Scientists have resurrected a glimmer of activity in human eyes after death

Scientists have resurrected a glimmer of activity in human eyes after death

Scientists have momentarily restored dying cells in the human eye to a faint twinkle of life.

To better understand how nerve cells succumb to a lack of oxygen, a team of US researchers measured the activity of retinal cells in mice and humans shortly after their deaths.

Amazingly, with a few changes to the tissue environment, they were able to revive the cells’ ability to communicate hours later.

The postmortem retina has been shown to emit specific electrical signals known as B-waves when stimulated by light.

These waves can also be seen in living retinas and indicate communication between all layers of the macular cells that allow us to see.

It’s the first time a deceased human donor’s eyes have responded to light in this way, and there are some experts who 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,” explains University of Utah biomedical scientist Fatima Abbas.

“In eyes obtained up to five hours after the death of an organ donor, these cells responded to bright light, colored light, and even very faint flashes of light.”

After death, it is possible to keep some organs in the human body for transplantation. But after circulation has stopped, the central nervous system as a whole reacts far too quickly for any form of long-term recovery.

However, not all types of neurons fail at the same rate. Different regions and different cell types have different survival mechanisms, which makes the whole issue of brain death much more complicated.

Learning how select tissues in the nervous system cope with a loss of oxygen might teach us a thing or two about restoring lost brain function.

Researchers have had some luck. In 2018, Yale University scientists made headlines when they kept pig brains alive for up to 36 hours after death.

Four hours after death they were able to resuscitate even a small reaction, although nothing organized or global that could be measured by an electroencephalogram (EEG).

The feats were accomplished by halting the rapid breakdown of mammalian neurons by using artificial blood, heaters, and pumps to restore circulation 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 some nutrients to the eyes of organ donors, University of Utah and Scripps Research researchers have been able to trigger synchronous activity between neurons after death.

“We were able to get the retinal cells to talk to each other like they do in the living eye to mediate human vision,” says visual scientist Frans Vinberg of 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.”

First, the experiments showed that retinal cells responded to light up to five hours after death. But the key intercellular b-wave signals quickly dropped, apparently due to the loss of oxygen.

Even when the retinal tissue is carefully protected from oxygen starvation, the researchers have not been able to fully restore robust b-waves.

In addition, of course, the temporary resuscitation of the retinal cells does not mean that the donor eyeballs could “see”. Higher visual centers in the brain are needed to revitalize full visual sensation and perception.

Still, some definitions of “brain death” require a loss of synchronous activity between neurons. If this definition is accepted, then the human retinas in the current study were not completely dead.

“Since the retina is part of the CNS, our recovery of the B-wave 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, it offers hope for future transplants that could help restore vision in eye diseases.

But there is still a long way to go until that day. Transplanted cells and patches from a donor retina would need to somehow seamlessly integrate with existing retinal circuitry, a formidable challenge that scientists are already addressing.

In the meantime, donor eyes and animal models will have to suffice, and testing for B waves could be a good way to determine whether a retinal transplant is viable or not.

“The scientific community can now study human vision in ways that are simply not possible with laboratory animals,” says Vinberg.

“We hope this will motivate organ donation societies, organ donors and eye banks by helping them understand the exciting new opportunities this type of research offers.”

The study was published in Nature.

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