Ethical aspects of research with chimera embryos

Vardit Ravitsky highlights a few ethical issues with human-nonhuman chimera research.


Assessing the potential benefits and risks of innovative health research is often a complex task. The higher the stakes, the more daunting such analysis becomes. This is the case with research on human-nonhuman chimera embryos, where human pluripotent stem cells (that can potentially develop into any kind of tissue or organ) are introduced into nonhuman embryos at a very early stage. These cells can replicate and then specialize, potentially being expressed in any part of the nonhuman animal as it develops, creating an organism that is part human and part nonhuman.

The potential benefits of human-nonhuman chimera research are immense. Such embryos, and the part-human animals that may develop from them, can be used to study human development, shed light on infertility, inform disease models, and test new drugs. If successful, this research might even lead to growing human organs inside nonhuman animals’ bodies that would be compatible with individual patients in need of an organ for transplantation. In light of ongoing organ shortages, chimera research could be a game changer for thousands of patients awaiting life-saving organs.


At the same time, the risks are significant. The effects of pluripotent human stem cells on organs and tissues in the chimeric animal are uncertain. Cells can go ‘off target’ and be expressed in an unintended part of the nonhuman animal’s body. There are, for example, concerns about the possibility that chimeric animals may develop human sperm and eggs, so that if two such animals breed with each other they might produce viable human embryos carried inside nonhuman animals. This research also raises animal welfare issues including concerns about the creation of nonhuman animals with human traits. For example, human cells might end up contributing to the nonhuman animal’s brain or central nervous system, altering its cognitive capacities and even accidentally endowing it with some aspect of human consciousness. In the words of ethicist David Resnik: “The specter of an intelligent mouse stuck in a laboratory somewhere screaming ‘I want to get out’ would be very troubling to people.” This leads to the controversial issue of defining the boundary between human and nonhuman, and the associated moral questions of how such creatures should be treated.

There is also the ‘chilling’ effect of crossing the line between humans and nonhuman animals. The term chimera, borrowed from Greek mythology where it denoted a part lion – part goat – part snake animal, evokes frightening images of monsters and Frankenstein-like creatures. For many, there is something unique, perhaps sacred, about the human creature that commands respect. Crossing this line thus entails not only moral issues, but also profound symbolic, existential, emotive and even spiritual issues related to what it is that makes us human.

In light of this complexity, in September 2015 the United States National Institutes of Health issued a moratorium on public funding in order to allow full consideration of the ethical concerns generated by crossing humans and nonhuman animals. Then, in early August 2016, following months of consultations with experts, the NIH announced that it was considering lifting the current moratorium to allow public funding of research on part-human chimera embryos, with some limitations.

For example, embryos of nonhuman primates, such as chimpanzees, will only be used later in their development, because they are considered close to humans and thus deserving special protections. Moreover, the ban will remain regarding research that could result in nonhuman animals with human sperm or eggs that might breed. The NIH also proposes to create a special committee composed of scientists, ethicists, and animal welfare experts, to review proposed studies and ensure heightened ethical scrutiny.

Funding from the NIH may not only give an important push to this research area, it may also create a more permissive environment, sending a message to researchers and to the public that the expected health benefits may outweigh the ethical concerns. This may lead to an increased acceptability of human-nonhuman chimera research in other countries as well.

Prior to making a final decision, the NIH has launched a period of public consultation. It is inviting comments from anyone willing to contribute to this important debate. This is a unique opportunity for the bioethics community to inform policy and shape the future of health research. Our expertise in weighing potential benefits and risks, and in developing well-argued nuanced positions on complex matter, is invaluable. This is an opportunity not to be missed. You can submit your comments before September 6th here.


Vardit Ravitsky is an Associate Professor, Bioethics Program in the School of Public Health, at University of Montreal and the Director of the Ethics and Health Branch at the Center for Research in Ethics. @VarditRavitsky

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