Scientists announced that they’ve been able to grow embryo-like structures made from human stem cells, featuring a heartbeat and traces of blood. The structures—made without the need for egg, sperm or fertilization—replicated some of the cells that would appear in early pregnancy. But these synthetic forms were designed intentionally to lack the tissue necessary to eventually turn into a fetus. The scientists behind the model embryos explained that this safeguard is what makes the research ethically sound. “I’d like to emphasize that these are neither embryos nor are we trying to make embryos actually,” Dr. Jitesh Neupane, of the University of Cambridge’s Gurdon Institute, said during the International Society for Stem Cell Research’s annual meeting in Boston on Saturday. “They are just models that could be used to look into specific aspects of human development.” Even so, Neupane was shocked to discover the heartbeat during the research. “I randomly took my plate under the microscope and when I saw the [heartbeat] for the first time I was scared, honestly. I had to look down and look back again,” he told The Guardian before his talk. “It was overwhelming for me. People get emotional when you see the heartbeat.”