Clinical Trials (Adult Cells)

STEM CELLS ARE specialized types of cells that differ from typical cells because of two distinct qualities. First, stem cells can renew themselves for long periods of time. Second, stem cells have the potential to differentiate into many types of cells. For example, hepatocytes or liver cells can regenerate to eventually return a liver to its original size following surgical removal of part of it. Stem cells can be divided into three main categories: embryonic, umbilical, and adult. One major difference among the three is that they are all obtained differently. For example, embryonic stem cells are collected from the very early stages of a fertilized egg, called a blastocyst—whereas umbilical stem cells are obtained from the umbilical cord of a newborn baby, adult stem cells can be derived from tissues or organs of living adults.

The advantage of using adult stem cells is that they are already somewhat differentiated, so inducement into a specific type of tissue may be easier. For example, embryonic stem cells can become differentiated into any cells in the body; hence they are called pluripotent. However, these differentiation procedures are timely and complicated. In contrast, hematopoietic stem cells, a type of adult stem cell, are called multipotent because they have the capacity to only become cells of the blood system and generally not other cell types making the differentiation relatively straightforward. Also, when recipients receive transplantation of their very own cells, these cells will usually be recognized by the recipient's immune system, and an immune response to the transplanted cells will most likely not be triggered, causing a rejection.

Although researchers are finding more and more locations from which adult stem cells can be isolated, they are still available only in select tissues and organs and are found in limited quantities. Among these locations are the bone marrow, the liver, and the brain. Another disadvantage of adult stem cells is that the harvested adult stem cells may carry genetic mutations that may be harmful to the receiving host.

Some very useful applications of adult stem cells in clinical medicine are bone marrow transplant (BMT) and peripheral bone stem cell transplant (PBSCT). BMT and PBSCT are used to facilitate patients' receiving very high dose chemotherapy or radiation therapy. Essentially, stem cells are [Page 105]used to restore cells that were destroyed by high—dose chemotherapy or radiation therapy. At present, BMT and PBSCT are most commonly used in the treatment of cancers such as leukemia, lym—phoma, and multiple myeloma, as well as other noncancerous diseases such as sickle cell anemia and aplastic anemia.

BMT is a lifesaving advance for many people, but there are numerous complications associated with it. One serious complication is graft—versus—host disease, or GVHD. GVHD is a potentially fatal incompatibility reaction mediated by antigens or small proteins found on the surface of the cells of the receiver that are not found on the cells of the donor. These antigens are usually recognized by the transplanted white blood cells (lymphocytes). These lymphocytes start attacking the recipient's cells and can lead to death in some cases.

...