In a challenge to the idea that brain death is final, researchers have revived the disembodied brains of pigs four hours after the animals were slaughtered. Although the experiments stopped short of restoring consciousness, they raise questions about the ethics of the approach — and, more fundamentally, about the nature of death itself. The current legal and medical definitions of death guide protocols for resuscitating people and for transplanting organs.

Details of the pig-brain experiments appear in a paper1 published on 17 April in Nature. Researchers at Yale University in New Haven, Connecticut, hooked the organs to a system that pumped in a blood substitute. The technique restored some crucial functions, such as the ability of cells to produce energy and remove waste, and helped to maintain the brains’ internal structures.

“For most of human history, death was very simple,” says Christof Koch, president and chief scientist of the Allen Institute for Brain Science in Seattle, Washington. ”Now, we have to question what is irreversible.”

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