequent implantation (this is called an autologous transplant). This process avoids any ethical or legal issues concerning sourcing, and also protects the patient from viral, bacterial, or other contamination from another individual. With proper manufacturing quality controls and testing, allogeneic adult stem cells (cells from a donor) may be practical as well. Already in clinical use are autologous and allogeneic transplants of hematopoietic stem cells that are isolated from mobilized peripheral blood or from bone marrow by positive selection with antibodies in commercial devices. In general, there is less ethical concern over their initial source. Additionally, since they normally differentiate into a narrower set of cell types, directing them to a desired fate is more straightforward. However, many cells of medical interest cannot, as of yet, be obtained from adult-derived cell types. Production of large numbers of these cells is much more difficult than is the case for ES cells. Based upon our present knowledge base, it appears unlikely that human adult stem cells alone will provide all of the necessary cell types required for the most clinically important areas of research.