The embryos of many species can stop developing when starved of nutrients, only to restart the process once these are restored – and scientists may have figured out how they do it.

In the early stages of pregnancy, a fertilised egg turns into a blastocyst, a tiny cluster of dividing cells. This then implants into the uterine wall, eventually differentiating into the various organ tissues of a fetus.

When some animals face extreme circumstances, such as when food is scarce or when it is really cold, blastocysts pause their growth and enter a state of dormancy called embryonic diapause. This can last for several months in some species, with activity resuming once conditions improve. “It is a strategy to maximise the reproductive process and thus the number of young born and their survival,” says at the University of Montreal in Canada, who wasn’t involved in the research.

Now, at the Chinese Academy of Science and his colleagues have uncovered how an embryo can tell when to stall development.

They put 14 newly pregnant mice in a cage with food and 11 others in a cage without food. After 3.5 days, they found that the blastocysts of the well-fed mice developed as usual, but those of the starved mice hadn’t implanted in the uterus, indicating embryonic diapause.

When these dormant blastocysts were then transplanted into the uteruses of well-fed mice, they started growing again.

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In another part of the experiment, the researchers grew mouse embryos in petri dishes with different nutrients. They found that embryonic diapause seems to be caused by a lack of carbohydrates and proteins, while embryos exposed to normal levels of these nutrients grow as expected.

Closer inspection revealed that a sensor called Gator1 in the blastocysts can detect drops in carbohydrate and protein levels in the uterus. This then prevents a molecule that controls protein synthesis from being activated, which is necessary for blastocyst development.

When the team injected the uteruses of pregnant mice that had been deprived of food with the necessary carbohydrates and proteins, embryonic growth resumed.

With a similar proces expected to occur in human embryos, Ye hope these findings could eventually be used to improve fertility treatments. Prior to in-vitro fertilisation (IVF), embryos are sometimes preserved by freezing, then transplanted into a uterus. This method of preservation is expensive and the embryos don’t always survive the thawing process. The team has shown it could be possible to preserve them through nutrient depletion, says Ye.

A team at the University of Cambridge has announced making the first human “synthetic embryos” – embryo-like structures made from stem cells – that have been grown to a stage equivalent to just past 14 days old, the legal limit to study natural embryos in the UK. The work could help researchers study the causes of early miscarriages and understand how organs develop, but some scientists want to tighten regulations covering this field.

What are synthetic embryos?

The term is somewhat misleading as these structures aren’t really synthetic, nor are they exactly the same as embryos. They are similar to early embryos, a tiny ball of cells arising from a sperm fertilising an egg, but created from stem cells grown in the lab.

These structures haven’t been wholly synthetically created, as the stem cells came from an embryo originally, and they seem to have certain differences to naturally formed embryos. Some therefore prefer to call these entities stem-cell-based embryo models.

How much of an advance is this?

Until recently, , although at the University of Cambridge had made it public that she was . On 14 June, she announced her team had grown the human versions to a stage at which a real embryo would implant into the uterus and start developing some different tissues, including the precursors of cells that go on to form sperm and eggs.

“In real embryos this [is equivalent to] a stage between day 7/8 and day 14,” says Żernicka-Goetz. She presented her work at the International Society for Stem Cell Research’s annual meeting in Boston, Massachusetts, as reported by , but hasn’t yet published the full report in a peer-reviewed paper

Why make synthetic embryos?

One reason is they can be made in unlimited numbers, while spare IVF embryos, which are used in early developmental research, are in short supply. All embryos in a batch made from the same stem cells are also more or less genetically identical, which is of great use for studying their response to different experimental conditions. It is also quicker and easier to genetically modify stem cells grown in a dish to understand the functions of different genes.

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Any other benefits?

One key advantage is that in most countries – including the UK and US – natural embryos studied in a dish must be destroyed after 14 days, to avoid ethical issues around working with something that could, in some circumstances, grow into a fetus. These rules don’t apply to synthetic embryos, so this could let researchers study embryo development for longer, a landmark step towards better understanding organ development and the causes of early miscarriages.

Should we make new rules that apply to synthetic embryos?

These are already being drawn up in the UK, but nothing has been announced elsewhere, including in the US. In the UK, scientists, such as Żernicka-Goetz, have formed a working group along with legal experts, patient representatives and ethicists to discuss what new rules should apply. The group has submitted recommendations to the UK’s Human Fertilisation and Embryology Authority, which regulates research involving natural embryos and will probably take on responsibility for synthetic embryos.

Policymakers may consider making a cut-off based on the developmental stage the synthetic embryo has reached rather than a certain time limit like the 14-day rule, says at The Francis Crick Institute in London. “Perhaps the threshold will be the formation of a certain organ system.”

What would happen if synthetic embryos were transferred into a uterus?

We don’t know because it has never been done, and it would be illegal. But animal research by a Chinese team suggests it would not, in the current state of the field, lead to pregnancy. When monkey synthetic embryos were created and transferred into eight females, a pregnancy seemed to begin in three of the individuals, , it was reported in April.

This may be because synthetic embryos aren’t identical to natural ones. “We can’t assume they completely recapitulate human embryonic development,” says at Hull York Medical School, UK.

How might synthetic embryos help advance medicine?

They should help us understand more about early embryonic development. We can’t see precisely what is going on inside the uterus at any stage of pregnancy and the 14-day rule prevents human embryos past this stage being studied in the lab. “That period is a black box,” says Sturmey.

Learning about this process may shed light on what goes wrong when early pregnancies miscarry. It could also help us understand how embryonic organs form and grow, which one day could lead to doctors being able to grow organs in the lab for transplants.

Scientists have visualised the early development of the nervous system in a chicken embryo.

at the University of California, San Francisco, dissected the 2-day-old embryo out of its egg and added fluorescent antibodies, which bind to specific proteins in the nervous system and make them visible.

An image of the embryo taken with a confocal laser scanning microscope shows its developing nerves in green.

The purple colour shows a group of cells called the neural crest that migrate through the embryo and form neurons in the gut, sensory nerves in the face and a diverse range of other cell types.

The cyan colour shows the presence of a protein called ELAVL1, which plays a role in neural crest development, says at the University of California, San Francisco, who oversaw the project.

“We use the chick embryo to investigate the molecular and cellular mechanisms of neural crest development because this model system develops similarly to human embryos, but develops outside the mother, allowing for easy manipulation of gene expression as well as live imaging approaches,” she says.

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Hutchins and her team are investigating neural crest development in embryos because some congenital conditions are caused by the abnormal migration of these cells. These include Hirschsprung’s disease, in which nerves are missing from parts of the intestine, and familial dysautonomia, which can affect the ability to feel pain among other things.

Embryos may develop more slowly in certain pregnancies that end in miscarriage, according to a study that uses virtual reality to visualise them using vaginal ultrasound scans.

The finding could be a step towards predicting from early in a pregnancy if it is likely to lead to a miscarriage, although the technique isn’t yet ready for clinical use, says at Erasmus University Medical Centre in Rotterdam, the Netherlands.

More than half of all pregnancies end in miscarriage, although in many cases it happens so early that people don’t even realise they were pregnant. Some people have multiple miscarriages without knowing why, meaning their pregnancies can be a time of great anxiety.

To find out more about why miscarriages occur, Rousian’s team developed a way of using vaginal ultrasound scans to build a highly detailed 3D image of an embryo. The image is magnified until it is about the size of an adult and then visually inspected by the researchers while they wear virtual reality headsets.

The team created these images for 644 pregnant women, 33 of whom went on to have miscarriages. Transgender people weren’t included in the study.

When looking at the 3D images, produced at around eight weeks post-conception, the researchers saw that, on average, the embryos that eventually miscarried were developing more slowly, compared with the pregnancies that continued to term.

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Embryo maturity was assessed via the so-called Carnegie staging system, which indicates which physical features have developed, such as limb buds and early facial features, according to a 23-stage scale.

The team found that the chance a woman would miscarry rose by 1.5 per cent per delayed Carnegie stage.

At eight weeks post-conception, this was equivalent to the embryos that went on to miscarry being delayed in development by about four days, according to the researchers. “Four days is quite a large gap in a very important period of life when all the organs are developing, all limbs are developing,” says team member .

If the findings are confirmed in larger studies, this may allow doctors to advise people whether or not their embryo is developing normally, says Rousian.

It is unknown exactly why embryos that develop more slowly may be more likely to miscarry. Other work has found that embryos or fetuses that miscarry often have alterations to or different numbers of chromosomes, the packages of DNA contained in nearly all our cells.

at the , a UK support charity, says that although the 3D embryo imaging may reassure some people, it could also cause uncertainty and anxiety.

Some people who have previously had a miscarriage may not want the vaginal ultrasound scan, while others could be desperate to check the progress of their pregnancy, she says.