Death may be the most uncanny topic to discuss for human beings. Even thinking about it is uncomfortable for some people.
To eliminate the mystery behind it, researchers worldwide are conducting scientific studies on death and coming up with surprising results, such as when researchers captured brainwaves during an individual's death and found semblance to high cognition activities.
And now, a team of scientists from the U.S. may have found a way to revive a glimmer of activity in human eyes after death. According to a study published yesterday (May 11, 2022) in the journal Nature, the team has managed to revive the connections between light-sensing neurons in organ donor eyes.
Overcoming oxygen deprivation
The research team measured the activity of retinal cells in both mice and humans shortly after their death. Initial experiments indicated that oxygen deprivation is the critical factor leading to photoreceptors' loss of communication with other cells in the retina.
To achieve their results, Anne Hanneken, Associate Professor at Scripps Research, procured organ donor eyes in less than 20 minutes after death. On the other hand, Frans Vinberg, an assistant professor at the John A. Moran Eye Center, developed a transportation unit to restore oxygenation and other nutrients to the organ donor eyes. Vinberg also built a device that stimulates the retina and measures its electrical activity.
Utilizing these devices, the team restored a specific electrical signal seen in living eyes, also known as the “b wave” in the postmortem retinas. After being triggered by light, the postmortem retinas emitted particular b-waves.
"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," told Fatima Abbas, a biomedical scientist and the lead author of the study.
Questioning the irreversible nature of death
Reviving photoreceptors also offers hope for future transplantations that could help restore vision in those with eye disease. However, transplanted cells and patches of a donor retina would have to be flawlessly integrated into existing retinal circuits. This is already a challenging problem on which scientists are working.
The study provides the first example of donated eyes responding to light. Therefore, it raises questions about death's irreversibility, partly related to the permanent loss of neural activity.
Death is defined as the irreversible cessation of circulatory, respiratory, or brain activity. Many peripheral human organs can be transplanted from deceased donors using protocols to optimize viability. However, tissues from the central nervous system rapidly lose viability after circulation ceases1,2, impeding their potential for transplantation. The time course and mechanisms causing neuronal death and the potential for revival remain poorly defined. Here, using the retina as a model of the central nervous system, we systemically examine the kinetics of death and neuronal revival. We demonstrate the swift decline of neuronal signaling and identify conditions for reviving synchronous in vivo-like trans-synaptic transmission in postmortem mouse and human retina. We measure light-evoked responses in human macular photoreceptors in eyes removed up to 5 h after death and identify modifiable factors that drive reversible and irreversible loss of light signaling after death. Finally, we quantify the rate-limiting deactivation reaction of phototransduction, a model G protein signaling cascade, in the peripheral and macular human and macaque retina. Our approach will have broad applications and impact by enabling transformative studies in the human central nervous system, raising questions about the irreversibility of neuronal cell death, and providing new avenues for visual rehabilitation.