Four teams of scientists have made models of human embryos that do not require an egg or sperm, they announced in June. While these “synthetic embryos” have raised questions about regulating this type of research, scientists hope the models will unlock answers about the early days of human development.
After an egg and sperm cell fuse, the fertilized egg travels through the fallopian tubes to the uterus, while its cytoplasm divides into smaller cells. This ball of cells then attaches to the lining of the uterus.
But scientists don’t know much about what happens to the cells in the next couple of weeks. This period is “truly a black box,” Jacob Hanna, a co-author of one the new studies and a stem cell biologist at the Weizmann Institute of Science in Israel, tells Science’s Mitch Leslie.
Studying this phase of an embryo’s development is difficult: Scientists can’t easily observe it when it’s developing inside a womb, and in the lab, several countries have laws against studying actual human embryos beyond 14 days after fertilization.
“We sought to develop a tool to ask specific questions about the second week of human embryo development, since using actual human embryos in research is ethically and technically challenging,” Magdalena Żernicka-Goetz, a co-author of one of the new studies and a developmental biologist at the University of Cambridge in the U.K., tells Nature News’ Philip Ball.
Instead of using real embryos, researchers have begun creating models using human stem cells, which can develop into many different types of cells. In the new research, the scientists coax stem cells to grow into the components of embryos. The stem cells were grown in a lab, but they came from embryos originally, writes New Scientist’s Clare Wilson.
Hanna and Żernicka-Goetz, with their respective teams, worked on separate model embryos that demonstrate human development through 14 days after fertilization. Hanna’s team’s model contains membranes within and outside of the embryo, and Żernicka-Goetz’s team’s model displays tissues that could later create the placenta and yolk sac, writes Laura Ungar of the Associated Press (AP).
The scientists want to use the models to study the period of development when many pregnancies fail, learn more about developmental disorders and provide insights into treatments for infertility and diseases. The stem cell embryos could also help researchers understand how organs develop or test drugs used during pregnancy.
“If we can experimentally model this period [nine to 14 days after fertilization], then we can finally start asking questions about how human development happens in those very early stages that are normally hidden within the body of the mother,” Berna Sozen, a study co-author and developmental stem cell biologist at Yale University, tells the AP.
Versions of two of the papers were posted online last week in the journal Nature while they are still undergoing editing. The two other studies were posted earlier in June on the preprint server bioRxiv and have not yet been peer reviewed.
This research remains controversial. For one, some scientists think the models are not yet practical representations of actual embryos.
“What we can see is masses of cells separated into compartments, but no embryo-like organization,” Alfonso Martinez Arias, a developmental biologist at Pompeu Fabra University in Spain who was not involved in the research, says to Nature News of the work by Żernicka-Goetz’s team.
“The work is in very early stages, and the current methods are far from reliable,” Alysson Muotri, a developmental biologist at the University of California San Diego who did not contribute to the research, tells the New York Times’ Carl Zimmer.
Other scientists wonder about the ethics surrounding the use of model embryos.
“If the whole intention is that these models are very much like normal embryos, then in a way, they should be treated the same,” Robin Lovell-Badge, a stem cell biologist at the Francis Crick Institute in the U.K. who wasn’t involved in the research, tells the Guardian’s Hannah Devlin. “Currently, in legislation they’re not. People are worried about this.”
Previously, researchers have made synthetic embryos from mouse and monkey stem cells. But even with human stem cells, some scientists say the synthetic embryos help avoid the ethical minefield of using real, fertilized eggs.
“They’re complete enough to give you a picture of what may be happening in the embryo during pregnancy, but they’re not so complete that you could actually use them for reproduction,” Insoo Hyun, an ethicist at Harvard Medical School who didn’t contribute to the studies, tells the AP. “It just will not work.”