In a laboratory dish, ordinary human stem cells sit in a chemical bath, no sperm involved, no egg, no act of conception. Left alone, a small fraction do something researchers still find hard to believe: they begin to sort themselves by type, cluster together, and build.
A yolk sac takes shape. An outer layer destined to become a placenta wraps around the rest. Two weeks in, the cluster has arranged itself, component by component, into something that looks remarkably like an embryo freshly lodged in a womb, and it secretes the hormone that turns a pregnancy test positive. Nothing here was conceived. It was constructed.
Earlier this year, a sprawling group of the world’s leading developmental biologists did something rare for a field running at full tilt.
Led by Dr. Alfonso Martinez Arias of Pompeu Fabra University in Barcelona and co-signed by more than 40 researchers spanning a dozen countries, they published a white paper, “Human Stem Cell-Based Embryo Models: Innovation, Ethics, and Policy,” in the journal Human Reproduction, calling for the rules they themselves have been working without.
It leaves the law facing a question it has no answer for: at what point does a replica of a human embryo stop being a replica?
