By: Emily Wang

Scientists are making human embryos out of stem cells to study why and how pregnancy goes awry. They are not made from egg and sperm—and therefore cannot bear life. However, “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,” said Insoo Hyun, a director of life sciences at Boston’s Museum of Science.

Using models also avoids the difficulty of having to use real embryos in research, he said. Scientists avoid using real embryos because it’s just not possible. Teams with researchers from the United States and England have shared their work in studies published on July 4 in the journal Nature. Other scientists in Israel and China posted reports on their work earlier in June 2023 as well.

While previous models mimicked pre-embryos, the latest ones model an embryo after it has implanted in the uterus. Real embryos can be very hard to see at that stage since they burrow into the uterus. The authors of ”Carnegie in 4D? Stem-cell-based models of human embryo development“ (2022) described models that resemble human embryos nine to 14 days after fertilization: “If we can experimentally model this period, 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,” said author Berna Sozen, who studies stem cell biology at Yale University.

Guidelines from the International Society for Stem Cell Research said that scientists cannot put a human embryo model into either a human or non-human uterus. For decades, the society had a related “14-day rule” that told researchers on how long actual embryos can be grown in the lab—which the group recommended not stressing under limited circumstances in 2021. Since the models are not real embryos, they don’t need to follow the rule.