The research could eventually lead to new sources of organs for transplant, but ethical and technical hurdles need to be overcome.
A Japanese stem-cell scientist is the first to receive government support to create animal embryos that contain human cells and transplant them into surrogate animals since a ban on the practice was overturned earlier this year.
Hiromitsu Nakauchi, who leads teams at the University of Tokyo and Stanford University in California, plans to grow human cells in mouse and rat embryos and then transplant those embryos into surrogate animals. Nakauchi's ultimate goal is to produce animals with organs made of human cells that can, eventually, be transplanted into people.
Until March, Japan explicitly forbid the growth of animal embryos containing human cells beyond 14 days or the transplant of such embryos into a surrogate uterus. That month Japan’s education and science ministry issued new guidelines allowing the creation of human-animal embryos that can be transplanted into surrogate animals and brought to term.
Human–animal hybrid embryos have been made in countries such as the United States, but never brought to term. Although the country allows this kind of research, the National Institutes of Health has had a moratorium on funding such work since 2015.
Nakauchi’s experiments are the first to be approved under Japan’s new rules, by a committee of experts in the science ministry. Final approval from the ministry is expected next month.
Nakauchi says he plans to proceed slowly, and will not attempt to bring any hybrid embryos to term for some time. Initially, he plans to grow hybrid mouse embryos until 14.5 days, when the animal’s organs are mostly formed and it is almost to term. He will do the same experiments in rats, growing the hybrids to near term, about 15.5 days. Later, Nakauchi plans to apply for government approval to grow hybrid embryos in pigs for up to 70 days.
“It is good to proceed stepwise with caution, which will make it possible to have a dialogue with the public, which is feeling anxious and has concerns,” says science-policy researcher Tetsuya Ishii of Hokkaido University in Sapporo, Japan.