US scientists have partially revived pig brains four hours after the animals were slaughtered.
The findings could fuel debate about the barrier between life and death, and provide a new way of researching diseases like Alzheimer's.
The study showed the death of brain cells could be halted and that some connections in the brain were restored, the BBC reported on Wednesday.
However, there were no signals from the brain that would indicate awareness or consciousness.
The surprise findings challenge the idea that the brain goes into irreversible decline within minutes of the blood supply being cut off. Thirty-two pig brains were collected from an abattoir.
Four hours later, the organs were connected to a system made by the team at Yale University. It rhythmically pumped (to mimic the pulse) a specially designed liquid round the brain, which contained a synthetic blood to carry oxygen and drugs to slow or reverse the death of brain cells.
The pig brains were given the restorative cocktail for six hours. The researchers found synapses - the connections between brains - were working.
The study, published in the journal Nature, showed a reduction in brain cell death, the restoration of blood vessels and some brain activity. The researchers found working synapses - the connections between brain cells that allow them to communicate.
The brains also showed a normal response to medication and used up the same amount of oxygen as a normal brain. This was all 10 hours after the pigs were decapitated.
Crucially there was no sign of the brain-wide electrical activity in an electroencephalogram (EEG brain scan) that would signal awareness or perception. Fundamentally they were still dead brains.
The research transforms ideas about how the brain dies, which many thought happened quickly and irreversibly without a supply of oxygen.
Nenad Sestan, a professor of neuroscience at Yale University, said: "Cell death in the brain occurs across a longer time window that we previously thought.
"What we are showing is the process of cell death is a gradual, stepwise process.
"And that some of those processes can be either postponed, preserved or even reversed."
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