TOKYO — Japan has become the first country to greenlight a controversial experiment involving hybrid human-animal embryos.

According to Nature magazine, experts at Japan's Science Ministry have approved a University of Tokyo proposal to grow human stem cells in mouse, rat, and pig embryos, and transplant them into surrogate animals.

Lead researcher Hiromitsu Nakauchi says the goal is to create animals with human organs than can then be used for transplants.

The proposal is the first one approved since Japan lifted a ban in March on developing animal embryos with human cells beyond 14 days, and bringing them to term.

Nakauchi told Nature that they will be creating animal embryos that lack a gene to produce certain organs. Once injected with human induced pluripotent stem cells or IPS cells, the embryo will use the IPS cells to make the organ.

He says he plans to grow mouse embryos for 14.5 days until they are almost to term. Rats will be grown to near term for 15.5 days, and pigs for up to 70 days.

Nakauchi's 2017 experiment saw him and his team successfully cure a diabetic mouse by growing a healthy new mouse pancreas using a rat embryo.

Science Alert reports that while the treatment is meant to target only the organ being grown, the experiment will be suspended if the team detects more than 30% of the animal's brain to be human.

This is apparently one of the Japanese government's conditions, in order to prevent humanized animals from ever coming into existence.