types, so they should be able to give rise to all the components of that tissue when transplanted into a patient. Second, some stem cells are able to migrate to injured tissue or other discrete sites in the body; for example, neural stem cells will migrate to tumor sites in the brain of a rodent (Aboody et al., 2000). This might provide more flexibility in choosing where to transplant them and more predictability in where they will localize after transplantation. Third, some adult stem cells are known to secrete growth factors that mobilize or protect other cells residing in the tissue that could increase the salutary effects of the transplant (Noble, 2000). It also might be possible to genetically engineer adult stem cells to produce higher levels of compounds normally produced in the body, to compensate for some deficiency in a patient’s own tissues. Similarly, the cells could be engineered to secrete a therapeutic agent, such as a drug.

In other situations, the use of adult stem cells would be inappropriate—for example, the isolation and autologous transplantation of a person’s stem cells suffering from a genetic disorder—in that case, the stem cells would carry the same incorrect genetic information. Transplantation of stem cells from a donor into another person will be subject to the problems of immune rejection, and this could be a substantial obstacle in time-critical situations, for example, spinal cord trauma or stroke, because characterizing the patient’s tissues and finding a match in a short period of time will be difficult.

However, because recent findings of adult stem cells are so new and studies of them raise so many questions, even the most preliminary generalizations and conclusions as to therapeutic potential are tentative. As was noted by James Thomson at the workshop, the hematopoietic stem cell is the most characterized cell in the body, and “The amount of knowledge we have on other adult stem cells goes down dramatically from there.”

First, human adult stem cells are rare and it is difficult to isolate a unique group of stem cells in pure form. So it is not surprising that what at first appears to be plasticity in a single adult stem cell type could be the result of a mixture of cells of different types, including different types of stem cells. At the workshop, Margaret Goodell explained how her

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