For the first time, an international research team led by Chinese scientists has created embryos that are part human and part monkey, sparking hopes of someday growing human organs for transplants, but also ethical debate.

In a paper published Thursday in the journal Cell, stem cell biologist Tan Tao of Kunming University of Science and Technology in the southwestern Yunnan province and his colleagues described in detail their creation of human-monkey chimeras — monkey embryos with human cells mixed in.

The team injected a type of human stem cell that has the potential to develop into any tissue depending on the biological instructions it receives, into monkey embryos. The idea behind creating human-animal chimeras is so that scientists can one day grow a human kidney inside a pig, for example, which could help meet high demand for organ transplants. According to China’s state-backed Xinhua News Agency, some 300,000 people in the country require organ transplants each year, though only one in 30 receives one.

“Our study is still in a very early stage,” Tan told Sixth Tone. “For example, we don’t have a way to figure out how to direct the stem cells to turn into a specific tissue on demand.”

Thursday’s paper, while still far from achieving the grand vision of animal-grown organs, advanced chimera research by successfully producing embryos in petri dishes for up to 20 days, all while keeping an unprecedented number of human cells alive. The team said that up to 7% of one monkey embryo’s cells had come from humans. By contrast, in previous research involving human-pig chimeras, only one out of every 100,000 or so cells was human.

Tan told Sixth Tone that this improvement was largely because of the team’s use of monkey embryos, which, compared with pig embryos, provide a growth environment that is much closer to that of humans.

The team terminated cell growth after 20 days. While part of the reason was because the technology wasn’t mature enough to grow monkey embryos in a petri dish much longer, “we were also worried about the ethical problems,” Tan said, explaining that the embryos would start developing nervous systems starting around day 23 — a step that is not ethically acceptable or even widely discussed.

In addition to the study’s potential for organ transplant, Wang Qiong, a stem cell biologist at Shanghai Jiao Tong University who is not affiliated with the research, told Sixth Tone that it could also shed light on the early development of human cells by growing them outside a womb.

Mixing human cells with those of animals has always been controversial. Tan said that even though his team’s study was vetted and approved by the lab’s ethics committee, research on human-monkey chimeras is ethically uncharted territory. One major concern is that the human cells could travel to the host animal’s brain or reproductive system during growth. This is theoretically possible, Tan said, because scientists have yet to figure out how to control the direction of stem cell development.

“That’s why I think we shouldn’t start doing such experiments before ethicists and lawmakers set guidelines,” he said. “Science and technology are likely to move faster than ethics, so we need to start talking about what’s acceptable … right now.”

Wang agreed. “Science experiments shouldn’t be conducted simply according to what’s possible,” she said. “It’s a challenge but also an opportunity.”

Editor: David Paulk.

(Header image: Human cells (red) in a lab-grown monkey embryo. Courtesy of Kunming University of Science and Technology)