We can use stem cells to make embryos. How far should we go?

Perhaps the biggest question rests on how these stem cell-derived structures look like embryos. For some scientists, this is a hopeless situation. If blastoids look too much like embryos, then many believe that research with them should be restricted in the same way that we control work with human embryos.

But if they don’t look enough like embryos, there’s no point in using them for research, says Chuva de Sousa Lopes. “At the moment, it’s so hard to understand how close they are or how different they are,” she says.

Scientists tend to look at the size and shape of structures, and the genes their cells express, to determine how similar they are to typical embryos. But there are other important aspects to consider.

“We first need to agree on what an embryo is,” says Naomi Moris, a developmental biologist at the Crick Institute in London. “Is this the thing that is only generated by the fusion of a sperm and an egg? Is it something to do with the types of cells it has, or the (shape) of the structure?

It is perhaps more related to the potential of the structure. A human embryo could go on to form a person. Human blastoids cannot become human. Again.

As technology advances, it seems increasingly likely that one day stem cell-derived embryos will be able to develop into living animals. “Theoretically, if you have all the right cell types…they could go further,” says Rossant. “Never say never.”

However we define blastoids and other embryo-like structures, now is the time to start regulating how we develop and study them. Rossant is one of many scientists I’ve spoken to who agree that, given how much these structures resemble embryos, they should probably be subject to the same rules and regulations that cover research on normal embryos.

“The big risk is… if we had a rogue player who went really fast (with human cells) and developed something that caused a public reaction,” Moris explains.