Scientists have momentarily restored a faint glow of life to dying cells in the human eye.
In order to better understand how nerve cells succumb to a lack of oxygen, a team of American researchers measured the activity of mouse and human retinal cells shortly after death.
Surprisingly, with a few adjustments to the tissue environment, they were able to revive the cells’ ability to communicate hours later.
When stimulated by light, postmortem retinas have been shown to emit specific electrical signals, called b waves.
These waves are also seen in living retinas and they indicate communication between all layers of macular cells that allow us to see.
This is the first time the eyes of a deceased human donor have reacted to light in this way, and some experts question the irreversible nature of death in the central nervous system.
“We were able to awaken 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 biomedical scientist Fatima Abbas from the University of Utah.
“In eyes obtained up to five hours after the death of an organ donor, these cells responded to bright light, colored lights, and even very faint flashes of light.”
After death, it is possible to preserve certain organs of the human body for transplantation. But after circulation stops, the central nervous system as a whole stops responding far too quickly for any form of long-term recovery.
Yet not all types of neurons fail at the same rate. Different regions and different cell types have different survival mechanisms, which makes the whole question of brain death much more complicated.
Learning how certain tissues in the nervous system cope with a loss of oxygen might teach us a thing or two about recovering lost brain function.
Researchers have already had some luck. In 2018, scientists at Yale University made headlines when they kept pig brains alive for up to 36 hours after death.
Four hours post-mortem, they were even able to revive a small response, although nothing organized or global that could be measured by an electroencephalogram (EEG).
The feats were achieved by blocking the rapid decay of mammalian neurons, using artificial blood, heaters and pumps to restore the flow of oxygen and nutrients.
A similar technique now seems possible in mice and human eyes, which are the only extruding part of the nervous system.
By restoring oxygenation and certain nutrients to the eyes of organ donors, researchers from the University of Utah and Scripps Research were able to trigger synchronous activity between neurons after death.
“We were able to get the retinal cells to talk to each other, as they do in the living eye to mediate human vision,” says visual scientist Frans Vinberg from the University of Utah.
“Previous studies have restored very limited electrical activity in the eyes of organ donors, but this has never been achieved in the macula, and never to the extent that we have now demonstrated.”
Initially, experiments showed that retinal cells continued to respond to light for up to five hours after death. Yet the crucial intercellular b-wave signals quickly dropped, apparently due to the loss of oxygen.
Even when retinal tissue is carefully protected from oxygen deprivation, researchers have been unable to fully restore robust b waves.
Also, the temporary rebirth of retinal cells does not mean that the donor eyeballs could “see”, of course. Higher visual centers in the brain are needed to revive full visual sensation and perception.
Nevertheless, some definitions of “brain death” require a synchronous loss of activity between neurons. If this definition is accepted, then the human retinas in the current study were not yet completely dead.
“Given that the retina is part of the CNS, our b-wave restoration in this study raises the question of whether brain death, as currently defined, is truly irreversible,” the authors write.
If specialized neurons, known as photoreceptors, can be revived to some extent, this offers hope for future transplants that could help restore vision in people with eye disease.
That day, however, is still far away. Cells and patches transplanted from a donor’s retina would have to somehow seamlessly integrate into existing retinal circuitry, which is a daunting challenge that scientists are already trying to overcome.
In the meantime, donor eyes and animal models will have to, and b-wave testing could be a good way to determine whether a retinal transplant is viable or not.
“The scientific community can now study human vision in a way that is simply not possible with laboratory animals,” says Vinberg.
“We hope this will motivate organ donor societies, organ donors and eye banks by helping them understand the exciting new possibilities that this type of research offers.”
The study was published in Nature.