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Document "Man or Mouse? Ethical aspects of chimaera research ": English Abstract
Chimeras are created in several fields of research, most commonly in connection with stem cell research. A chimera is an organism in which one animal’s own cells and that of another are present side by side in the same body. A related organism is a hybrid, in which genetic material from animals of different species is present in each individual cell. These can be formed by fusing germ cells from animals of different species.

Some types of research on chimeras imply the mixing of cells from humans and nonhuman animals. This could give rise to ethical concerns for several reasons. One important reason is that the boundary between humans and animals is fundamental in our everyday practices, in society and in current legislation. Therefore, the Danish Council of Ethics and the Danish Ethical Council for Animals formed a joint working party in 2006, which is now finishing its report on ethical aspects of human-animal chimeras.


The members [of the Danish Council of Ethics] give different weight to these arguments but find that none of them rules out every creation of human–nonhuman chimeras. There is, however, a need to modify current regulation to ensure that chimeras difficult to place biologically, ethically and legally will not be created. Thus the Council members agree that research should not be allowed if it e.g.:

- crucially affects an animal’s cognitive functions in a human direction (e.g. transfer of human embryonic or human neural stem cells to the brain of early fetuses or born experimental animals (particularly primates) or transplantation of parts of brains between animals and humans)

- could impact on a human brain in some way that reduces the cognitive capacities (transfer of neural stem cells from animals, or parts of animal brains to born humans for therapeutic purposes)

- could lead to the formation of human germlines in animals (e.g. by the transfer of human embryonic stem cells to early animal embryos or transplantation of human germline-producing tissue to animal fetuses or born experimental animals)

- could give rise to extensive mixtures between animals and people (hybrids or embryo fusion)

- brings into the world experimental chimeras or hybrids that have been so crucially altered that justified doubt can arise as to whether the hybrid creature can still be classified as an animal and can thus be put down if the experiment has an adverse outcome (e.g. when transferring human embryonic stem cells to early animal embryos)

- entails born hybrid creatures being given an opportunity to multiply other than in closed systems, corresponding to what applies to experiments on genetically modified organisms, and thus pass on any changes in the gene pool to their descendants

- involves a chimeric experimental animal with the ability to form human germlines being permitted to multiply

- involves the implanting of a human embryo into an animal womb or of an animal embryo into a woman’s womb.

Whether a specific research project will conflict with these recommendations should be assessed by a research ethics committee in each separate case. This implies an adjustment to the law, since the current Danish legal framework does not take sufficient account of research which combines elements from human and nonhuman animals.

The members therefore recommend that existing regulation be revised on the basis of the ethical principles laid out in the report. This would imply that the accreditation scheme is adjusted and committees are given a mandate to decline research projects that will potentially lead to the creation of human–nonhuman chimeras (including fetuses), which from an ethical viewpoint are altered to an unacceptable degree, as in the above-mentioned examples. The members also suggest that politicians consider whether it would be desirable to combine the assessment of research involving elements from animals as well as humans in one review body.

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