Stem Cells Cloning and Related Techniques Offer Exciting Possibilities but Raise Some Concerns

Identical twins occur naturally when the mass of cells composing an early embryo divides into two parts, each of which develops and grows into an individual animal. Each cell in an eight-cell-stage mouse embryo has the potential to give rise to any part of the entire animal. Cells with this capability are referred to as embryonic stem (ES) cells. As we learn in Chapter 22, ES cells can be grown in the laboratory (cultured) and will develop into various types of differentiated cells under appropriate conditions.

The ability to make and manipulate mammalian embryos in the laboratory has led to new medical opportunities as well as various social and ethical concerns. In vitro fertilization, for instance, has allowed many otherwise infertile couples to have children. A new technique involves extraction of nuclei from defective sperm incapable of normally fertilizing an egg, injection of the nuclei into eggs, and implantation of the resulting fertilized eggs into the mother.

In recent years, nuclei taken from cells of adult animals have been used to produce new animals. In this procedure, the nucleus is removed from a body cell (e.g., skin or blood cell) of a donor animal and introduced into an unfertilized mammalian egg that has been deprived of its own nucleus. This manipulated egg, which is equivalent to a fertilized egg, is then implanted into a foster mother. The ability of such a donor nucleus to direct the development of an entire animal suggests that all the information required for life is retained in the nuclei of some adult cells. Since all the cells in an animal produced in this way have the genes of the single original donor cell, the new animal is a clone of the donor (Figure 1-8). Repeating the process can give rise to many clones. So far, however, the majority of embryos produced by this technique of nuclear-transfer cloning do not survive due to birth defects. Even those animals that are born live have shown abnormalities, including accelerated aging. The "rooting" of plants, in contrast, is a type of cloning that is readily accomplished by gardeners, farmers, and laboratory technicians.

The technical difficulties and possible hazards of nuclear-transfer cloning have not deterred some individuals from pursuing the goal of human cloning. However, cloning of humans per se has very limited scientific interest and is opposed by most scientists because of its high risk. Of greater scientific and medical interest is the ability to generate specific cell types starting from embryonic or adult stem cells. The scientific interest comes from learning the signals that can unleash the potential of the genes to form a certain cell type. The medical interest comes from the possibility of treating the nu-

▲ FIGURE 1-8 Five genetically identical cloned sheep. An early sheep embryo was divided into five groups of cells and each was separately implanted into a surrogate mother, much like the natural process of twinning. At an early stage the cells are able to adjust and form an entire animal; later in development the cells become progressively restricted and can no longer do so. An alternative way to clone animals is to replace the nuclei of multiple single-celled embryos with donor nuclei from cells of an adult sheep. Each embryo will be genetically identical to the adult from which the nucleus was obtained. Low percentages of embryos survive these procedures to give healthy animals, and the full impact of the techniques on the animals is not yet known. [Geoff Tompkinson/Science Photo Library/Photo Researchers, Inc.]

merous diseases in which particular cell types are damaged or missing, and of repairing wounds more completely.

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