Finally, promising research has been conducted with adult stem cells (Lanza et al., 2004; Wagers and Weissman, 2004). Adult stem cells can be obtained from various tissues of adults or in some cases from neonatal tissues. A well-known example of the use of adult stem cells is bone marrow transplantation. Hematopoietic (blood-forming) adult stem cells from bone marrow or from umbilical cord blood give rise to all the cells of the blood. Skin cell transplants similarly rely on the transfer of skin stem cells. In both examples, the tissue involved naturally renews itself from its pool of stem cells—a property that can be exploited for medical use. It is possible that similar approaches can be developed for other tissues (such as muscle). However, in many other tissues, natural self-renewal appears to be a slow process, and stem cells for such tissues are correspondingly harder to characterize and isolate. There is also the possibility that some tissues may not contain a distinct subpopulation of undifferentiated stem cells at all. Furthermore, the anatomic source of the cells (such as brain or heart muscle) might preclude easy or safe access.

There are important biological differences between embryonic and adult stem cells. Embryonic stem cells show a much greater capacity for self-renewal, can be cultured to generate large numbers of cells, and are pluripotent—they have the potential for differentiation into a very wide variety of cell types. In contrast, adult stem cells appear to be capable of much less proliferation and, in general, have a restricted range of developmental capacities; that is, they can differentiate into only a limited array of cells (Wagers and Weissman, 2004). Thus most experts consider “adult stem cell research” not to be an alternative to hES and hEG cell research, but rather a complementary and important line of investigation.

hES cells currently can be derived from three sources: blastocysts remaining after infertility treatments and donated for research, blastocysts generated from donated gametes (oocytes and sperm), and the products of NT. Cadaveric fetal tissue is the only source of hEG cells. hES and hEG cells offer remarkable scientific and therapeutic possibilities, involving the potential for generating more specialized cells or tissue. This could allow the generation of new cells to be used to treat injuries or diseases involving cell death or impairment, such as Parkinson’s disease, diabetes, heart disease, spinal cord injury, and hematologic and many other disorders. In addition, understanding the biology of hES and hEG cells is critical for understanding the earliest stages of human development. Ethical concerns about the sources of hES and hEG cells, however, and fears that use of NT for research could lead to the use of NT to produce a child have fostered a great deal of public discussion and debate. Concern has also been expressed about whether and how to restrict the production of human/nonhuman chimeras when conducting research with hES cells. Such research could be tremendously useful in understanding the etiology and progression of human disease and in testing new drugs, and will be necessary in preclinical testing of both adult and embryonic stem cells and their derivatives. However, some are concerned that creating chimeras would violate social conventions built around the notion of species (Robert and Baylis, 2003).

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