Scientists have kept human embryos alive in a laboratory for longer than ever before, but the major breakthrough has reignited an old ethical debate.
Research teams from the University of Cambridge and The Rockefeller University in New York cultured the embryos for 13 days, almost double the previous record and past the point at which a foetus would naturally implant in the womb.
"It opens a new window on the earliest stages of human development while pushing the limits of embryonic research," says the Wall Street Journal.
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The time after implantation is when "some of the most important [biological] decisions are made", said Cambridge professor Magdalena Zernicka-Goetz, a senior author of one of the studies, which used a new technique originally tested on mice in 2014.
"It was entirely a black box of development that we were not able to access until now," she said.
It is hoped that the discovery could help improve IVF success rates, shed light on birth defects and give a greater insight into why miscarriages occur.
But, as The Guardian points out, it also puts scientists into "direct conflict" with laws prohibiting donated embryos from being grown in a laboratory for more than 14 days. "Until now, the barrier has been science, not law," the newspaper says.
Some in the scientific community are now calling for those rules to be reconsidered.
The 14-day time limit is pegged to the time at which human embryos can no longer fuse or split, says Science Magazine. "That, in the eyes of some religious bioethicists, marks the threshold at which an embryo is a distinct human."
But critics argue going beyond that point would open a new window into human biology. It could help researchers address "deeply compelling questions", such as how the nervous system is formed, said Harvard University stem cell researcher George Daley.
However, there continues to be a lack of consensus among experts. Zernicka-Goetz agrees that the scientific benefits of extending the period would be significant, but says: "It's not for us now to decide whether we should do it or not."
She added: "Rules are very useful, we would always adhere to them."