Researchers from the University of Cambridge have harnessed mouse stem cells to create model “synthetic” embryos that comprise a brain, a beating heart, and the foundations of all the other organs of the body.
The team, led by Magdalena Zernicka-Goetz, PhD, mimicked natural processes, in the lab,without the use of eggs or sperm, by guiding the three types of stem cells found in early mammalian development to the point where they start interacting. By inducing the expression of a particular set of genes and establishing a unique environment for their interactions, the researchers were able to get the stem cells to “talk” to each other.
The stem cells self-organized into structures that progressed through the successive developmental stages until they had beating hearts and the foundations of the brain, as well as the yolk sac where the embryo develops and gets nutrients in its first weeks. Unlike other synthetic embryos, the Cambridge-developed mouse embryo models reached the point where the entire brain, including the anterior portion, began to develop. This is a further point in development than has been achieved in any other stem cell-derived model.
“Our mouse embryo model not only develops a brain, but also a beating heart, all the components that go on to make up the body,” said Zernicka-Goetz, who is a professor in mammalian development and stem cell biology in Cambridge’s department of physiology, development, and neuroscience.
The scientists say their results, which culminate from more than a decade of research that has progressively led to more and more complex embryo-like structures, could help scientists understand why some embryos fail while others go on to develop into a healthy pregnancy. Additionally, the results could be used to guide the development of synthetic human organs for transplantation. “It’s just unbelievable that we’ve got this far,” Zernicka-Goetz continued. “This has been the dream of our community for years, and the major focus of our work for a decade and finally we’ve done it.”
Zernicka-Goetz, together with first author Gianluca Amadei, PhD, described their work in Nature, in a paper titled, “Synthetic embryos complete gastrulation to neurulation and organogenesis,” in which they concluded, “… these complete embryoids are a powerful in vitro model for dissecting the roles of diverse lineages and genes in development … Because ETiX-embryoids capture extensive aspects of development, they provide a significant opportunity to uncover mechanisms of development and disease.”
For a human embryo to develop successfully there needs to be dialogue between the tissues that will become the embryo, and the tissues that will connect the embryo to the mother. In the first week after fertilization, three types of stem cells develop: one will eventually become the tissues of the body, and the other two support the embryo’s development. One of these extraembryonic stem cell types will become the placenta, which connects the fetus to the mother and provides oxygen and nutrients; and the second is the yolk sac, where the embryo grows and where it gets its nutrients from in early development.
“In natural development, the zygote develops into the epiblast, which will form the organism; the extraembryonic visceral endoderm (VE), which contributes to the yolk sac; and the extraembryonic ectoderm (ExE), which contributes to the placenta,” the authors explained. “Stem cells corresponding to these three lineages offer the possibility to completely regenerate the mammalian organism from multiple components, instead of from a single totipotent zygote.” In vitro embryonic stem cells can undergo many aspects of mammalian embryogenesis, the team continued, “… but their developmental potential is substantially extended by interactions with extraembryonic stem cells, including trophoblast stem cells (TSCs), extraembryonic endoderm stem cells (XEN), and inducible-XEN cells (iXEN).”
Many pregnancies fail at the point when the three types of stem cells begin to send mechanical and chemical signals to each other, which tell the embryo how to develop properly. “So many pregnancies fail around this time, before most women realize they are pregnant,” said Zernicka-Goetz, who is also a professor of biology and biological engineering at Caltech. “This period is the foundation for everything else that follows in pregnancy. If it goes wrong, the pregnancy will fail.”
Over the past decade, Zernicka-Goetz’s group ihas been studying these earliest stages of pregnancy, in order to understand why some pregnancies fail and some succeed. “The stem cell embryo model is important because it gives us accessibility to the developing structure at a stage that is normally hidden from us due to the implantation of the tiny embryo into the mother’s womb,” said Zernicka-Goetz. “This accessibility allows us to manipulate genes to understand their developmental roles in a model experimental system.”
To guide the development of their synthetic embryo, the researchers put together cultured stem cells representing each of the three types of tissue in the right proportions and environment to promote their growth and communication with each other, eventually self-assembling into an embryo. They discovered that the extraembryonic cells signal to embryonic cells by chemical signals but also mechanistically, guiding the embryo’s development. “This period of human life is so mysterious, so to be able to see how it happens in a dish—to have access to these individual stem cells, to understand why so many pregnancies fail, and how we might be able to prevent that from happening—is quite special,” said Zernicka-Goetz. “We looked at the dialogue that has to happen between the different types of stem cell at that time—we’ve shown how it occurs and how it can go wrong.”
A major advance in the reported study is the ability to generate the entire brain, in particular the anterior part, which has been a major goal in the development of synthetic embryos. “Our embryo model displays head-folds with defined forebrain and midbrain regions …” the investigators noted. The team’s previous studies had used the same component cells to develop into embryos at a slightly earlier stage. Now, by pushing development just one day further, they say that their model is the very first to signal development of the anterior, and in fact the whole, brain.
“This opens new possibilities to study the mechanisms of neurodevelopment in an experimental model,” said Zernicka-Goetz. “In fact, we demonstrate the proof of this principle in the paper by knocking out a gene already known to be essential for formation of the neural tube, precursor of the nervous system, and for brain and eye development. In the absence of this gene, the synthetic embryos show exactly the known defects in brain development as in an animal carrying this mutation. This means we can begin to apply this kind of approach to the many genes with unknown function in brain development.”
As the authors noted, “Importantly, we were able to replicate the consequences of Pax6 knockout in neurulating embryoids, which illustrates the potential of this complete embryo model to dissect the genetic factors that regulate development without the need of experimental animals.”
In conclusion, they stated, “Here, we show that we can assemble mouse embryonic and extraembryonic stem cells to form an embryo model that develops the brain, neural tube, heart, foregut, somite, allantois, primordial germ cells, and yolk sac structures. This embryo model is able to achieve this entirely through self-organization of these three stem cell types, without the need to provide any additional external signalling cues.”
While the current research was carried out in mouse models, the researchers are developing similar human models, potentially enabling the development specific organ types that could help scientists understand mechanisms behind processes that would be otherwise impossible to study in real embryos. At present, under UK law, human embryos can be studied in the laboratory only up to the fourteenth day of development.
If the methods developed by Zernicka-Goetz’s team are shown to be successful with human stem cells, they could feasibly be used to guide development of synthetic organs as human transplants. “There are so many people around the world who wait for years for organ transplants,” said Zernicka-Goetz. “What makes our work so exciting is that the knowledge coming out of it could be used to grow correct synthetic human organs to save lives that are currently lost. It should also be possible to affect and heal adult organs by using the knowledge we have on how they are made.
“This is an incredible step forward and took 10 years of hard work of many of my team members—I never thought we’d get to this place. You never think your dreams will come true, but they have.”
The newly reported work comes weeks after the publication, in Cell, of a study by a team led by co-author Jacob Hanna, PhD, at the Weizmann Institute. James Briscoe, PhD, principal group leader, assistant research director, Francis Crick Institute, said, that similar to the research recently reported by Hanna and colleagues, the study by Zernicka-Goetz and colleagues represented “valuable proof of concept” demonstration that a synthetic mouse embryo-like structure can be assembled from stem cells. “By combining these cells together, the study shows that it is possible to coax the development of something that resembles a mouse embryo at a stage when the main organs of the body are beginning to be established, including the nervous system, heart, and gut,” Briscoe said.
However, Briscoe pointed out that that formation of the synthetic embryos was very inefficient, that even the successful synthetic embryos “appeared not as well organized as natural embryos,” and that they didn’t develop beyond what would be day 8.5 of normal embryonic development, which is just under halfway through a normal mouse pregnancy.
“This emphasizes how much we still have to learn about how embryos build themselves,’ he noted. “The technique reported in this study is a promising approach to provide new insights into how mammalian embryos organize and construct organs. Nevertheless, the study has broad implications as, although the prospect of synthetic human embryos still requires further research (as human embryos are not identical to mouse embryos), now is a good time to engage in wider discussions about the legal and ethical implications of such research.”