Credit: Weizmann Institute of Science
No egg, sperm, or matrix: synthetic mouse embryo models created solely from stem cells
An egg meets a sperm: this is a necessary first step in the beginnings of life. In embryonic development research, it is also a common first step. However, in a new study published on August 1, 2022 in the journal Cell, researchers at the Weizmann Institute of Science have grown synthetic mouse embryo models outside the womb starting solely with cultured stem cells in a petri dish. This means that 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 also one day allow tissues and organs to be grown for transplant using synthetic embryo models.
A video showing a synthetic mouse embryo model on day 8 of its development; it has a beating heart, a yolk sac, a placenta, and an emerging blood circulation.
“The embryo is the best machine for making organs and the best 3D bioprinter – we tried to emulate 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 the “margin.” In fact, the pioneers of this cellular reprogramming won a Nobel Prize in 2012. However, going in the opposite direction, that is, making the stem cells differentiate into specialized body cells , not to mention whole organs, has proved much more difficult.
“Until now, in most studies, specialized cells were often difficult to produce or aberrant, and tended to form a mixture rather than well-structured tissue suitable for transplantation. We have managed to overcome these obstacles by unleashing the potential of self-organization encoded in stem cells”.
(Left to right): Noah Noverschtern, Prof. Jacob Hanna, Alexander Aguilera-Castrejon, Shadi Tarazi and Carine Joubran. Credit: Weizmann Institute of Science
Hanna’s team built on two previous advances in his lab. One was an efficient method to reprogram stem cells back to a naïve state, that is, at their earliest stage, when they have the greatest potential to specialize into different cell types. The other, described in a scientific paper in Nature in March 2021, was the electronically controlled device the team had developed over seven years of trial and error to grow natural mouse embryos outside the womb. The device keeps embryos bathed in a nutrient solution in continuously moving beakers, simulating the way nutrients are supplied by material blood flow to the placenta and closely monitors oxygen exchange and atmospheric pressure . In previous research, the team had successfully used this device to grow natural mouse embryos from day 5 to day 11.
Here’s 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 grow a synthetic embryo model solely from naïve mouse stem cells that had been grown for years in a Petri dish, bypassing the need to start with a fertilized egg This approach is extremely valuable because it could largely avoid the technical and ethical problems involved in the use of natural embryos in research and biotechnology. Even in the case of mice, some experiments are currently infeasible because they would require thousands of embryos, while access to models derived from mouse embryonic cells, which grow in laboratory incubators by the millions, is virtually impossible. limited
“The embryo is the best machine for making organs and the best 3D bioprinter; we tried to emulate 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 become embryonic organs, the cells were left as they were. Cells in the other two groups were pretreated for 48 hours only to overexpress one of two types of genes: placental or yolk sac master regulators. “We gave these two groups of cells a transient push to give rise to extraembryonic tissues that sustain the developing embryo,” says Hanna.
Development of synthetic embryo models from day 1 (top left) to day 8 (bottom right). All of its early progenitor organs had formed, including a beating heart, an emerging blood circulation, a brain, a neural tube, and an intestinal tract. Credit: Weizmann Institute of Science
Shortly after mixing within the device, the three groups of cells gathered into aggregates, the vast majority of which did not develop properly. But about 0.5 percent, 50 out of about 10,000, went on to form spheres, each of which later became an elongated, embryo-like structure. Because the researchers had labeled each group of cells with a different color, they were able to observe the placenta and yolk sacs forming outside the embryos, and the development of the model proceeded as in a natural embryo. These synthetic models developed normally until day 8.5, nearly halfway through the mouse’s 20-day gestation, at which stage all the progenitors of early organs had formed, including a beating heart, circulation of blood stem cells, a brain with well-formed folds, a nerve tube and an intestinal tract. Compared to natural mouse embryos, the synthetic models showed 95% similarity in both the shape of the internal structures and the gene expression patterns of different cell types. The organs seen in the models gave every indication of being functional.
Day 8 in the life of a mouse embryo: a synthetic model (top) and a natural embryo (bottom). The synthetic models showed 95 percent similarity in both the shape of the internal structures and the gene expression patterns of different cell types. Credit: Weizmann Institute of Science
For Hanna and other stem cell and embryonic development researchers, the study opens up a new field: “Our next challenge is to understand how stem cells know what to do: how they assemble into organs and they find their way to their assigned sites within an embryo. And because our system, unlike the uterus, is transparent, it may prove useful for modeling birth defects and implantation of 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 for growing 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 won’t have to dictate to nascent organs how to develop. The embryo itself does it better.”
A diagram showing the innovative method for growing synthetic models of mouse embryos from stem cells – without egg, sperm or matrix – developed in the laboratory of Prof. Jacob Hanna. Credit: Weizmann Institute of Science
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-Ces , 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, 2022 , Cell.DOI: 10.1016/j.cell.2022.07.028
This research was co-led by Shadi Tarazi, Alejandro Aguilera-Castrejon and Carine Joubran of the Weizmann 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 Noverstern of Weizmann’s Genecular’s Mole. Department; Montaser Haddad and Professor Tsvee Lapidot of the Weizmann Department of Immunology and Regenerative Biology; the Dr. Merav Kedmi of Weizmann’s Department of Life Sciences Core Facilities; Dr. Hadas Keren-Shaul of the Nancy and Stephen Grand Israel National Center for Personalized Medicine; and Dr. Nadir Ghanem, Dr. Suhair Hanna, and Dr. Itay Maza from the Rambam Health Care Campus.
Professor Jacob Hanna’s research is supported by the Institute Dr. Barry Sherman of Medicinal Chemistry; the Helen and Martin Kimmel Institute for Stem Cell Research; and Pascal and Ilana Mantoux.