In a breakthrough that redefines how life can be created, embryologists working at the University of Cambridge in the UK have grown realistic-looking mouse embryos using only stem cells. No egg. No sperm. Just cells plucked from another embryo.

The researchers placed the cells carefully in a three-dimensional scaffold and watched, fascinated, as they started communicating and lining up into the distinctive bullet shape of a mouse embryo several days old.

“We know that stem cells are magical in their powerful potential of what they can do. We did not realize they could self-organize so beautifully or perfectly,” Magdelena Zernicka­-Goetz, who headed the team, told an interviewer at the time.

Zernicka-Goetz says her “synthetic” embryos probably couldn’t have grown into mice. Nonetheless, they’re a hint that soon we could have mammals born without an egg at all.

That isn’t Zernicka-Goetz’s goal. She wants to study how the cells of an early embryo begin taking on their specialized roles. The next step, she says, is to make an artificial embryo out of human stem cells, work that’s being pursued at the University of Michigan and Rockefeller University.

Synthetic human embryos would be a boon to scientists, letting them tease apart events early in development. And since such embryos start with easily manipulated stem cells, labs will be able to employ a full range of tools, such as gene editing, to investigate them as they grow.

Artificial embryos, however, pose ethical questions. What if they turn out to be indistinguishable from real embryos? How long can they be grown in the lab before they feel pain? We need to address those questions before the science races ahead much further, bioethicists say.