Scientists Revive Human Retinas after Death

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When we die, it takes time for cells in our body to completely shut down. Having enough oxygen and nutrients is important. It's not like a whole-body switch tells every cell to stop functioning all at once. that's why we can remove certain organs for transplantation; those organs are still in good condition but only for a short time (a few hours) after death. Nerve cells have the greatest problems in surviving when circulation is cut off, though. The retina of the the eye is tied directly to the optic nerve, and recently, researchers from the University of Utah learned that retina cells from the back of eyes can still function for a while after death. What's going on there?

In humans, the retina is a complex layer of 10 types of cells that covers the back two-thirds of the eye. When light hits the eye, it goes through the cornea and is focused by the lens through a large pocket of liquid making up the bulk of the eye's mass before striking the retina. That's when the seeing process begins.

Anatomy of the eye, showing layers of retinal cells

Corneas have been transplanted, and in rare cases so have tear ducts and eyelids. But how long do retinas survive death? It is known that within seconds of animals or humans dying, electrical activity between cells and oxygen supplies are depleted. At the same time, it takes longer (minutes) before blood sugar or energy molecules called ATP to be used up. Some experiments have shown partial brain recovery in various animals hours after death. The University of Utah researchers examined euthanized mice by implanting electrodes and measuring retinal responses to flashes of light every 5 seconds. In just 100 seconds after death, there is no response.

Then they euthanized the mice, removed the eye quickly, and surgically cut out the retina. This was fed an incoming stream of nutrients to feed the cells, give them oxygen, and control the pH. Again, they measured responses to flashes of light to see whether the retinal cells could be revived and respond like those in the whole eye. They examined the responses of the actual photoreceptor cells, which change the light energy into a signal, and the binary cells, which send that signal to nerves (ganglion cells) that then transmit the signal to the brain through the optic nerve.

Responses of retina cells kept alive after removal from the eye (Univ. of Utah)

What they saw was a clear difference between cells in the whole eye that didn't have life support vs. the same types of cells kept alive after removing them from the mouse eye. The photoreceptors seemed to keep a response longer than the binary cells, which could be important. If photoreceptor cells can function, but the binary cells can't send the message, that's the time limit to worry about.

So, for at least an hour after death, the binary cells could send a signal, but the photoreceptor cells stayed alive, and their function recovered, as long as 2-3 hours after death.

The researchers controlled the oxygen levels they delivered to the retinal cells. When they exposed them to less than 2.5% oxygen (what is expected in the body after death), they saw a nearly identical pattern of signal loss to what they measured in whole eye retinas. So, oxygen plays a very important role in maintaining the functionality of photoreceptor cells. Acidity showed a similar effect, too.

Mouse retinas have a greater number of photoreceptor cells called rods than humans. They help in low light conditions (useful for mice, because they are active at night). Humans have more cone photoreceptors (useful for distinguishing color). So, the Utah researchers also wanted to investigate retina tissue closer to humans in structure. They saw that nearly identical responses from eyes removed from macaques or human eyes donated 45 minutes to 2 hours after death by heart attack. Neither could not show a revival of retinal function, but they explained that in humans as due to the severe lack of oxygen caused by the heart attack. If the time after death was 20 minutes or less, recovery was possible.

The bottom line for future work is that transplantations will have to remove eyes extremely soon after death if they are going to be functional.

Here is a cool website with descriptions of the eye's anatomy. It has short videos showing the parts of the eye in 3D.