Without eggs, sperm or uterus: synthetic mouse embryo models created only from stem cells
An egg meets a sperm – it’s a necessary first step at the beginning of life. In research on embryonic development, it is also a common first step. However, in a new study published August 1, 2022, in the journal Cell, researchers at the Weizmann Institute of Science have developed models of synthetic mouse embryos outside the womb starting with just stem cells grown in a Petri dish. This means they are grown without the use of fertilized eggs. This method opens up new horizons for studying how stem cells form various organs in the developing embryo. It may one day allow the cultivation of tissues and organs for transplantation using synthetic embryo models.
A video showing a synthetic mouse embryo model at day 8 of its development; it has a beating heart, yolk sac, placenta, and emerging blood circulation.
“The embryo is the best organ-building machine and the best 3D bio-printer – we tried to mimic what it does,” says Professor Jacob Hanna of Weizmann’s Department of Molecular Genetics, who led the Research Team.
Hanna explains that scientists already know how to restore mature cells to “stem.” In fact, the pioneers of this cellular reprogramming won a Nobel Prize in 2012. However, to go in the opposite direction, that is, to cause stem cells to differentiate into specialized body cells, let alone form whole organs, proved to be much more difficult.
“Until now, in most studies, specialized cells were often difficult to produce or aberrant, and they tended to form a hodgepodge instead of well-structured tissue suitable for transplantation. We have succeeded in overcoming these obstacles by unlocking the self-organizing potential encoded in stem cells.
Hanna’s team built on two previous advances in her lab. One was an effective method for reprogramming stem cells to a naïve state – that is, their earliest stage – when they have the greatest potential to specialize in different cell types. The other, described in a scientific article in Nature in March 2021, was the electronically controlled device the team had developed over seven years of trial and error for growing natural mouse embryos outside the womb. The device keeps embryos bathed in nutrient solution inside continuously moving beakers, simulating how nutrients are delivered by material blood flow to the placenta, and tightly controls the exchange of oxygen and atmospheric pressure. In previous research, the team had successfully used this device to grow natural mouse embryos from day 5 to day 11.
This is how synthetic mouse embryo models were grown outside the womb: a video showing the device in action. Continuously moving beakers simulate the natural supply of nutrients, while oxygen exchange and atmospheric pressure are tightly controlled.
In the new study, the team set out to develop a synthetic embryo model solely from naïve mouse stem cells cultured for years in a Petri dish, without having to start with a fertilized egg. This approach is extremely valuable because it could, to a large extent, circumvent the technical and ethical problems associated with the use of natural embryos in research and biotechnology. Even in the case of mice, some experiments are currently unfeasible because they would require thousands of embryos, while access to models derived from mouse embryo cells, which grow by the millions in laboratory incubators, is virtually unlimited.
“The embryo is the best organ building machine and the best 3D bio-printer – we tried to imitate what it does.”
Before placing the stem cells in the device, the researchers separated them into three groups. In one, which contained cells destined to develop into embryonic organs themselves, the cells were left as they were. The cells of the other two groups were pretreated for only 48 hours to overexpress one of two types of genes: the master regulators of the placenta or the yolk sac. “We gave these two groups of cells a transient boost to give rise to extraembryonic tissues that support the developing embryo,” says Hanna.
Shortly after being mixed inside the device, the three groups of cells came together into clumps, the vast majority of which failed to grow properly. But about 0.5% – 50 out of about 10,000 – then formed spheres, each of which later became an elongated structure resembling an embryo. Since the researchers had labeled each group of cells with a different color, they were able to observe the formation of the placenta and yolk sacs outside the embryos and the development of the model taking place as in a natural embryo. These synthetic models developed normally until day 8.5 – almost halfway through the mouse’s 20-day gestation – the stage at which all the first organ progenitors were formed, including a beating heart, circulation blood stem cells, brain with well formed folds, nervous system tube and intestinal tract. Compared to natural mouse embryos, the synthetic models displayed 95% similarity in both the shape of the internal structures and the gene expression patterns of the different cell types. The organs seen in the models gave every indication of being functional.
For Hanna and other stem cell and embryonic development researchers, the study presents a new area: “Our next challenge is to understand how stem cells know what to do – how they self-assemble into organs and find their way to their assigned points inside an embryo.And because our system, unlike a uterus, is transparent, it can prove useful in modeling birth and implantation defects in human embryos.
In addition to helping to reduce the use of animals in research, synthetic embryo models could in the future become a reliable source of cells, tissues and organs for transplantation. “Instead of developing a different protocol to grow each type of cell – for example, kidney or liver – we may one day be able to create a synthetic embryo-like model and then isolate the cells we need. We will not need to dictate to emerging organs how they should develop. The embryo itself does it better.
Reference: “Post-Gastrulation Synthetic Embryos Generated Ex Uterine from Naive Mouse ESCs” by Shadi Tarazi, Alejandro Aguilera-Castrejon, Carine Joubran, Nadir Ghanem, Shahd Ashouokhi, Francesco Roncato, Emily Wildschutz, Montaser Haddad, Bernardo Oldak, Elidet Gomez-Cesar , Nir Livnat, Sergey Viukov, Dmitry Lukshtanov, Segev Naveh-Tassa, Max Rose, Suhair Hanna, Calanit Raanan, Ori Brenner, Merav Kedmi, Hadas Keren-Shaul, Tsvee Lapidot, Itay Maza, Noah Novershtern and Jacob H. Hanna, August 2022, Cell.
This research was co-directed by Shadi Tarazi, Alejandro Aguilera-Castrejon and Carine Joubran from Weizmann’s Department of Molecular Genetics. Study participants also included Shahd Ashouokhi, Dr. Francesco Roncato, Emilie Wildschutz, Dr. Bernardo Oldak, Elidet Gomez-Cesar, Nir Livnat, Sergey Viukov, Dmitry Lokshtanov, Segev Naveh-Tassa, Max Rose and Dr. Noa Novershtern Weizmann’s molecular genetics. Department; Montaser Haddad and Professor Tsvee Lapidot of Weizmann’s Department of Immunology and Regenerative Biology; Dr. Merav Kedmi from Weizmann’s Central Life Sciences Facilities Department; Dr. Hadas Keren-Shaul of the National Center for Personalized Medicine Nancy and Stephen Grand Israel; and Dr. Nadir Ghanem, Dr. Suhair Hanna and Dr. Itay Maza from Rambam Health Care Campus.
Professor Jacob Hanna’s research is supported by the Dr. Barry Sherman Institute for Medicinal Chemistry; the Helen and Martin Kimmel Institute for Stem Cell Research; and Pascal and Ilana Mantoux.