While the latest work was well within the limit - the embryos were grown for 13 days to the critical time when balls of cells are poised to start the process of sculpting the human form - the achievement has led to calls to revisit the legal limit .
The 14-day rule is enshrined in law in at least a dozen countries, the UK included, and while extending the allowed period for embryo research would be welcomed by some scientists, the move would be resisted by many, including religious groups already opposed to embryo research.
We can now, for the very first time, study human development at this very critical stage of our lives, at the time of implantation, said Magdalena Zernicka-Goetz, who led the UK research at Cambridge University. The longest that human embryos had previously been grown in the lab was nine days, though seven days was far more common.
The procedure used to grow the embryos, developed at Cambridge and Rockefeller University in New York, promises scientists fresh new insights into early human development; the causes of early stage miscarriages, and ways to produce stem cells to treat diseases. But the work has also ignited a vigorous debate around the laws that govern human embryo studies.
In the UK and many other countries, scientists are allowed to study spare, donated IVF embryos, but they can only be grown in the lab for 14 days. After that, the embryos must be destroyed. The 14-day stage marks the point when the individuality of an embryo is assured, because they can no longer split into twins. At about the same time, embryos form what is called the primitive streak, a faint band of cells that starts to distinguish the head from the tail.
Introduced in Britain 30 years ago, the 14-day rule aimed to give scientists room to study human embryos, while respecting wider views on embryo research. And while it has served scientists well, it has never held them back. Until now, the barrier has been science, not law.
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