Experts hope the breakthrough will lead to a better understanding of early miscarriages - but ethical questions have also been raised
Scientists have succeeded in growing human embryos in a laboratory for nearly two weeks, in a breakthrough likely to bring advances in stem-cell therapies and assisted reproduction.
It was previously thought human embryos could not mature beyond a few days outside a mother's womb.
But the findings of the study - published in the journal Nature - may help explain early miscarriages and the high failure rate of in vitro fertilisation.
Experts believe it could also open a window onto the first steps in the creation of a human, since little was known about how the bundle of cells called a blastocyst attaches to the uterus, allowing an embryo to take shape.
Ali Brivanlou, a professor at The Rockefeller University in New York, said this region of human development was previously "a complete black box".
Rockefeller scientist and lead author Alessia Deglincerti added: "We were able to create a system that properly recapitulates what happens during human implantation."
The scientists saw the blastocyst grow in the laboratory, then begin to divide into the different types of cells that eventually give rise to a foetus and its placenta.
But unlike in earlier experiments, in which growth has rarely continued beyond seven days, the embryos showed an unexpected ability to self-organise.
The scientists destroyed the embryos to avoid breaching the so-called "14-day rule" - which says that human embryos cannot be cultured in the lab for more than two weeks.
Aside from the potential scientific and medical implications, the breakthrough also opens up difficult ethical questions, according to experts.
Regulators may be faced with a decision on whether to lift or extend the 14-day rule, which is law across a dozen countries, and accepted practice in five others, including the United States and China.
"If we do not use a 14-day rule, what limit will we use?" asked Henry Greely, the director of the Centre for Law and the Biosciences at the Stanford School of Medicine in California.
"Twelve weeks or so as in many European abortion laws? Viability - at around 23 weeks - as in US abortion law?
"Human development is a seamless process. But ultimately lines need to be drawn."