The technology could be a starting point to grow organs from scratch, Jacob Hanna of Weizmann's Molecular Genetics Department, who headed the research team, said in a statement.
Independent experts said a lot more research would be needed before even considering growing a human embryo this way.
Still, this research makes this possibility a little more feasible, adding urgency to the ethical question, they said.
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A synthetic embryo is shown growing from day 1 to day 8. Scientists could see the beginnings of a beating heart, a blood circulation, a brain, a neural tube and an intestinal tract in the synthetic embryo at day 8.Courtesy of the Weizmann Institute of Science
Cracking the synthetic embryo code
"The embryo is the best organ-making machine and the best 3D bioprinter — we tried to emulate what it does," said Hanna.
Most of Hanna's synthetic embryos died early on in the process. But a few managed to grow for 8.5 days, about half of the gestational time for a mouse.
At that point, they were 95% similar to normal mouse embryos and had grown a placenta and the beginnings of a spine and brain, a digestive tract, a beating heart, per the study.
However, these are not "real" embryos, Hanna told The Guardian. For one, they were not able to grow to term when they were put in a mouse uterus, he said.
A synthetic model (top) and a natural embryo (bottom) are shown at day 8. Structures of emerging organs are shown with arrows.Courtesy of the Weizmann Institute of Science
Growing organs from synthetic embryos
Because these synthetic embryos are made from stem cells, rather than via fertilization, it is easier to scale the process and make lots at once.
That could be invaluable for science, because it could make huge amounts of synthetic embryos available for research without relying on lab animals.
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If these cells can be coaxed into making the beginning of organs, studying them might reveal the building blocks to make organs from scratch to transplant them into humans without a need for donors, Hanna said.
"Our next challenge is to understand how stem cells know what to do — how they self-assemble into organs and find their way to their assigned spots inside an embryo," said Hanna.
This is what the synthetic mouse embryos look like when they are being grown, from day 5 (top left) to day 8 (bottom right).Courtesy of the Weizmann Institute of Science
Still a long way from synthetic human embryos
James Briscoe, a group leader at the Francis Crick Institute in London not involved in the research told The Guardian that the research raises ethical questions.
"Now is a good time to consider the best legal and ethical framework to regulate research and use of human synthetic embryos and to update the current regulations," he said.
We aren't going to see human embryos grown from stem cells anytime soon, Briscoe said. These synthetic mouse embryos were not capable of developing into a live mouse. We also know a lot less about human embryos, which take a lot longer to come to term and are a lot bigger.
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Still, this innovation could put this field of research in motion, Paul Tesar, a developmental biologist at Case Western Reserve University School of Medicine who was not involved in the study, told STAT News.
"This is just one step, but a very important step for us to be able to study early development," Tesar said.
"We're crossing into the realm of being able to generate an embryo from scratch, and potentially a living organism. It's been a really notable switch for the field."
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