at the workshop, for example, David Prentice cited many reports as supporting the argument that research on adult stem cells has all the necessary scientific potential and represents a morally less problematic alternative that obviates the need for research on ESCs. But Prentice also pointed out that much of this evidence is suggestive rather than definitive and that the hurdles so far encountered in research on adult stem cells suggest that predictions of success are highly speculative. As discussed in Chapter 2, the evidence indicates that there are substantial potential problems in realizing this goal. Stem cells in adult mammalian tissues are rare and difficult to isolate, and very few stem cell types have been confirmed to exist in adult human tissues. Most types of adult stem cells are difficult to grow in culture, and their potential plasticity has not been clearly established. Much of the work that is used to support the argument that adult stem cells can substitute for ESCs was done only in mice or other animal models, which might or might not prove applicable to humans (Chen et al., 2001; Clarke et al., 2000; Jackson et al., 2001; Kocher et al., 2001; Krause et al., 2001; Orlic et al., 2001; Ramiya et al., 2000; Torrente et al., 2001; Wang et al., 2000), or reported work performed with human hematopoietic stem cells (Bhardwaj et al., 2001; Cashman and Eaves, 2000; Colter et al., 2000; Gilmore et al., 2000; Laughlin et al., 2001), which is not generalizable to other cell types. It should also be noted that the study of human ESCs is likely to advance some applications of adult stem cells in the future.

Second, the creation of stem cells with the SCNT technique is problematic to some because the technique is similar to that used for reproductive cloning. There is a scientific rationale for not foreclosing this avenue of research and for distinguishing clearly between SCNT to prevent transplant rejection and SCNT to create a fetus. Theoretically, the SCNT technique could produce genetically identical stem cells that could give rise to tissues that would not be rejected by a transplant recipient’s immune system. That is an attractive option because such a histocompatible transplant would not prompt the types of medically serious and potentially life-threatening immunological responses encountered by transplants of tissue from foreign donors.

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