The writer is a science commentatorEgg plus sperm equals embryo. It is hard to think of a more basic rule in biology. Still, rethink it we must. Last week, at a stem cell conference in Massachusetts, researchers revealed they had created “synthetic” human embryos using neither eggs nor sperm. These structures, more correctly called embryo models, were instead grown from single living stem cells, which were derived from a real human embryo. It was not a one-off achievement: another team created an embryo model with a heartbeat.

The apparent breakthroughs — yet to be published in peer-reviewed journals — are intended to shed light on infertility, pregnancy loss and birth defects. But they also raise moral and legal concerns. Science, and society more widely, now needs to consider how closely these lab-cultivated ersatz embryos, essentially tapestries of tissue, approximate to the real thing. An open discussion — along with updated regulation and careful consideration of how such work might be commercialised — will encourage public support for valuable research that could lead to treatments. 

About one in five pregnancies ends in miscarriage. Yet science affords surprisingly little insight into the reasons, partly because studying embryos in vivo is difficult. The alternative is to scrutinise embryo development in the lab, including using leftover embryos donated by IVF patients. The research use of human embryos is governed by strict laws that bow to a developmental timeline.