J-STORIES | 日本発の革新的な挑戦を世界に伝えるメディア

J-STORIES - Scientists at a Japanese university have successfully used stem cells to generate the precursors of eggs of one of the globe's most threatened species.

Led by Katsuhiko Hayashi, a professor at Osaka University's faculty of medicine who is well known for his stem cell research, the team succeeded in generating germ cells, the precursors of eggs, from induced pluripotent stem (iPS) cells of the northern white rhino, of which there are only two in the world.

The northern white rhino is extinct in the wild, and despite a decades-long effort to preserve the species through a breeding program only two remain in captivity. However, Najin and Fatu, who live at the Ol Pejeta conservation park in Kenya, are both female, the last bull of their species having died in 2018.

Determined to ensure that they won't be the last of their kind, Hayashi and his team developed the stem cell technique in the hope that if these immature reproductive germ cells successfully develop into eggs, they could be used in in vitro fertilization with frozen sperm from deceased bulls to produce new rhino calves.

Although mouse and human germ cells have been produced before, this is the first time the technique has been successfully used in wild animals. Not only is it more difficult to generate wild animal germ cells and eggs, but collecting the stem cells themselves is complicated, and the practical challenges are different for each species.

But Hayashi believes that if such obstacles can be overcome, it might be possible to breed and save not just the endangered rhino, but also other animals facing extinction.

Najin and Fatu are the last two remaining northern white rhinos.     Source: Osaka University — Najin and Fatu are the last two remaining northern white rhinos. Source: Osaka University

Research has shown that cells from the skin, or blood can be reprogrammed back into an embryonic-like state and Hayashi's method is based on skin cells saved from deceased rhinos, potentially opening a window for a greater source of eggs than was previously available.

This has attracted global attention, leading to Hayashi's participation in BioRescue, an international research project involving scientists from Germany, Italy, the Czech Republic, and Japan. Led by Thomas Bernd Hildebrandt, a professor at the Leibniz Institute for Zoo and Wildlife Research in Germany, the group aims to preserve and increase populations of critically endangered species, in particular the northern white rhino.

The group has already recovered immature egg cells (oocytes) from the two last northern white rhinos and fertilized them with sperm to produce embryos. They plan to implant these in a surrogate mother from a closely related species, the southern white rhinoceros, to give birth to northern white rhinoceros calves.

Nonetheless, experts say there are still hurdles to address, including whether or not those surrogates can successfully carry the embryos to produce northern white rhino offspring. Another challenge is ensuring the genetic diversity that is necessary to sustain a population, without which the undesirable consequences of inbreeding can result.

There have also been calls to reflect and act on why this species, and indeed all six species of rhinoceros living in Asia and Africa, have been placed at risk of extinction. A main reason is that in countries such as China and Vietnam, rhino horns are highly sought after as a traditional remedy for a variety of ailments, and consequently command extremely high prices. Despite the best efforts of rangers, rhinos are still being hunted by poachers.

“We want to use our technology to save endangered species," says Hayashi, who in 2016 led a group that transformed mouse skin cells into eggs in a dish — the first-ever creation of eggs entirely outside a mouse. "In particular, I feel it is our human duty to save the northern white rhino, which is being driven to extinction at the hands of humans.”