Scientists have temporarily revived dying cells in the human eye.
A team of US researchers examined activity in mouse and human retinal cells just after death to better understand how nerve cells succumb to a shortage of oxygen.
Amazingly, they were able to restore the cells' capacity to interact hours later with a few changes to the tissue's surroundings.
The postmortem retinas were found to exhibit unique electrical signals known as b-waves when triggered by light.
These waves may also be detected in living retinas, indicating communication across all of the layers of macular cells that allow humans to see.
It's the first time that dead human donor eyes have ever reacted to light in this fashion, leading some researchers to ask whether death in the central nervous system is irrevocable.
"We were able to wake up photoreceptor cells in the human macula, which is the part of the retina responsible for our central vision and our ability to see fine detail and color," says University of Utah biomedical scientist Fatima Abbas.
"In eyes obtained up to five hours after an organ donor's death, these cells responded to bright light, colored lights, and even very dim flashes of light."
Some organs in the human body can be saved for transplantation after death. However, if circulation is lost, the central nervous system as a whole ceases to respond far too rapidly for long-term recovery.
However, not all neuron types perish at the same rate. Distinct areas and kinds of cells have different survival strategies, thus complicating the whole brain-death problem.
Learning how certain nervous system tissues respond to oxygen deprivation might teach us a thing or two about regaining lost brain functions.
Researchers have already had some success. Yale University scientists made news in 2018 when they discovered a way to keep pig brains alive for up to 36 hours after death.
They were even able to resurrect a modest reaction four hours after death, albeit nothing coordinated or global enough to be assessed by an electroencephalogram (EEG).
The accomplishments were accomplished by halting the fast degeneration of mammalian neurons and restoring oxygen and nutrient circulation using artificial blood, heaters, and pumps.
A comparable procedure appears to be viable in mice and human eyes, the nervous system's solitary extruding portion.
Researchers from the University of Utah and Scripps Research were able to activate synchronized activity among neurons after death by restoring oxygenation and certain nutrients to organ donor eyes.
"We were able to make the retinal cells talk to each other, the way they do in the living eye to mediate human vision," explains visual scientist Frans Vinberg from the University of Utah.
"Past studies have restored very limited electrical activity in organ donor eyes, but this has never been achieved in the macula, and never to the extent we have now demonstrated."
The research first revealed that retinal cells might respond to light for up to five hours after death. However, due to a lack of oxygen, the critical inter-cellular b-wave signals gradually faded.
The researchers were unable to fully restore strong b-waves even when retinal tissue was properly safeguarded from oxygen deprivation.
Furthermore, the transient resurrection of retinal cells does not imply that the donor eyes could 'see.' To restore complete visual sensation and perception, higher visual centers in the brain are required.
Despite this, some definitions of 'brain death' call for the lack of synchronous activity among neurons. If that criterion is adopted, the human retinas studied in this study were not entirely dead.
"Since the retina is part of the CNS, our restoration of the b-wave in this study raises the question of whether brain death, as it is currently defined, is truly irreversible," the scientists add.
If specialized neurons known as photoreceptors can be resurrected to some extent, it opens the door to future transplants that might help people with eye illness regain their eyesight.
However, that day is still a long way off. Transplanted cells and portions of a donor retina would have to be flawlessly incorporated into existing retinal circuits, which is a difficult task that scientists are now attempting to solve.
In the meantime, donor eyes and animal models will have to suffice, and checking for b-waves might be a useful tool for determining whether or not a retinal graft is feasible.
"The scientific community can now study human vision in ways that just aren't possible with laboratory animals," Vinberg argues.
"We hope this will motivate organ donor societies, organ donors, and eye banks by helping them understand the exciting new possibilities this type of research offers."
