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Japan Approves Controversial Experiment to Grow Human Organs Inside Animals

Bioethicists are worried about the implications of the research, including the possibility of creating a human-animal hybrid.
Gavin Butler
Melbourne, AU
13-day old mouse embryo
A 13-day old mouse embryo. Image via Wikimedia

A Japanese scientist has been given the green light to embark on a controversial experiment. Hiromitsu Nakauchi, a stem-cell scientist who leads teams at the University of Tokyo and Stanford University in California, wants to produce a number of human–animal hybrids—that is, animals with organs made of human cells—in the hope that those organs could eventually be transplanted into people. And in a landmark move that’s already ruffling the feathers of bioethicists, the Japanese government has thrown its support behind the research.


The practice of growing animal embryos containing human cells for any longer than 14 days, as well as the transplant of those embryos into a surrogate uterus, was banned in Japan until earlier this year, according to the Nature international journal of science. But in March, the nation’s education and science ministry issued new guidelines that allowed the creation of human–animal embryos which could be transplanted into surrogates and brought to term. Hiromitsu’s experiments are the first to be approved under this new set of rules.

For some, this research signals a misguided step onto a slippery ethical slope. Experts and academics have previously condemned any research that involves injecting human stem cells into animal embryos and allowing for “the creation of part-human and part-animal organisms known as chimeras”. Alan Moy, CEO of biopharmaceutical company Cellular Engineering Technologies, published a paper in 2017 in which he suggested that lifting the moratorium on this kind of research would mean “spending taxpayer dollars on the creation and manipulation of new organisms that would blur the line between humans and animals”.

The National Institutes of Health in the United States, where Alan is from, has had a moratorium on funding such work since 2015. Things are different in Japan these days, though—and in the case of Hiromitsu’s research, bioethicists are worried about the possibility that human cells might stray beyond the targeted organ and travel to and develop other parts of the animal’s body, such as its brain, which could potentially affect the specimen’s cognition.

Hiromitsu insists he is proceeding with caution, though, claiming that he won’t attempt to bring any hybrid embryos to term for a number of years. He plans to start by growing hybrid mouse embryos until 14.5 days and hybrid rat embryos until 15.5 days—when the animals’ organs are mostly formed and they are almost to term—and in the future he intends to apply for government approval to grow hybrid embryos in pigs for up to 70 days.

“It is good to proceed stepwise with caution,” said Hokkaido University’s science-policy researcher Tetsuya Ishii. “[This] will make it possible to have a dialogue with the public, which is feeling anxious and has concerns.”

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