Cambridge University scientists have created synthetic mouse embryos in the laboratory, without fertilization of the egg by sperm, with a brain and a beating heart.
Mouse embryos, developed using stem cells, lasted only eight days.
But the research team says it could improve understanding of the earliest stages of organ development - and why some pregnancies fail.
Other scientists caution that while the technique holds promise, there are still many hurdles to overcome.
Researchers from the University of Cambridge and the California Institute of Technology recently published their results in the journal The nature (Nature).
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The Cambridge team has been studying the early stages of pregnancy for the past decade, but much of it is obscured by the uterus, so it can't even be seen clearly.
By mimicking natural processes in the laboratory, scientists have found a way to integrate three types of mouse stem cells and grow them into embryo-like structures.
These embryos created in the laboratory lasted only eight days due to defects, but they reached the level where brain development begins.
"It's a dream come true," said Magdalena Zernika-Getz, professor of stem cell biology and mammalian development at Cambridge.
As she explained, this discovery can provide insight into how organs are formed.
"This period of human life is so mysterious.
"That's why this is special because it can help us look at how it all happens, we also have access to these individual stem cells, and it can help us understand why so many pregnancies fail and how it can be prevented," she said. .
The progress could also mean that fewer animals will be used in research, and at the same time could be useful for testing new drugs.
'Rana stage'
However, Alfonso Martínez Arias, a professor from Pompeu Fabra University in Barcelona, says that "this is progress, but it is an early stage of development, a rare case, where although it looks like an embryo, it has flaws that should not be ignored."
The scientists plan to work on keeping the synthetic embryos developing for a day or two longer, which is difficult to do without creating a synthetic placenta.
Their ambition is to develop similar embryos from human stem cells.
It is, however, ethically much more complicated by a long shot.
British law currently allows human embryos to be studied in laboratories up to day 14 of development.
However, there are no rules when it comes to synthetic embryos.
Robin Lovell-Badge, a professor at the French Cric Institute, believes that this should be changed.
"Since they are similar to real embryos, it is necessary to consider in what way and how such integrated models of embryos based on stem cells will be regulated," adds the professor.
The important thing, he believes, is not to think of embryo-like models as the real thing, even though they are very similar.
"If they were made from human stem cells, and it was accepted that they were never transplanted into the uterus, we would never know if they were equivalent."
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