Embryo models might offer a way to go down that path with even fewer legal and ethical restrictions. They are not legally considered to be embryos because they do not have the potential to grow into viable organisms. So even under present guidelines and regulations in many countries, if embryo models can be grown through gastrulation and beyond, it could become legal for the first time to experimentally study human development and perhaps lead to a better understanding of defects that cause miscarriages or deformities.

But if embryo models can indeed grow that far, at what point do they stop being models and become equivalent to the real thing? The better and further along the models get, the blurrier the biological and ethical boundaries become.

That dilemma was hypothetical when embryo models could only recapitulate the very earliest stages of development. It isn’t anymore.

Turning Stem Cells Into Embryos

Embryo models are generally made from embryonic stem cells, “pluripotent” cells derived from early embryos that can develop into every tissue type in the body. By the time an embryo has reached the blastocyst stage — around day 5 or 6 in human development — it consists of several cell types. Its hollow shell is made of cells that will form the placenta (called trophoblast stem cells, or TSCs) and the yolk sac (the extra-embryonic endoderm, or XEN cells). The pluripotent cells that will become the fetus are confined to a blob on the inside of the blastocyst wall, and it is from them that embryonic stem cells can be cultured.

Experiments in the 1990s and early 2000s showed that embryonic stem cells extracted from one blastocyst and transferred into another can still become an embryo capable of developing all the way to full-term birth as a healthy animal. But the support provided by TSCs and XEN cells is essential — embryonic stem cells alone can’t get past the first few days of development unless they are in a blastocyst.

More recent research, however, shows that embryolike structures can be made from scratch from the respective cell types. In 2018, Zernicka-Goetz and her colleagues showed that assemblies of embryonic stem cells, TSCs and XEN cells from mice could self-organize into a hollow form shaped like a peanut shell and comparable in appearance to a regular embryo undergoing gastrulation. As gastrulation proceeded, some of the embryonic stem cells showed signs of getting more specialized and mobile as a prelude to the development of internal organs.

But those early embryo models were flawed, Zernicka-Goetz said, because the added XEN cells were at too late a developmental stage to wholly fulfill their role. To solve that problem, in 2021 her group found a way to convert embryonic stem cells into early-stage XEN cells. “When we placed [embryonic stem cells], TSCs and these induced-XEN cells together, they could now undergo gastrulation properly and initiate development of organs,” she said.

Last summer in Nature, Zernicka-Goetz and her collaborators described how they had used a rotating bottle incubator to extend the growth of their mouse embryo models by another crucial 24 hours, to day 8.5. Then the models formed “all regions of the brain, beating hearts and so on,” she said. Their trunk showed segments arising for development into different parts of the body. They had a neural tube, a gut and the progenitors of egg and sperm cells.

In a second paper published around the same time in Cell Stem Cell, her group induced embryonic stem cells to become TSCs as well as XEN cells. Those embryo models, cultivated in the rotating incubator, developed to the same advanced stage.

Meanwhile, Hanna’s team in Israel was growing mouse embryo models in a similar way, as they described in a paper in Cell that was published shortly before the paper from Zernicka-Goetz’s group. Hanna’s models too were made solely from embryonic stem cells, some of which had been genetically coaxed to become TSCs and XEN cells. “The entire synthetic organ-filled embryo, including extra-embryonic membranes, can all be generated by starting only with naïve pluripotent stem cells,” Hanna said.

Hanna’s embryo models, like those made by Zernicka-Goetz, passed through all the expected early developmental stages. After 8.5 days, they had a crude body shape, with head, limb buds, a heart and other organs. Their bodies were attached to a pseudo-placenta made of TSCs by a column of cells like an umbilical cord.

“These embryo models recapitulate natural embryogenesis very well,” Zernicka-Goetz said. The main differences may be consequences of the placenta forming improperly, since it cannot contact a uterus. Imperfect signals from the flawed placenta may impair the healthy growth of some embryonic tissue structures.

Without a better substitute for a placenta, “it remains to be seen how much further these structures will develop,” she said. That’s why she thinks the next big challenge will be to take embryo models through a stage of development that normally requires a placenta as an interface for the circulating blood systems of the mother and fetus. No one has yet found a way to do that in vitro, but she says her group is working on it.

Hanna acknowledged that he was surprised by how well the embryo models continued to grow beyond gastrulation. But he added that after working on this for 12 years, “you are excited and surprised at every milestone, but in one or two days you get used to it and take it for granted, and you focus on the next goal.”

Jun Wu, a stem cell biologist at the University of Texas Southwestern Medical Center in Dallas, was also surprised that embryo models made from embryonic stem cells alone can get so far. “The fact that they can form embryolike structures with clear early organogenesis suggests we can obtain seemingly functional tissues ex utero, purely based on stem cells,” he said.

In a further wrinkle, it turns out that embryo models do not have to be grown from literal embryonic stem cells — that is, stem cells harvested from actual embryos. They can also be grown from mature cells taken from you or me and regressed to a stem cell-like state. The possibility of such a “rejuvenation” of mature cell types was the revolutionary discovery of the Japanese biologist Shinya Yamanaka, which won him a share of the 2012 Nobel Prize in Physiology or Medicine. Such reprogrammed cells are called induced pluripotent stem cells, and they are made by injecting mature cells (such as skin cells) with a few of the key genes active in embryonic stem cells.

So far, induced pluripotent stem cells seem able to do pretty much anything that real embryonic stem cells can do, including growing into embryolike structures in vitro. And that success seems to sever the last essential connection between embryo models and real embryos: You don’t need an embryo to make them, which puts them largely outside existing regulations.

Growing Organs in the Lab

Even if embryo models have unprecedented similarity to real embryos, they still have many shortcomings. Nicolas Rivron, a stem cell biologist and embryologist at the Institute of Molecular Biotechnology in Vienna and one of Zernicka-Goetz’s collaborators, acknowledges that “embryo models are rudimentary, imperfect, inefficient and lack the capacity of giving rise to a living organism.”

The failure rate for growing embryo models is very high: Fewer than 1% of the initial cell clusters make it very far. Subtle abnormalities, mostly involving disproportionate organ sizes, often snuff them out, Hanna said. Wu believes more work is needed to understand both the similarities to normal embryos and the differences that may explain why mouse embryo models haven’t been able to grow beyond 8.5 days.

Still, Hanna is confident that they will be able to extend that limit by improving the culture device. “We can currently grow [IVF] mouse embryos ex utero until day 13.5 — the equivalent for human embryos will be around day 50 to 60,” he said. “Our system opens the door.”

He added, “When it comes to studying early human development, I believe this is the only possible way.”

Marta Shahbazi, a cell biologist at Cambridge who works on embryogenesis, agrees. “For humans, an equivalent system [to mouse embryo models] would be really useful, because we don’t have an in vivo alternative to study gastrulation and early organogenesis,” she said.