in the future. 6 TYPES OF STEM CELLS Through nuclear transfer, scientists could produce a blastocyst by inserting the nucleus from an adult cell (for example, a skin cell) into an egg without a nucleus. All the stem cells derived from this blastocyst are genetically matched to the adult cell. 1Claims by Korean scientists of successful derivation of human embryonic stem cells using nuclear transfer have been found to be invalid and were retracted. Scientists believe that if they are able to use nuclear transfer to derive human stem cells, it could allow them to study the development and progression of specific diseases by creating stem cells containing the genes responsible for certain disorders. In the future, scientists may also be able to create “personalized” stem cells that contain only the DNA of a specific patient. The embryonic stem cells created by nuclear transfer would be genetically matched to a person needing a transplant, making it far less likely that the patient’s body would reject the new cells than it would be with traditional tissue transplant procedures. Although using nuclear transfer to produce stem cells is not the same as reproductive cloning, some are concerned about the potential misapplication of the technique for reproductive cloning purposes. Other ethical considerations include egg donation, which requires informed consent, and the possible destruction of blastocysts. Adult Stem Cells Adult stem cells are hidden deep within organs, surrounded by millions of ordinary cells, and may help replenish some of the body’s cells when needed. In fact, some adult stem cells are currently being used in therapies. They have been found in several 7 Producing Embryonic Stem Cells TYPES OF STEM CELLS Using Nuclear Transfer Is Not the Same as Reproductive Cloning The use of nuclear transfer to develop disease-specific stem cells can be called research cloning, and the use of this technique for personalized tissue transplants is sometimes called therapeutic cloning. These terms must be carefully distinguished from reproductive cloning, in which the intent is to implant a cloned embryo in a female’s womb and allow it to develop fully into an individual. This was the technique by which Dolly the sheep was made and is now widely used for reproductive cloning in animals. In humans, however, reproductive cloning has been actively discouraged by most in the scientific community. The National Academies concluded,“Human reproductive cloning should not now be practiced. It is dangerous and likely to fail” in the 2002 report Scientific and Medical Aspects of Human Reproductive Cloning. Some of the known sources of adult stem cells. 8 TYPES OF STEM CELLS organs that need a constant supply of cells, such as the blood, skin, and lining of the gut, and have also been found in surprising places like the brain, which is not known to readily replenish its cells. Unlike embryonic stem cells, adult stem cells are already somewhat specialized. For example, blood stem cells normally only give rise to the many types of blood cells, and nerve stem cells can only make the various types of brain cells. Recent research however, suggests that some adult stem cells might be more flexible than previously thought, and may be made to produce a wider variety of cell types. For example, some experiments have suggested that blood stem cells isolated from adult mice may also be able to produce liver, muscle, and skin cells, but these results are not yet proven and have not been demonstrated with human cells. Nevertheless, scientists are working on finding a way to stimulate adult stem cells, or even other types of adult cells, to be more versatile. If they succeed, it could provide another source of unspecialized stem cells. Attributes Limitations Ethical Concerns In Vitro Fertilization • can produce all cell types • relatively easy to identify, isolate, maintain, and grow in the laboratory • large source of “excess” blastocysts from IVF clinics • limited number of cell lines available for federally funded research • risk of creating teratomas (tumors) from implanting undifferentiated stem cells • destruction of human blastocysts • donation of blastocysts requires informed consent Nuclear Transfer • can produce all cell types • relatively easy to identify, isolate, maintain, and grow in the laboratory • stem cells may be genetically matched to patient • not yet achieved with human cells • risk of creating teratomas (tumors) from implanting undifferentiated stem cells • destruction of human blastocysts • donation of eggs requires informed consent • concern about misapplication for reproductive cloning Adult Tissues • demonstrated success in some treatments • stem cells may be genetically matched to patient • produce limited number of cell types • not found in all tissues • difficult to identify, isolate, maintain, and grow in the laboratory • no major ethical concerns have been raised COMPARISON OF THE DIFFERENT SOURCES OF STEM CELLS Embryonic Stem Cells Adult Stem Cells The day-to-day work that goes on in the laboratories across the country studying stem cells begins with developing ways to identify stem cells, culture cell lines, and stimulate stem cells to differentiate. Once these first steps have been achieved, work on animals plays an important role in furthering basic research and developing medical applications. This work is