Inside a lab in Vienna, cells are dividing to form a hollow sphere. Although the fragile ball has all the characteristics of an early human embryo, it isn’t quite what it seems. It didn’t, in fact, begin with an egg meeting a sperm. Instead, it was created entirely in the lab.

The very first days of pregnancy have long been an enigma. Scientists are unable to peer inside the uterus during pregnancy, meaning we know little about why so many fail. This is now beginning to change, thanks to embryo models created from stem cells, which are lifting the lid on one of the great mysteries of human biology.

In the five years since early human embryo models known as blastoids were first created in several labs – including the one in Vienna – researchers have dramatically advanced our understanding of the early days of life. This is already leading to improvements for in vitro fertilisation (IVF) and treatments for serious conditions that occur during pregnancy. Blastoids are allowing scientists to recreate early pregnancy in a dish and then “poke it, perturb it and see how the system copes”, says Peter Rugg-Gunn, a developmental biologist at the University of Cambridge. As the science evolves and researchers are able to sustain embryo models in the lab for longer, they are beginning to find themselves in ethically uncharted territory. They’re faced with a quandary: just how far should they go?

Peering into the black box of human pregnancy

After an egg is fertilised, it begins dividing rapidly, forming a ball of cells that becomes a blastocyst. To keep developing, the blastocyst must dig into and attach to the uterus, which happens in humans around a week after fertilisation. This process, called implantation, often goes wrong. Only around a third of embryos successfully implant into the uterus, while 60 per cent of IVF embryo transfers fail.

Understanding why has long proved challenging. Scientists have previously studied mice – which have predictably different pregnancies from humans – and human embryos surgically removed in hysterectomies or expelled in miscarriages. These provide snapshots of embryo development. But there has been no way to watch human embryo growth or to see the crucial moment when the embryo embeds in the uterus.

In 2021, several research teams – including one led by Nicolas Rivron, a stem cell biologist and founder of the lab at the Institute of Molecular Biotechnology in Vienna – successfully created blastoids from human stem cells. Researchers learned to develop human pluripotent stem cells that have early embryonic cells’ capacity to create many other types of cell. When placed in the right environment, these stem cells organised themselves into embryo models. It was a major breakthrough and the platform for significant work since.

In two studies this year, for instance, scientists witnessed implantation as it happened by moving this key moment into a dish. Their innovation: a three-dimensional model for the endometrium, the lining of the uterus.

Rugg-Gunn and his team built a model endometrium from biopsy samples taken from healthy women, and also developed blastoids, meaning that both sides of the equation – the embryo and the endometrium – finally met. Within three days, more than 80 per cent of the blastoids had successfully implanted into the artificial endometrium.

Across the ocean, meanwhile, Jun Wu at the University of Texas Southwestern Medical Center and his colleagues created “endometrioids”, postage stamp-sized chips that nourished a bioengineered endometrium model made from donated tissue samples. When they added blastoids, about 60 per cent implanted, but that rate fell to 20 per cent when the donated tissue came from people who had undergone several failed rounds of IVF.

The researchers then tested whether more than 1000 different drugs previously approved by the US Food and Drug Administration for a variety of conditions could improve implantation, finding a handful that successfully increased rates by up to 60 per cent. But the drugs only worked for some samples, so the team is now screening to find a common drug that works for most people.

Turning research into treatments

The ability to watch implantation outside the uterus is rapidly building to new treatments to improve IVF success.

A single round of IVF is both physically and emotionally draining. On average, eight eggs are extracted per cycle, of which 70 to 80 per cent are successfully fertilised. When embryos are transferred into the uterus, typically one at a time, around 35 to 40 per cent develop into pregnancy – but the numbers at every stage vary significantly. “The emotional strain is extremely high,” says Christina Fadler, founder of Austrian fertility advocacy group Die Fruchtbar, who experienced infertility herself. Her sense of depression increased with each negative test result, she says. Although some people get the costs covered, such as through the National Health Service in the UK or health insurance, others have to pay £8000 per cycle in the UK, or up to $30,000 in the US.

Researchers are learning from blastoid implantation to improve this experience. In Texas, Simbryo Technologies developed a test that predicts the chance that the next embryo transfer will be successful, helping people make an informed choice about whether to go ahead with further IVF cycles. The start-up creates endometrium models grown from clients’ tissue samples and tests whether blastoids are able to embed. “When things go wrong, we know the problem is on the endometrial side, not the embryo side,” says Aryeh Warmflash, a bioscientist at Rice University in Texas and chief science officer at the company.

Fadler says that, though she’s wary of companies profiteering on infertility, many people would find such tests valuable. “We still don’t even know what can go wrong during implantation – it’s all such a black box,” she says. “Everything that’s researched and every bit of additional knowledge generated is important and beneficial for patients in the long run.”

Others are working on the embryo side of the equation. The start-up dawn-bio, which was co-founded by Rivron, wants to improve IVF procedures by optimising the growth conditions for embryos before they are transferred into the uterus. Only 20 per cent of fertilised eggs develop sufficiently in time to be transferred. “We’re not giving the embryos what they need,” says Peter Greiner, a biochemist who is chief executive of the company. “Fundamentally, in terms of what is in the medium for growing embryos, nothing new has happened [since the first IVF baby was born] because we don’t know what human embryos need and we can’t do experiments on human embryos.”

The company identified 150 human metabolites that it believes are valuable, but that aren’t currently used when cultivating embryos for IVF. Testing on both blastoids and donated embryos identified seven metabolites that improved embryo quality – which is determined by markers such as symmetry and the number of cells – by day five of development.

“Blastoids made a tectonic shift possible for the field of IVF,” says Greiner. “We are aiming for a 100 per cent own healthy baby rate: that 100 per cent of the people who want to have a baby can have their own, healthy baby.”

Unravelling the complexity of embryo implantation

Blastoids are also enabling a far deeper understanding of how, exactly, embryos function. At the University of Cambridge, Rugg-Gunn and his team made blastoids express a fluorescent protein, so they glowed as they became implanted in endometrial models. This allowed the researchers to see something surprising: the embryo models, shortly after burrowing into the artificial uterus, sent out cells into the endometrium.

“We don’t know what these cells are. But now that we can see them, we can study them,” says Rugg-Gunn. The cells may help to anchor the embryo. Or they could pave the way for communication signals between the embryo and the endometrium. Rugg-Gunn suspects early communication is key for implantation and, when it goes awry, it may be responsible for some miscarriages.

An even more unexpected discovery came from Heidar Heidari Khoei, a stem cell biology researcher at the Institute of Molecular Biotechnology, who uncovered a “pause button” in human blastoids. This is likely to be related to some mammals’ ability to slow embryo development for weeks before implantation, only continuing pregnancy once the chances of survival are better. Khoei similarly pushed the pause button in human blastoids by blocking specific signalling pathways, before successfully restarting their development by reactivating the pathways.

Meanwhile, Rivron’s colleague, stem cell biology researcher Anna Osnato, is using gene editing to better understand blastoids. Because embryo models are grown from stem cells, scientists can manipulate this starting material, and Osnato wants to figure out which genes allow implantation to proceed smoothly, influencing how the embryo burrows in to the right depth. Recently, she identified genes connected with the embryonic cell layer that sticks to the uterus. When she removed those genes, the blastoids then attached a lot less frequently.

Studying implantation is also key to preventing later problems, such as pre-eclampsia, a condition affecting 5 to 8 per cent of pregnancies that causes high blood pressure, organ failure, strokes and seizures and can be life-threatening. There is growing evidence that the issue has its origins in implantation, when the placenta starts to develop, and Rugg-Gunn believes research could lead scientists to identify biomarkers that point to increased risk. So, analysing implantation under the microscope, from every angle, holds the potential to resolve a host of pregnancy complications.

A menagerie of embryo models

Scientists now want to go beyond implantation and have developed embryo models mirroring a later stage than the blastocyst. Jacob Hanna at the Weizmann Institute of Science in Israel has published research on stem cell-based embryo models equivalent to embryos 14 days after fertilisation.

He says he can now grow embryo models to the equivalent of 21 days post-fertilisation. Hanna and other scientists want to push that even further. The question is: should they?

In theory, it could one day be possible to create a blastoid that would develop into a human if implanted in a uterus. In practice, that’s currently impossible, and even mouse and monkey blastoids fail to develop for long when implanted into animal uteruses. Scientists can’t try to cross that line: both the guidelines of the International Society for Stem Cell Research (ISSCR) and national frameworks such as the UK Code of Practice forbid inserting a human embryo model into a uterus.

When it comes to human embryos, there are strict regulations on how long they are allowed to develop in the lab. For instance, the UK and Australia allow 14 days of development, while there’s a complete ban on embryo research in countries such as Germany and Austria. But embryo models tend not to fall under those regulations, and there are few clear limits. ISSCR guidance simply states that embryo model research must be justified and have defined endpoints.

As embryo models are currently unable to develop into humans, several bioethicists say many significant ethical issues around human embryos don’t apply. Tsutomu Sawai, a bioethicist at Hiroshima University in Japan, says Japan’s Cabinet Office on Bioethics is “very sceptical of the potentiality to be humans when it comes to stem cell-based embryo models”. And some researchers argue there are benefits to growing later-stage embryo models. Wu envisions developing embryo models that are equivalent to a 3-week-old embryo, when the first organs begin to form. The seed cells giving rise to organs could be extracted and used for improved mini-organs (organoids) or bioprinting to construct living tissue models.

Blood stem cells could also be used to help people with leukaemia. “A 30-day-old human embryo has the best transplantable blood stem cells in the liver,” says Hanna. But stem cell transplants require a tissue-type match, and it is currently unfeasible for patients to find a human embryo that’s a match. Embryo models offer a solution, as they could be built from the patient’s own stem cells.

There is currently no limit on how long embryo models in Israel are allowed to develop. Through his company, Renewal Bio, Hanna says he wants to grow embryo models to 70 days, at which point ovaries form, and use the eggs inside for IVF treatments. This would allow people without eggs – or without good-quality eggs – to generate new ones using their stem cells.

These ambitions are controversial. “It is not going to be acceptable to generate near-complete structures to use a small part of it and discard the rest,” says Rivron.

Hanna acknowledges the moral questions. “But the benefit in the scenario given is an infertile woman who needs her eggs, and this is a way to give [them] to her. Or a patient with leukaemia who is about to die because he cannot find a [blood stem cell] donor,” he says. “Ethics is not just abstract.”

Any day 30 or day 70 models are likely to be incomplete, meaning the focus would be on developing one or two embryonic tissues, rather than the whole embryo. These models are akin to adult stem cell-based organoids, and should have similar ethical oversight that focuses on the purpose of research, rather than a blanket cutoff point, says Emma Cave, professor of healthcare law at Durham University, UK, and chair of the Nuffield Council on Bioethics review of embryo models. If later-stage embryo models do start to more closely resemble human embryos, then that could raise concerns about sentience and the ability to feel pain. Such work is currently impossible, but Cave says research shouldn’t be approved if it crosses those boundaries. The level of developmental sophistication is also important, even if a model lacks the ability for consciousness, she says, and limits will need to be carefully assigned.

The idea of human ectogenesis – growing an embryo or embryo model entirely outside the uterus until it is fully developed – is far more contested.

Wu expects ectogenesis to happen for mice embryo models within the next five years, but human ectogenesis isn’t yet conceivable. “It’s like we haven’t even landed on Mars, but are already talking about going to another galaxy,” he says.

Human ectogenesis research is prohibited by the ISSCR. Rivron agrees with this decision, noting that, to be successful, scientists would inevitably have to first create highly advanced fetuses outside the uterus that wouldn’t survive. “I think it’s ethically unacceptable,” he says. Even so, dawn-bio has already been approached by billionaires hoping to fund work on ectogenesis. The company has turned them away.

Even as some experiments remain firmly off-limits, embryo models have already transformed our understanding of embryos and fertility in just the few years since they were first developed. In the next five to 10 years, Wu expects to “fill most of the gaps of our early human development”, dramatically advancing the success rates of IVF. By studying pregnancy outside the uterus, scientists are fast uncovering its secrets.