Developmental biologists have devised a method for growing mouse embryos outside a uterus for longer than ever before, giving them an unprecedented view of how mammalian organs and limbs form—a process previously hidden inside a mother’s body. Researchers in Israel report today that the new system, which includes rotating bottles filled with nutrients, kept the mouse embryos alive from roughly day five of development until day 11, about halfway through the animals’ 20-day gestation. By that time the embryos have formed hind limbs and all their major organs.
“It looks very spectacular,” says Max Planck Institute for Molecular Genetics developmental biologist Alexander Meissner, who was not involved in the work. “The fact that [the researchers] can culture these embryos and keep them alive for such a long time—it’s amazing.”
To develop the new technique, Weizmann Institute of Science developmental biologist Jacob Hanna and his colleagues engaged in more than 7 years of trial and error. Previously, scientists could grow mouse embryos in the lab for the first 3 or 4 days of development. In normal mouse pregnancies, when the embryo implants in the wall of the uterus, the placenta starts to form, and the embryo’s cells start to differentiate into more specific types of stem cells that will form different tissues. Beyond that point, it was difficult to grow developing mouse embryos outside the uterus for more than a day or two.
But the new system increased that time by nearly 1 week, Hanna and colleagues report today in Nature. Beyond nutrients in which they bathed embryos, the rotating glass bottles helped provide the tiny embryos with sufficient oxygen and atmospheric pressure. Keeping the air pressure and oxygen saturation at the right levels was the hardest part, Hanna says. “We learned how to control the ventilation system,” he adds with a laugh.
The team’s two-step process starts by growing the embryos, extracted from a pregnant mouse just before implantation, on culture plates from day five to day seven of development. That is when the embryos undergo a process called gastrulation, in which they transform from a hollow ball of cells into a multilayered structure with specific cells destined to form different tissues. The researchers then transfer the embryos to a set of rotating jars, where they are able to keep them alive for an additional 4 days of development.
Hanna’s team also introduced genetic tags to certain cells in the mouse embryos that let the scientists follow the fates of those cells as development progressed. Finally, they also showed that they could add certain kinds of human neural cells to the embryos, which partly incorporated them into the developing brain. Both techniques can be used to help understand how normal development happens—and how it sometimes goes wrong.
The technique “opens new doors by making embryos accessible for a detailed study of many aspects of their development,” says Magdalena Zernicka-Goetz, a developmental biologist at the California Institute of Technology. She and her colleagues have also developed ways to culture mouse embryos through gastrulation, but Hanna’s system pushes the process even further. “[It] will make a big contribution to the field, which we are certainly planning to exploit,” Zernicka-Goetz says.
Hanna says the next step is to attempt to grow mouse embryos created by in vitro fertilization, rather than ones from natural pregnancies. That would “combine everything so we can go from day zero to day 11,” he says.
Meissner doesn’t think the technique could be stretched much longer. “There’s a natural limit in terms of how big [the embryos] can get without nutrients and blood supply. It’s not obvious to see how you can go from here to an actual ex utero delivery,” he says. But the technique will make it possible to answer a range of new questions about development, he says. “The toolbox has become quite powerful.”
The new finding comes as other researchers are developing systems to grow human cells into clusters resembling an early stage of embryo development, the relatively undifferentiated balls of cells called blastocysts. Two groups reported their techniques in Nature today.
Hanna says he has not yet attempted to use the rotating bottle method to grow human embryos—something that would violate guidance from the International Society for Stem Cell Research that recommends against culturing human embryos for longer than 14 days of development, when gastrulation takes place. Those guidelines are being revised, however, and updated recommendations are expected in May. “Of course it’s scientifically very important to do such experiments with human embryos, because we have so little information about these stages of human development,” Hanna says. Israeli law would not prohibit such experiments, he said, if they received ethical approval.
sciencemag.org, 17 March 2021