Menu Close

Will the NIH Fund Research on Human-Animal Chimeras?

By Michael Smith*

What was, until recently, a dream of science fiction moved firmly into the realm of scientific fact in January of this year when scientists at the Salk Institute announced—for the first time—the successful production of a viable pig embryo containing human cells.[1]  Such a part-pig-part-human organism is called a chimera.[2]

Chimeras are created when a pig embryo is injected with pluripotent human stem cells.[3]  Pluripotent stem cells are a special type of cell that can “differentiate” (that is, “transform”) into any form of human tissue.[4]  Successfully introducing such cells into an animal embryo is the greatest source of medical and scientific promise the technology provides—but also generates ethical concerns.  While successful introduction of enough pluripotent human stem cells theoretically allows any human organ to be grown inside an animal and subsequently transplanted into a human recipient,[5] it is also theoretically possible that some human stem cells will migrate into the growing pig embryo’s brain, with unknown effects on the resulting organism’s intelligence or even consciousness.[6]

The National Institutes of Health (NIH) currently prohibits funding of research on human-animal chimeras due to ethical concerns stemming from this theoretical capacity of human pluripotent stem cells.[7]  The NIH’s ethical concerns are not unique. Prior to the NIH’s funding ban, Hiromitsu Nakauchi, one of the pioneers in the race to create viable, part-human chimeras, relocated from Japan to Stanford University to pursue research on the subject free from research restrictions imposed by the Japanese government.[8]

The NIH’s recent funding moratorium is not, however, the final word concerning human-animal chimera research in the United States. Nakauchi’s research was funded, in part, by a grant of six million dollars from the California Institute of Regenerative Medicine—a California state agency founded, in part, to circumvent federal research restrictions concerning stem cells.[9]  The Salk Institute scientists elied on private funding.[10]  Daniel Garry, a cardiologists conducting similar chimeric studies at the University of Minnesota, received a $1.4 million grant from the U.S. Army to research the possibility of growing human hearts in animal hosts.[11]  In short, the tremendous promise of the technology all but ensures funding sources will be available for human-animal chimera research: no matter what the NIH ultimately decides.

The NIH’s concern about human-animal chimeras first became apparent when, in September of 2015, it reversed prior policy and instituted a funding moratorium to allow for review of the scientific, social, and ethical implications of the research.[12]  In November of 2015, the NIH held a workshop to gather additional information about the state of the science.[13]  This workshop, in combination with vocal scientific opposition to the funding moratorium,[14] moved the NIH to propose research guideline changes that would finally permit NIH funding for research into human-animal chimeras, albeit, with additional oversight.[15]  Comment closed on these proposed guideline changes in September of 2016.[16]  While the NIH indicated it planned to lift the funding moratorium in January of 2017,[17] that has not yet happened.[18]  Additionally, it is unclear what—if any—impact the Trump Administration will have on the NIH’s proposed rule change.[19]

Barring a wider policy shift regarding stem-cell research—perhaps driven by the new Administration—it is likely the NIH’s ethical concerns about human-animal chimera research can be mitigated.  The Salk Institute scientists responsible for the most recent breakthrough consciously limited the chimeric embryo’s development to twenty-eight days—well below the 114-day pig gestation period —out of an abundance of ethical caution.[20]  Twenty-eight days marks the earliest point in development that the scientists could check to see if human cells are contributing to the half-inch embryonic pig’s organ development, and allowing development to progress further would only add unnecessary ethical risk.[21]  Additionally, current scientific data fails to show viable formation of human brain and nervous system cells in pig embryos.[22]  Existing NIH restrictions also prohibit the breeding of animals in which human pluripotent stem cells have been introduced—eliminating any ethical concerns that could arise if reproduction between human-animal chimeras was possible.[23]

Because the inherent ethical concerns can be effectively mitigated, and because of the vast potential for scientific and medical advancement human-animal chimera research holds, there is good reason to hope that the NIH will soon lift its funding moratorium.  This would open a promising new field of scientific study and perhaps, in the not-too-distant future, provide a key to addressing the chronic organ shortages transplant patients face in the United States and around the world.

The views and opinions expressed in this blog are those of the authors only and do not reflect the official policy or position of the Michigan Journal of Environmental and Administrative Law or the University of Michigan.

*Michael Smith is a Junior Editor on MJEAL. He can be reached at

[1] Jun Wu et al., Interspecies Chimerism with Mammalian Pluripotent Stem Cells, 168 Cell 473–486 (2017); see also, Hannah Devlin, First Human-Pig ‘Chimera’ Created in Milestone Study, The Guardian (Jan. 26, 2017, 12:00 PM),

[2] Antonio Regalado, Human-Animal Chimeras are Gestating on U.S. Research Farms, MIT Tech. Rev. (Jan. 6, 2016),

[3] Jocelyn Kaiser, NIH Moves to Lift Moratorium on Animal-Human Chimera Research, Science (Aug. 4, 2016, 5:00 PM),

[4] Id.

[5] Id.

[6] See Regalado, supra note 2.

[7] National Institutes of Health Guidelines for Human Stem Cell Research, 74 Fed. Reg. 32,170, 32,175 (July 7, 2009).

[8] Regalado, supra note 2.

[9] Id.

[10] Erin Blakemore, Human-Pig Hybrid Created in the Lab—Here Are the Facts, Nat’l Geographic (Jan. 26, 2017),

[11] Regalado, supra note 2.

[12] Regalado supra note 2; National Institutes of Health Guidelines for Human Stem Cell Research, 74 Fed. Reg. at 32,175.

[13] Carrie Wolinetz, Next Steps on Research Using Animal Embryos Containing Human Cells, Nat’l Insts. of Health, Office of Sci. Pol’y: Under the Poliscope (Aug. 4, 2016),; Workshop on Research with Animals Containing Human Cells, Nat’l Insts. of Health, Office of Sci. Pol’y, (last visited Mar. 12, 2017).

[14] See e.g., Arun Sharma et al., Lift NIH Restrictions on Chimera Research, 350 Science 640, 640 (2015).

[15] Request for Public Comment on the Proposed Changes to the NIH Guidelines for Human Stem Cell Research and the Proposed Scope of an NIH Steering Committee’s Consideration of Certain Human-Animal Chimera Research, 81 Fed. Reg. 51,921 (proposed July 28, 2016).

[16] Id.

[17] Kaiser, supra note 3.

[18] Gretchen Vogel, Human Organs Grown in Pigs? Not So Fast, Science (Jan. 26, 2017, 1:00 PM),

[19] Id.

[20] Regalado, supra note 2.

[21] See id.

[22] See Sharma, supra note 14.

[23] Id.

Leave a Reply

Your email address will not be published. Required fields are marked *

%d bloggers like this: