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Scientists put human cells in a sheep embryo. Here's what that means.

Illustration: Rebecca Zisser / Axios

Researchers have successfully incorporated human cells into a sheep embryo — the first step in a process that they hope may one day allow them to grow replacement organs in other animals. The advance is one of several recent ones in a burgeoning field that is raising ethical questions.

The bottom line: Advances in the past year are important proofs-of-principle that this is even a plausible approach but an organ hasn't been grown in a large animal.

"We’re a long way away from getting a human tissue in a pig or sheep," says Thorold Theunissen, a stem cell biologist at Washington University School of Medicine. "It's a tantalizing prospect and important that more experiments are done.”