The recipe for mammalian life is simple: take an egg, add sperm, and wait. But two new articles demonstrate that there is another way. Under the right conditions, stem cells can divide and self-organize into an embryo on their own. In studies published in Cell1 and Nature2 this month, two groups report that they have been growing synthetic mouse embryos for longer than ever before. The embryos grew for 8.5 days, long enough for them to develop distinct organs – a beating heart, an intestinal tube and even neural folds.
The process is far from perfect. Only a tiny fraction of cells develop these characteristics and those that do do not fully mimic a natural embryo. But the work still represents a major breakthrough that will help scientists see organ development in unprecedented detail. “It’s very, very exciting,” says Jianping Fu, a bioengineer at the University of Michigan in Ann Arbor. “The next big step in this field will most likely be a synthetic stem cell-based human embryo,” he says.
Both research teams achieved the feat using similar techniques. Magdalena Zernicka-Goetz, a developmental and stem cell biologist at laboratories at the University of Cambridge, UK, and the California Institute of Technology in Pasadena, has been working on this problem for a decade. “We started with only embryonic stem cells,” she says. “They can mimic the early stages of development, but we couldn’t go any further.” Then, a few years ago, his team discovered3 that when they added stem cells that give rise to the placenta and yolk sac, their embryos grew further. Last year they demonstrated4 that they could use this technique to grow embryos up to day 7. In their latest paper, published in Nature Today, Zernicka-Goetz’s team describes how they grew embryos for another 1.5 days.
Zernicka-Goetz’s team did this using a technique developed by Jacob Hanna, a stem cell biologist at the Weizmann Institute of Science in Israel, who has also been working on this problem for years. Last year, Hanna’s team reported5 that they had developed a device that allowed them to grow natural mouse embryos for an unprecedented length of time outside the womb. This incubator, which held embryos from day 5 to day 11, takes aspects of earlier technology — in which embryos reside in glass flasks that spin on a Ferris wheel-like system — and adds ventilation. The ventilation system controls the mixture of oxygen and carbon dioxide entering the vials, as well as the pressure.
After Hanna’s paper was published last year, her team shared part of their incubator with other developmental and stem cell biologists. “We shared the brain of this machine with everyone who asked for it,” he says, including Zernicka-Goetz and his colleagues, who modified it slightly for their experiments. In an article published in Cell on August 1, Hanna’s team describes how they used the system to also grow embryos for 8.5 days. Full gestation in mice lasts about 20 days.
This period is long enough for regions of the brain to develop, the heart to start beating, and the neural and intestinal tubes to form. These synthetic embryos look a lot like the natural embryos that form when mouse sperm meet the egg, but they “were not 100% identical,” says Hanna. “You can see some defects and some changes in organ size.”
Each team grew their embryos by combining three different cell types, and Hanna’s team also managed to create all three types from naïve embryonic stem cells. “It offers a way to simplify the process,” says Hanna. “You can start everything from one population.” The Zernicka-Goetz team reported a similar achievement in a published preprint6 on bioRxiv (In their Nature article, the researchers relied on placental precursor cells from a cell line to create the embryos.)
Zernicka-Goetz’s team also conducted an experiment in which they knocked out a gene called Pax6, which plays a key role in brain development. When they knocked out this gene, the mouse heads did not develop properly, mimicking what happens in natural embryos that lack this gene. The result demonstrates “that the system is actually functional,” says Zernicka-Goetz.
“These two papers reinforce each other,” says Martin Pera, a stem cell biologist at the Jackson Laboratory Center for Precision Genetics in Bar Harbor, Maine. “Two very competent groups can really produce quite similar results independently.”
For researchers, these synthetic models have many advantages over natural embryos created from eggs and sperm. Because they grow outside the uterus, they are much easier to observe. They are also easier to manipulate using genome editing tools. “We can disrupt, we can manipulate, we can knock out all possible mouse or human genes,” Fu says. This could make them useful for discovering the role of different genes in birth defects or developmental disorders. Zernicka-Goetz plans to use this model to understand why pregnancies fail.
Hanna hopes to use the technique to grow synthetic human embryos that can be a source of new organs and tissues for people who need them.
What about humans?
But translating this work into humans will not be easy. Researchers have persuaded human stem cells to become blastocysts and even mimic some aspects of gastrulation – when the early embryo arranges itself into distinct layers made up of different cell types. But reaching the stage of organ formation in human cells, which occurs about a month after fertilization, presents a significant technical challenge. Still, Ali Brivanlou, a developmental biologist at Rockefeller University in New York, is optimistic. “The field is not too far.”
And the more advanced these embryos are, the greater the ethical concerns. A key question is whether these synthetic structures should be considered embryos, a point of debate in the field. The International Society for Stem Cell Research has long advised against culturing human embryos beyond day 14 (equivalent to day 6 in a mouse) – around the time the “primitive steak” appears, the structure that marks the onset of gastrulation. In 2021, the society removed the limit and issued guidelines stating that such research should have a compelling scientific rationale and should use the minimum number of embryos necessary to achieve the scientific goal.
Still, Pera sees the need for an ongoing conversation about the ethics of these role models. Researchers have been working on human embryo models for years without much opposition. But he worries about a backlash as researchers begin to develop models of human embryos that begin to develop organs. “The reaction to this could jeopardize this whole area of research,” he says. “It’s important that people know what’s on offer and that it’s done with some sort of ethical consensus,” adds Pera. “You have to go there carefully”