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Scientists have successfully grown monkey embryos containing human cells for the first time — the latest milestone in a rapidly advancing field that has drawn ethical questions.

In the work, published on 15 April in Cell1, the team injected monkey embryos with human stem cells and watched them develop. They observed human and monkey cells divide and grow together in a dish, with at least 3 embryos surviving to 19 days after fertilization. “The overall message is that every embryo contained human cells that proliferate and differentiate to a different extent,” says Juan Carlos Izpisua Belmonte, a developmental biologist at the Salk Institute for Biological Studies in La Jolla, California, and one of the researchers who led the work.

Researchers hope that some human–animal hybrids — known as chimaeras — could provide better models in which to test drugs, and be used to grow human organs for transplants. Members of this research team were the first to show in 20192 that they could" data-track-category="body text link">grow monkey embryos in a dish for up to 20 days after fertilization. In 2017, they reported a series of" data-track-category="body text link">other hybrids: pig embryos grown with human cells, cow embryos grown with human cells, and rat embryos grown with mouse cells3.

But the latest work has divided developmental biologists. Some question the need for such experiments using closely related primates — these animals are not likely to be used as model animals in the way that mice and rodents are. Nonhuman primates are protected by stricter research ethics rules than are rodents, and they worry such work is likely to stoke public opposition.

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