Researchers will soon have the means to study embryos beyond the 14-day legal limit. Does the potential for advances in medicine outweight ethical concerns?
A human zygote at 18-20 hours after insemination. The male and female pronuclei (the two discs in the centre) have not yet formed a single nucleus. Photograph: © 2010 Nicoli et al; licensee BioMed Central Ltd
It is not birth, marriage or death, but gastrulation that is truly the most important time in your life.”
When the eminent embryologist Lewis Wolpert wrote these words 30 years ago in From Egg to Embryo: Determinative Events in Early Development, human gastrulation – the most momentous of all embryonic transformations – was something of an unknown quantity. Indeed, it is still referred to as the “black box” of human development, occurring about 16 or 17 days after fertilisation and more than a week after the free-floating embryo has anchored itself to the lining of the womb.
For all its importance, no one has actually witnessed gastrulation in a human embryo. We can infer it happens from studies on the embryos of other mammalian species and from something called the Carnegie collection of human embryos (around 8,000 of which had been amassed by the 1940s) from the wombs of women undergoing hysterectomies who had conceived at various time points before their operations.
More recently, the IVF revolution has enabled early human embryos to be studied in unprecedented detail outside the body, but still gastrulation has remained a mystery. Technical limitations have meant that no one has been able to keep them alive much beyond nine days, about halfway to the key developmental stage when the simple ball of embryonic cells begins to take on the identity of a proper body plan, with a top and a bottom, and the first signs of the distinctive triple layer of tissues that form the disc‑shaped “gastrula”.
This now looks set to change following two studies published last month, showing that it is possible to keep human IVF embryos alive and developing for up to 13 days after fertilisation. The scientists involved believed they could have gone further but stopped short because of the 14-day rule widely adopted around the world that states that no human IVF embryo should be allowed to live beyond this point outside a woman’s body.
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In Britain, breaching the 14-day rule is a criminal offence, but many scientists are questioning whether the law should be changed. Gastrulation is just too important, they say, to be left unobserved, unstudied and, ultimately, unknown.
Allowing IVF embryos to develop through this critical stage of development could shed light on a spectrum of medical problems, from infertility and recurrent miscarriages to congenital conditions such as spina bifida, cancer and even age-related diseases, they say.
“We could be missing out on discovering some very key aspects of human development because we have this rule,” says Azim Surani, a cell biologist at Cambridge University’s Gurdon Institute. “It results in a waste of material because we have lot of embryos left over from IVF treatment, so by extending the time limit we can use them in research to obtain first-hand information about early human development that has an impact much later on in life. Why not make optimum use of this precious material, extend the time limit and use it for something that could benefit advances in medicine?”
By allowing some of these spare IVF embryos to reach gastrulation, scientists would be able to understand some of the most important events in early human development. It could, for instance, transform our understanding of how to use stem cells in regenerative medicine or inform us about key stages in the development of the “primordial germ cells”, which go on to produce sperm and eggs. These germ cells are effectively immortal because they are continually passed on through the generations and knowing how they form during gastrulation could help to make sperm and eggs in the lab from skin cells, already achieved in the mouse.
An embryo at 26-30 days. At this stage it measures 3-5mm and the heart is beating. Development of the digestive system is underway and limb buds have appeared. Photograph: UNSW Embryology
“My particular interest is the origin of primordial germ cells and those are probably specified at around the 17th day, just at the time of gastrulation. So because we can’t study human embryos at this point because of the 14-day rule we are missing out on key aspects of human development,” Surani explains.
“The precursor cells that are going to make blood, muscle, gut or whatever are all being made here. This is the starting point of the key decisions in life, so Lewis Wolpert was right – this is where the real business of development starts.”
Many scientists see the 14-day rule as an anachronism. It can be traced to 1979, when it was proposed as a limit by the ethics advisory board of the US Department of Health, Education and Welfare, just a year after the birth of Louise Brown, the world’s first IVF baby. In 1984, it was endorsed in Britain by the Warnock committee, whose seminal report became the basis of the 1990 Human Fertilisation and Embryology Act.
The logic behind a 14-day limit was a peculiar combination of religious and scientific thought. It was the point when twinning – the natural splitting of an embryo into two further embryos – became impossible, so some theologians were happy with it on the grounds that prior to this the “soul” had not entered the embryo, otherwise twins would have to share the same soul, a theological conundrum too far.
A more scientific reason for 14 days was that it was before the appearance of the “primitive streak”, a set of tissues that unambiguously become the foetus and baby, rather than the extra-embryonic tissues of the placenta.
“It’s before the formation of the primitive streak, and the first signs of the nervous system development, so the argument was it was well before embryos could possibly feel anything. Of course, they are not in fact going to feel anything until months later because that’s when you have differentiated nerves,” explains Robin Lovell-Badge, a mammalian geneticist at the Francis Crick Institute in London.
“The 14-day rule was a compromise between the scientific and the non-scientific arguments. But certainly at that time it was a philosophical rather than a practical limit, given that it is only now that it has been possible to maintain approximately normal development beyond seven days.”
This changed dramatically last month with the publication of two studies showing that it was possible to culture human embryos in vitro right up to the legal limit. One was led by Magdalena Zernicka-Goetz at Cambridge, who built on her work of two years ago showing how mouse embryos can be made to live long outside the body with a little help from 3D scaffolding material and some additional growth factors and hormones added to the IVF culture medium.
What was surprising, according to her paper in Nature Cell Biology, was the innate ability of the human embryo to “self-organise” itself in the absence of a womb into the distinctive, disc-shaped layers of tissues that appear several days after implantation has taken place.
“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,” Dr Zernicka-Goetz said at the time of publication.
“It is also the stage of pregnancy at which many development 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.”
A rush of excitement followed the publication of the two studies, the second (in Nature) being from the lab of Ali Brivanlou at the Rockefeller University in New York. For the first time, scientists seemed to have acquired the technical ability to breach the legal limit, a bit like suddenly having cars that could go faster than 70mph on a motorway. So the obvious question was, why shouldn’t they?
A human embyro (now a foetus) at 56-60 days, measuring about 27-31mm. A head, eyelids, external ears and limbs have developed. Photograph: © Dr Mark Hill 2016, UNSW Embryology
Peter Donovan, an expert in molecular genetics and stem cell development at the University of California, Irvine, summed up the vista of possibilities: “Perhaps we could begin to understand the consequences of foetal alcohol syndrome, study the potential causes of autism and find out why some environmental chemicals can affect development. Perhaps we might, for example, be able to more quickly understand what the Zika virus does to embryos to cause major problems with brain development.”
In the US, where the anti-abortion movement has had more influence historically on this kind of scientific research than in the UK, there are siren voices urging caution. Henry Greely, director of the Center for Law and the Biosciences at Stanford School of Medicine, says the 14-day limit shouldn’t be breached just because it is now possible.
“Frankly, I am not convinced… Given the questionable scientific value of the research, no case has been made for even revisiting the line, let alone changing it,” he says.
Philosopher Françoise Baylis, a bioethicist at Dalhousie University in Canada, is more scathing: “Isn’t it somewhat ironic that when the agreed upon limit might finally be practically relevant – meaning that it could function to stop scientists from doing something they might otherwise do – the suggestion is that now might be a good time to change the limit?”
In the noisy cafe of the Wellcome Collection in London, Lovell-Badge takes out a fine-tipped felt pen and gently touches the pages of my notebook to leave a barely visible spot of ink. “If that’s a fertilised egg at day one, then,” he says making a slightly larger full stop next to it, “that’s an embryo at day 14.”
Size, of course, matters not to religious moralists concerned about the soul. The Catholic church takes the moral line that the soul begins at conception, which is why it opposes all research on IVF embryos, no matter how many days old they are. Other religions have different views on when a human life starts – in China, for instance, many consider that it takes full effect the day after birth.
Lovell-Badge and Surani are more concerned with the medical benefits that might come out of research on older IVF embryos. They believe these benefits are most likely to be felt first in improvements in fertility treatment, with better IVF pregnancy rates, fewer miscarriages and instances of eclampsia.
They also feel there will be much to learn about how genetic disorders are triggered in early embryonic development.
Like so many scientists around the world, they have been astounded by the power and yet simplicity of a new gene-editing technique called Crispr-Cas9, which has been likened to the “find and replace” command of word-processing software – only working on the DNA of the genome. The Crick Institute has the first licence to use Crispr-Cas9 on day-old embryos, donated for research purposes, specifically to understand the causes of recurrent miscarriages.
Both Lovell-Badge and Surani are quick to bring Crispr-Cas9 into the conversation about the reasons for extending the 14-day limit. Each argues that it will enable scientists to understand which genes are active during the crucial phase of gastrulation when the body plan is laid down and the three main tissue layers – the ectoderm, mesoderm and endoderm – are formed as the biological foundations for the specialised tissues of the nervous system, muscle and blood, and lungs and intestines, respectively.
“By introducing mutations in early human embryos with Crispr-Cas9 we can then ask how these genes affect these early stage of development. One particular benefit we can envisage is in regenerative medicine. We are trying to make a variety of cell types and some of the key decisions are happening around here,” Surani says.
Surani and Lovell-Badge also point out that if there is one thing we have learned by studying mouse embryos it is that they are different to human embryos. As a model, the mouse has proved itself, but it also has its limitations, they say. “The mouse has been a key model for mammalian studies but it has become increasingly clear that we can’t extrapolate to human. We really need to get into human embryology to really understand what the differences are,” Surani says.
The “black box” of human development starts at day 14 and ends at around day 28 when scientists can study the tissues of aborted foetuses. A new embryo limit of 28 therefore would not result in a slippery slope, says Surani, assuming that it would ever be technically possible to reach this point, especially given that reaching day 21 in a dish is “not trivial”.
As for the possibility of completing a full-term pregnancy completely outside the womb, Lovell-Badge simply says it won’t happen, at least not in his lifetime. The spectre of a brave new world of full-term, test-tube babies will remain fiction. “No one is close to doing it in any mammal species,” Lovell-Badge says.
In reality, the pressing need is to see and understand gastrulation, which can only be done by extending the 14-day limit by a week or two, he says. “I think it ought to be looked at again. I’m in favour of that, but as a scientist I also think we’ve got to take people along with you. The arguments for extending the limit are pretty strong.”
And on the 14th day, some light could finally be shed on the black box of human existence.[1]
[1]- Inside the ‘black box’ of human development, the Guardian, Sunday 5 June 2016
