Some of the promise of stem cell therapy has been realized. A prime example is bone marrow transplantation. Even here, however, manyproblems remain to be solved.

Challenges facing stem cell therapy include the following:

Adult stem cells Tissue-specific stem cells in adult individuals tend to be rare. Furthermore, while they can regenerate themselves in an animal or person they are generally very difficult to grow and to expand in the laboratory. Because of this, it is difficult to obtain sufficient numbers of many adult stem cell types for study and clinical use. Hematopoietic or blood-forming stem cells in the bone marrow, for example, only make up one in a hundred thousand cells of the bone marrow. They can be isolated, but can only be expanded a very limited amount in the laboratory. Fortunately, large numbers of whole bone marrow cells can be isolated and administered for the treatment for a variety of diseases of the blood. Skin stem cells can be expanded however, and are used to treat burns. For other types of stem cells, such as mesenchymal stem cells, some success has been achieved in expanding the cellsin vitro, but application in animals has been difficult. One major problem is the mode of administration. Bone marrow cells can be infused in the blood stream, and will find their way to the bone marrow. For other stem cells, such as muscle stem cells, mesenchymal stem cells and neural stem cells, the route of administration in humans is more problematic. It is believed, however, that once healthy stem cells find their niche, they will start repairing the tissue. In another approach, attempts are made to differentiate stem cells into functional tissue, which is then transplanted. A final problem is rejection. If stem cells from the patients are used, rejection by the immune system is not a problem. However, with donor stem cells, the immune system of the recipient will reject the cells, unless the immune system is suppressed by drugs. In the case of bone marrow transplantation, another problem arises. The bone marrow contains immune cells from the donor. These will attack the tissues of the recipient, causing the sometimes deadly graft-versus-host disease.

Pluripotent stem cells All embryonic stem cell lines are derived from very early stage embryos, and will therefore be genetically different from any patient. Hence, immune rejection will be major issue. For this reason, iPS cells, which are generated from the cells of the patient through a process of reprogramming, are a major breakthrough, since these will not be rejected. A problem however is that many iPS cell lines are generated by insertion of genes using viruses, carrying the risk of transformation into cancer cells. Furthermore, undifferentiated embryonic stem cells or iPS cells form tumors when transplanted into mice. Therefore, cells derived from embryonic stem cells or iPS cells have to be devoid of the original stem cells to avoid tumor formation. This is a major safety concern.

A second major challenge is differentiation of pluripotent cells into cells or tissues that are functional in an adult patient and that meet the standards that are required for 'transplantation grade' tissues and cells.

A major advantage of pluripotent cells is that they can be grown and expanded indefinitely in the laboratory. Therefore, in contrast to adult stem cells, cell number will be less of a limiting factor. Another advantage is that given their very broad potential, several cell types that are present in an organ might be generated. Sophisticated tissue engineering approaches are therefore being developed to reconstruct organs in the lab.

While results from animal models are promising, the research on stem cells and their applications to treat various human diseases is still at a preliminary stage. As with any medical treatment, a rigorous research and testing process must be followed to ensure long-term efficacy and safety.

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Stem Cell FAQ