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Because of medical and ethical challenges, there’s much we don’t know about the first few weeks of human embryo development, says biologist Jacob Hanna of Israel’s Weizmann Institute of Science. Hanna’s team used naive stem cells treated with chemicals to nudge them into becoming four types of cells found in early embryos. One percent of the treated stem cells spontaneously formed a structure similar to a human fetus, which researchers allowed to grow for 14 days. While distinctly not human, the model is close enough to give researchers potential insights into fertility, miscarriage, and more, for the first time.

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