Scientist Hiromitsu Nakauchi is the first researcher in Japan to receive permission from the government to create animal embryos that contain human cells human-animal chimeras and transplant them into surrogate animals for gestation, as explained in an article in the journal Nature .

The practice was prohibited in Japan until March this year, when the ban limiting the growth of chimeric animal embryos beyond 14 days of development or the transplantation of such embryos into a surrogate animal was lifted. That month, the Japanese Ministry for Education, Culture, Sports and Science (MEXT) issued new guidelines that would allow human-animal embryos to be obtained, implanted in surrogate animals and brought to term.

Could be possible obtain mature organs formed entirely of human cells?

Human-animal hybrid embryos have already been created in other countries such as the United States, but their birth has never been allowed, interrupting the animal’s pregnancy. Nakauchi’s experiments are the first to be approved under the new Japanese rules by a committee of experts in the Science Ministry, with final approval expected next month.

Nevertheless, Nakauchi says that he will not attempt to bring any hybrid embryo to term for a while. He initially plans to culture hybrid mouse embryos until 14.5 days, when the animal’s organs have mostly formed and it has almost reached term. He will then perform the same experiments in rats, growing the hybrids until 15.5 days. Later, Nakauchi plans to seek approval from the government to culture hybrid embryos in pigs for up to 70 days.

Human–animal hybrid for organ transplant is very promising, also to acquire models of human diseases and to study embryonic development. Nevertheless, there are still several technical obstacles to be overcome, as the proportion of human cells obtained the animal is very low.

Additionally, this research also poses serious ethical concerns, such as the possibility that the human cells may stray beyond development of the target organ and reach the developing animal’s brain, possibly affecting its cognition, or its gonads, potentially giving rise to human reproductive cells in the animal. The latter raises the disturbing possibility that two animals could conceive a human being. Researchers are working to design methods to prevent this from happening, such as genetic modification of the animal so that it does not produce the target organ, but rather a niche that directs the human cells to form that organ. This technical possibility is very positive from an ethical point of view. Another difficulty is that these experiments, which require human stem cells, sometimes use human embryonic stem cells. However, obtaining these necessitates the use of in vitro human embryos and their destruction. This use — ethically unacceptable — can be avoided, however, by using another type of human stem cells (see HERE) that do not pose any ethical drawbacks for application.

Work with non-embryonic stem cells, in this field, should be encouraged

In conclusion, experiments with human-animal chimeras could bring various important benefits to society, but the ethical questions  raised mean that progress must be cautious, ensuring the efficacy of the methods excluding the use of embryos (read our comment on American restriction of human embryos experiments) and controlling the differentiation fate of the human cells in the animal. Likewise, work with non-embryonic stem cells should be encouraged (see more about the ethical assessment of human-animal chimeras HERE.

Our special report of tne possiblity to obtain a mature organ formed entirely of human cells