A human embryo has been grown in a laboratory for twice the length of time than was previously possible in a breakthrough that could “revolutionise” medicine but also raises fresh ethical questions about when life begins.
Since scientists first fertilised an embryo in a test tube in 1969, they have never managed to keep one alive for long after the point at which the foetus implants in the womb, normally about seven days.
However researchers at Cambridge University have now grown embryos for 13 days -- a process they only stopped to avoid breaking the current legal limit of about 14 days.
The ability to observe a human embryo as it grows during this “most enigmatic and mysterious” stage of life in a lab should shed new light on genetic diseases and disabilities.
And it could help improve the dismal failure rate of IVF embryos -- currently up to 70 per cent do not successfully implant – and lead to better understanding of miscarriages.
Greater knowledge of our early embryonic development might also hasten the arrival of one of the medical world’s greatest hopes: regenerative medicine using stem cells. These can turn into any different kind of cell and doctors believe they could one day be used to treat Alzheimer’s, heart disease and a host of other conditions.
The breakthrough immediately led to calls for the Government to consider extending the legal limit, but such a move would be hugely controversial.
About 14 days – the limit is also linked to biological changes -- was chosen in the 1980s because it was deemed to be the point at which “individuality” is assigned as twins do not develop after this point.
Professor Magdalena Zernicka-Goetz, one of the lead researchers, stressed the benefits of being able to observe an embryo up to 13 days.
“We can now, for the very first time, study human development at this very critical time, the time of implantation,” she said.
“It is really most enigmatic and mysterious stage of our own development.
“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 plan is decided.
“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.”
It had been “absolutely unbelievable”, she said, to see the “amazing, self-organising ability” of the cells.
“They have to talk to themselves and together they make something more beautiful,” Professor Zernicka-Goetz said.
But she accepted the research raised ethical questions, particularly “how far we can go?”
“I think that longer cultures [of embryos] could provide absolutely critical information for basic human biology. They can improve IVF success, they can improve stem cell differentiation, so this is very important,” she said.
“But this would raise the next question, where should we put the next limit? I think this is really not my place to say, one way or the other.
"Even if I believe there’s a scientific case for it, [the decision] should be taken with other scientists, ethicists and the general public.”
Professor Zernicka-Goetz added that she thought they might only be technically able to grow an embryo for perhaps a day past the limit and dismissed suggestions that it could lead to the lab-based creation of human life.
“There’s no way to grow [even] a mouse outside the womb. It’s really just science fiction, it’s not possible,” she said.
This point was echoed by Professor Allan Pacey, of Sheffield University, who was not involved in the research.
"It will not open the door to couples being able to grow babies in the laboratory; this is not the dawn of a Brave New World scenario," he said.
"But it does open up exciting opportunities to understand the nature of human disease and disability and for that reason the scientists involved should be congratulated."
Professor Robin Lovell-Badge, a leading geneticist and stem cell expert, said there should be a public debate about the 14-day limit.
He suggested it could be extended only slightly to allow scientists to study a key process in embryonic development called gastrulation.
“We don’t need to take it beyond that, but just that few days [after 14] would be useful,” he said.
“I think you could do a lot of valuable research. Other scientists are arguing for maybe just a couple more days, maybe a week.
"Proposing to extend the 14-day limit might be opening a can of worms, but would it lead to Pandora’s box or a treasure chest of valuable information ? This is not a question to be left to scientists alone.”
Religious figures expressed concern about how any new limit would be set.
The Rev Dr Brendan McCarthy, the Church of England's national adviser on medical ethics, welcomed the scientists' calls for a debate.
But he added: "The present limit recognises the importance of the emergence of the ‘primitive streak’ heralding the beginning of neural development.
"If the beginnings of neural development do not provide a watershed for research, it is difficult to see where else the limit might be placed in order properly to respect the developing human embryo and foetus.”
However Professor David Jones, director of the Roman Catholic Anscombe Bioethics Centre, described the 14-day limit as "completely arbitrary" and pointed out abortions were legal up to 24 weeks.
He said it was likely that a scientist somewhere in the world would now go further than the Cambridge team.
"In the future this may even be seen as the first step towards culturing babies outside the womb, where the child is not only conceived outside the protection of his or her mother's body but no such protection is even envisaged at any stage," said Professor Jones, who opposes any research on human embryos.
"Human life and human pregnancy should not be separated in this way. On a technical level it is a scientific breakthrough but it is also a further step away from humane and ethical science and a further step towards an increasingly inhuman future."
The Nuffield Council on Bioethics has already decided to hold a meeting of experts "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".
"The Council has agreed to explore whether arguments for reviewing the 14-day limit are gaining force," it said, "in the expectation that any move to review this limit will be likely to generate significant moral controversy and would require careful analysis."
The research was reported in the journals Nature and Nature Cell Biology.
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