Originally published 2022-03-16.
In April 2019 a team at the Yale School of Medicine reported that it had restarted some basic cell activity in pig brains four hours after the animals were slaughtered. The brains did not think, feel or wake up. What came back was circulation, metabolism and the structure of the tissue, which had been expected to collapse within minutes of losing oxygen. That alone was enough to unsettle long-held ideas about how quickly and how finally brain cells die.
The work, led by neuroscientist Nenad Sestan, appeared in Nature on 17 April 2019 with the first author Zvonimir Vrselja. Here is what the experiment did, what it did not do, and why ethicists, doctors and transplant specialists paid such close attention.
The short answer: cells, not minds
The researchers used 32 brains from pigs killed for food at a pork-processing plant. The brains would otherwise have been thrown away. Each one was connected to the system four hours after death and kept on it for six hours.
During that time the treated tissue used glucose and oxygen, gave off carbon dioxide, kept its cell architecture and showed far less cell death than brains left untreated. Blood vessels widened when given a drug that normally relaxes them, and certain cells mounted an inflammatory response when challenged. Untreated control brains, by comparison, broke down quickly.
The team saw no organised, brain-wide electrical activity of the kind linked to awareness. Sestan put the distinction plainly to reporters: it was "not a living brain," but "a cellularly active brain."
How the BrainEx system worked
BrainEx is a closed loop of pumps, filters and heaters, roughly comparable to the heart-lung machines used in cardiac surgery. It pushes a warm, synthetic fluid through the brain's arteries at normal body temperature, imitating the pulse of real blood flow.
The fluid was designed to carry oxygen, nourish cells and protect them. It also contained compounds that block nerve signalling. That choice was deliberate: by damping neural firing, the scientists lowered the chance that any coordinated activity could arise. They also kept anaesthetics and cooling on hand to stop the experiment at once if electrical recordings showed organised patterns. None did.
Some small slices of tissue were later taken out of the system and washed free of the blockers. Individual neurons in those slices, from the hippocampus, could still fire electrical spikes. Neuroscientist Christof Koch told National Geographic that this was the most surprising part of the study, because it showed that single cells kept more capacity than anyone had assumed.
Why the result surprised neuroscientists
The textbook picture of the brain after its blood supply stops is grim. Within seconds a person loses consciousness; within minutes, cells begin a cascade of damage that was long thought to be largely irreversible. Much of stroke and cardiac-arrest care is built around racing that clock.
The Yale results suggested that at least part of the decline is slower and more reversible than believed, provided the right mix of oxygen, nutrients and protective chemicals arrives. Cell death, in other words, may be a process rather than a single moment.
That matters for research. A whole, intact large-mammal brain with working cells could let scientists study circuits, drugs and disease in three dimensions, rather than in thin slices or cells grown in dishes. Duke University bioethicist Nita Farahany, who co-wrote a commentary in the same issue of Nature, called the study a breakthrough for neuroscience while also noting that it challenged assumptions about the point of no return.
What it could mean for medicine
None of this is a treatment, and no one has tried it in people. Still, researchers pointed to several possible long-term uses:
- Stroke and cardiac arrest. Learning what keeps oxygen-starved cells alive could guide new ways to limit damage after blood flow stops.
- Brain disease research. A preserved whole brain could become a model for testing ideas about conditions such as Alzheimer's or Parkinson's, though that remains a distant prospect.
- Organ preservation. Similar perfusion methods might keep donated organs healthier for longer before transplant.
The US National Institutes of Health, which helped fund the work through its BRAIN Initiative, described the platform mainly as a research tool for understanding injuries in which the brain loses its blood supply.
The ethical questions it raised
Few lab studies have drawn as much ethical commentary. Three questions stood out.
Could a revived brain become aware?
The Yale team deliberately prevented it, but critics asked what would happen if a later experiment skipped the nerve blockers. A brain that regained some form of experience while cut off from a body would raise welfare questions with no clear precedent. University of Pennsylvania bioethicist Jonathan Moreno told The New York Times: "This is wild."
What counts as dead?
Brain death is a legal and medical standard in many countries, based on the irreversible loss of all brain function. If some functions can be partly restored hours later, the word "irreversible" needs careful thought. In their Nature commentary, bioethicists Stuart Youngner and Insoo Hyun warned that more advanced forms of such technology could complicate decisions around organ donation.
Do these brains need protections?
Research rules for animals were written with living animals in mind. A brain that is neither alive in the ordinary sense nor fully dead sits in a grey zone, and Farahany and colleagues argued that guidelines should be written before the science moves further.
Common misreadings of the study
Headlines about "zombie pigs" and "brains brought back to life" spread quickly. A few corrections help:
- The pigs were not revived. Only isolated brains were treated; the animals themselves remained dead.
- Consciousness was not restored. Recordings across the brain showed no organised activity, and the fluid was designed to suppress it.
- The brains were not kept going for days. Each perfusion lasted six hours, after which the experiment ended.
- It does not apply to humans yet. No human brain has been treated this way, and the method is nowhere near clinical use.
What came next: OrganEx
The same Yale lab pushed the idea further. On 3 August 2022 it reported in Nature a system called OrganEx, adapted from BrainEx, that was connected to whole pigs one hour after their hearts had stopped under anaesthesia. Six hours later, key cell functions were active in the heart, liver and kidneys, the heart showed electrical activity, and circulation reached the whole body. Some cell activity returned in parts of the brain, but again no organised brain activity suggesting awareness was detected.
The researchers framed OrganEx mainly as a way to protect organs for transplant and to study damage after heart attacks or strokes. Both projects point the same way: the boundary between living and dead tissue is more gradual than once believed, and that knowledge brings real promise along with serious questions that society, not only scientists, will need to answer.
Sources: Nature, April 2019: Pig experiment challenges assumptions around brain damage in people; Live Science, 17 April 2019: Hours after pigs' death, scientists restore brain cell activity; National Geographic: Pig brains partially revived; The New York Times, 17 April 2019; Yale News, 3 August 2022: Yale-developed technology restores cell, organ function in pigs after death; earlier reporting by Big Think.
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