interspecies boundaries. There is general agreement in the scientific community that these boundaries are to some extent arbitrary. As discussed in Chapter 2, some chimeras are viewed with equanimity (for example, pig heart valve transplants into humans), and one must be careful to distinguish legitimate concerns from discomfort arising from unfamiliarity. Although moral intuitions about the creation of chimeras may vary, it is a subject of deep moral concern to many thoughtful people for whom the creation of animals with certain kinds or quantities of human tissues, such as neural or germline cells, would be offensive. Accordingly, such research requires careful consideration and review.

Among the issues to be considered in the review of such proposals will be the number of hES cells to be transferred, what areas of the animal body would be involved, and whether the cells might migrate through the animal’s body. The hES cells may affect some animal organs rather than others, raising questions about the number of organs affected, how the animal’s functioning would be affected, and whether some valued human characteristics might be exhibited in the animal, including physical appearance.

Perhaps no organ that could be exposed to hES cells raises more sensitive questions than the animal brain, whose biochemistry or architecture might be affected by the presence of human cells. Human diseases, such as Parkinson’s disease, might be amenable to stem cell therapy, and it is conceivable, although unlikely, that an animal’s cognitive abilities could also be affected by such therapy. Similarly, care must be taken lest hES cells alter the animal’s germline. Protocols should be reviewed to ensure that they take into account those sorts of possibilities and that they include ethically sensitive plans to manage them if they arise.

Various precautions seem reasonable in studies that involve the transfer of hES cells into nonhuman animals and should be considered in any prior review of a protocol. Questions that should be raised in this context include the following

  • Are hES cells required, or can cells from other primates or animals be used?

  • Has sufficient animal work preceded the proposed work involving hES cells?

  • Might the cell transfer result in the animal’s acquiring characteristics that are valued as distinctly human?

  • If hES cells are to be transferred into an animal embryo or fetus, have studies (for example, with ES cells from other species or interspecies chimeras) suggested that the resulting creature would exhibit human characteristics that would be ethically unacceptable to find in an animal?

  • If visible human-like characteristics might arise, have all those involved in these experiments, including animal care staff, been informed and educated about this?

Furthermore, donors of gametes and embryos should be informed that some of the hES cells derived from their donated cells and tissues might be transferred into nonhuman animals in the course of developing and testing their therapeutic potential (see Chapter 5).

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