But within 20 days of transfer, the monkey blastoids stopped developing and seemed to come apart, say Liu and colleagues, who published their results in the journal Cell Stem Cell. This suggests the blastoids still aren’t perfect replicas of normal embryos, says Alfonso Martinez Arias, a developmental biologist at Pompeu Fabra University in Barcelona, Spain. For the time being, “it clearly doesn’t work,” he says.
That might be because a typical embryo is generated from an egg, which is then fertilized by sperm. A blastoid made from stem cells might express genes in the same way as a normal embryo, but it may be missing something crucial that normally comes from an egg, says Martinez Arias.
There’s also a chance that the team might have seen more progress if the experiment had been done in more monkeys. After all, of the 484 blastoids that were developing at day seven, only five survived to day 17. And getting an embryo to implant in the uterus is a tricky business, says Chuva de Sousa Lopes. “Even when you do IVF in humans, it’s one of the bottlenecks in getting pregnant,” she says. “Perhaps if you did this with 100 monkeys, you would have two that would get pregnant further.”
Monkey lives are precious, though, says Martinez Arias, and such large experiments would probably not be considered ethical.
A model embryo
None of this means that the blastoids are not useful. They still provide a good model of what happens in the earliest stages of embryo development in monkeys—and potentially in humans.
Researchers hope that monkey blastoids will help us learn more about human embryos. We know very little about how the union of sperm and egg eventually leads to the development of our organs and nervous system—and why things can sometimes go wrong. Scientists are generally not allowed to study human embryos in a lab beyond 14 days after fertilization. And recently published international guidelines stress that human blastoids should never be implanted into a person or any other animal.
“We want to understand human development, and it is not safe to transfer human blastoids [into people],” says Rivron. “We have to find an alternative. And nonhuman primates are the closest relatives to humans.”
Scientists hope that this type of research can tell us more about human pregnancy, including why some people struggle to conceive and why some miscarriages happen. Because scientists could generate infinite numbers of blastoids, they wouldn’t need to rely on animals as embryo donors. And they would be able to test drugs on hundreds or thousands of blastoids in the hope of discovering ways to improve IVF, says Naomi Moris, who researches embryo development at the Crick Institute in London.