The development of human pluripotent stem cells has opened up the possibility to analyse the function of human cells and tissues in animal hosts, thus generating chimeras. Although such lines of research have great potential for both basic and translational science, they also raise unique ethical issues that must be considered.
Several broad categories of experimental investigation are now making use of human-animal chimeras. Recent work from Jacob Hanna's lab has used the mouse embryo as an in vivo system to test the potential of human pluripotent cells: creating chimeras by microinjection of hESCs or iPSCs into a mouse morula and analysing the chimeric embryo shortly afterwards (Gafni et al., 2013). Given that these experiments were limited to early embryos (10 days; within the limit allowed for research on human embryos), the ethical concerns here are limited, but it is possible that central nervous system (CNS) tissue containing both mouse and human cells will be found in this chimera.
One key ethical question is whether crossing species boundaries is in principle or prima facie ethically wrong. If so, we need to consider whether the generation of stem cell chimeras represents a particular and controversial instance of such boundary crossing. The reason why such issues are raised is that humans and animals are treated differently in our culture and have different ethical and legal status: animals are not legal entities and do not have rights in the way that humans do.
In 1974 the National Research Act (NRA) was passed (National Research Act 1974). This act created the National Commission for the Protection of Human Subjects in Biomedical and Behavioral Research in response to a history of unethical medical research that was conducted in the United States. The commission created a set of required rules that govern human-subject research (NCPHS 1979). First, the commission created the rules that there must be a respect for subjects as “autonomous agents, and second, that persons with diminished autonomy are entitled to protection” (5-12). Research subjects must be offered and provided informed consent.