Human embryos have been kept alive in the lab for a record breaking 13 days - giving scientists new insight into the early development of foetuses.
The study authors claim the technique could open up new avenues of research aimed at helping improve IVF success rates, as well as investigations into the causes of early miscarriages, and ways to produce stem cells to treat diseases.
The experiments were deliberately ended after 13 days - to avoid breaking the current legal limit of 14 days. This has led to debate about the ethics of extending the legal limit.
Professor Magdalena Zernicka-Goetz, who led the UK research and is an author on both studies, explained why this development is being hailed as a "milestone":
"Implantation is a milestone in human development as it is from this stage onwards that the embryo really begins to take shape and the overall body plans are decided," she said.
"It is also the stage of pregnancy at which many developmental defects can become acquired.
"But until now, it has been impossible to study this in human embryos.
"This new technique provides us with a unique opportunity to get a deeper understanding of our own development during these crucial stages and help us understand what happens, for example, during miscarriage."
Dr Simon Fishel, founder and president of CARE fertility group, added: "Knowledge of these processes could help improve the chances of success of IVF, of which only around one in four attempts are successful."
Until now it had been practically impossible to study human embryos in the laboratory beyond around day seven - when the embryo must implant into the uterus of the mother to survive and continue to develop.
The researchers created a nutrient solution that mimics the conditions in the womb and a structure the embryo can "attach" to, which allows it to begin dividing into groups of cells, which will eventually form the foetus, placenta and yolk sac.
The embryos involved in the research were donated by couples who were undergoing IVF treatment.
The UK is one of many countries where scientists are allowed to study spare, donated IVF embryos. But they must be destroyed after 14 days.
This "14-day rule' was set in the 1980s because it was deemed to be the point at which “individuality” is assigned, as embryos do not split into twins after this point.
The Nuffield Council of Bioethics - an independent body that examines ethical issues in biology and medicine - has announced it will review the issue.
Chair of the Council Jonathan Montgomery said:
"Later this year, the Council intends to bring together invited participants with a range of perspectives on embryo research in order to evaluate whether, after 25 years, there may be persuasive reasons to review this legal limit, or whether the reasons for its introduction remain sound.
"It will publish a note of that meeting and then consider whether to undertake further work."
However Professor Zernicka-Goetz told The Guardian she was not pushing for a change in the law:
“To be able to culture embryos for a couple of days longer would provide an enormous body of information, but it’s not for us now to decide whether we should do it or not.
"Rules are very useful, we would always adhere to them, and they should be set out by the wider community."
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