Synthetic Humans — Should They Be Used for Risky Experiments?

Researchers have succeeded in creating synthetic embryos for the first time, without stopping to first answer the question of if they should be created at all. The embryos exist without the need for egg, sperm or sexual reproduction of any kind. They were engineered from stem cells and provide a window into the earliest days of human development.¹

The scientists behind the synthetic embryos, including Magdalena Żernicka-Goetz, of the University of Cambridge and the California Institute of Technology, hope to study this so-called “black box” development period, as researchers are only legally allowed to grow human embryos up to 14 days.²

“We can create human embryo-like models by the reprogramming of [embryonic stem] cells,” Żernicka-Goetz said at the 2023 International Society for Stem Cell Research meeting in Boston.³ Further, the synthetic human embryos aren’t subject to the 14-day rule.⁴

Synthetic Human Embryos Form Distinct Cell Lines

The embryos were grown to the gastrulation stage of development, when distinct cell lines develop. While the embryos do not have a beating heart, gut or beginnings of a brain, they have primordial cells that are the precursors to egg and sperm. Żernicka-Goetz told The Guardian:⁵

“Our human model is the first three-lineage human embryo model that specifies amnion and germ cells, precursor cells of egg and sperm. It’s beautiful and created entirely from embryonic stem cells.”

The preprint study, which hasn’t been peer-reviewed yet, was published in bioRxiv,⁶ alongside a similar study by stem-cell biologist Jacob Hanna and colleagues at the Weizmann Institute of Science in Rehovot, Israel.⁷ Both studies have received some criticism that the synthetic embryos aren’t as advanced as they initially appear. Nature reported:⁸

“Alfonso Martinez Arias, a developmental biologist at Pompeu Fabra University in Barcelona, Spain, says there is ‘nothing’ in the results described by Zernicka-Goetz and her colleagues that can be considered analogous to real 14-day embryos.

‘What we can see is masses of cells separated into compartments, but no embryo-like organization,’ he says. He thinks that the over-expression of some genes needed to produce the extra-embryonic cell types ‘confuses what cells do,’ and argues that the results do not show anything that goes beyond earlier work.”

However, others have praised the work. Hanna’s team, which also produced a synthetic embryo-like structure from human stem cells, also stopped their experiment at the 14-day cutoff for human embryos, but Żernicka-Goetz and others have argued that allowing the synthetic embryos to develop longer would be useful to science.⁹

Speaking with Science, developmental biologist Jesse Veenvliet of the Max Planck Institute of Molecular Cell Biology and Genetics said of the synthetic embryo developed by Hanna’s team, “The similarity to the natural embryo is remarkable, almost uncanny.”¹⁰

Prior to the synthetic human embryo, researchers created synthetic mouse embryos.¹¹ This was less than a year ago, showing how rapidly the field is moving. While the implications for research are exciting, there are significant ethical implications, since the synthetic embryos could, at least theoretically, grow into a human. Robin Lovell-Badge, told The Guardian:¹²

“The idea is that if you really model normal human embryonic development using stem cells, you can gain an awful lot of information about how we begin development, what can go wrong, without having to use early embryos for research.”

Legal and Ethical Implications Are Significant

While it’s currently against the law to attempt to implant a synthetic embryo into a human womb, the science is rapidly outpacing related regulations. “If the whole intention is that these models are very much like normal embryos, then in a way they should be treated the same,” Lovell-Badge told The Guardian. “Currently in legislation they’re not. People are worried about this.”¹³

In animal studies, synthetic embryos implanted into mice wombs did not survive. Similarly, when synthetic monkey embryos were implanted into monkey wombs, pregnancies were induced, although the embryos spontaneously stopped developing after a few days.¹⁴

However, if the synthetic embryos could one day grow into adults, we’d be entering into uncharted legal and ethical territory. Ethicist J. Benjamin Hurlbut of Arizona State University told Science that synthetic embryos represent “a matter of significant moral discussion and of significant moral concern.”¹⁵