Nerve cells in the retinas of elderly mice show an unexpected and purposeful burst of growth late in life, according to researchers at UC Davis.

"Mostly, the older you are, the more neurons shrivel up and die. This gives us a more optimistic view of aging," said Leo Chalupa, professor of ophthalmology and neurobiology, chair of neurobiology, physiology and behavior at UC Davis, and senior author on the paper, which was published by the journal Proceedings of the National Academy of the U.S.A.

Because the nerves of the eye are a part of the brain, Chalupa said, this discovery indicates that there is a possibility of encouraging other parts of the aging brain to grow back. The research group has preliminary evidence that the same process takes place in the eyes of elderly people.

The nerve cells, or neurons, in the retina form a layer over another layer containing the light-sensitive cells. The neurons collect signals from the light-sensitive layer and relay them back to the brain.

Researcher Lauren Liets noticed that in mice more than a year old — roughly 70 to 80 years old in human terms — the neurons sprouted tendrils into the photoreceptor layer, and the older the mice, the more growth took place.

At the same time, the photoreceptor cells were shrinking and pulling back, so the neurons appeared to be following them, perhaps compensating for those effects, Chalupa said.

Similar sprouting occurs in damaged or detached retinas, Liets said. But the new research marked the first time that such an effect had been seen in the normal, aging eye, she said.

Other members of the research group were Kasra Eliasieh and Deborah A. van der List.