The National Institutes of Health recently announced that it expects to lift a ban on research involving animal embryos that have been injected with human stem cells.  Embryonic stem cells are already lightning rods for controversy, but part-human “chimeras” are saddled with their own set of ethical dilemmas.  Some worry, for example, that chimeras could develop human-like brains that might afford the animal a rudimentary level of human-like consciousness.

If this situation makes you anxious, it may be comforting to learn that a similar technique employing full-grown mice (and other animals) is common in modern biomedical research.  Any attempt to introduce human cells into mice must circumvent the mouse’s immune system, which will reject most non-mouse cells it finds.  Scientists can work with mice that have had their entire immune systems destroyed, but a more elegant solution goes one step further and uses human blood stem cells to repopulate the mouse with a human immune system.  These “humanized” mice are commonly used to test the efficacy and safety of new drugs before clinical trials take place.

Because they involve human and non-human cells developing side-by-side, chimeras are uniquely poised to address several important questions about human biology.  One particularly exciting avenue for future research involves the “farming” of human organs for transplantation from sheep, pigs, and other livestock.  Chimeras could also eventually supplant humanized animals as the more relevant (and, therefore, valuable) models of human disease.  The first batch of experiments with chimeras will take place under strict regulatory scrutiny, but prudence and excitement need not be mutually exclusive.

Acknowledgments: Many thanks to Jamie Lahvic, a graduate student in the Biological and Biomedical Sciences Program at Harvard University, for providing her expertise and commentary on the topic.

Managing Correspondent: Christopher Gerry

Media Coverage: NIH Plans to Lift Ban On Research Funds For Part-Human, Part-Animal Embryos – NPR; NIH moves to lift moratorium on animal-human chimera research – Science