Bird retinas function without oxygen—solving a centuries-old biological mystery

Neural tissue normally dies quickly without oxygen. Yet bird retinas—among the most energy-demanding tissues in the animal kingdom—function permanently without it. This may be relevant in future treatment of stroke patients.

In a study published today in Nature , an international research team reveals how birds have solved a biological paradox. The researchers show that the inner parts of the bird retina operate under chronic oxygen deprivation, relying instead on anaerobic energy production. At the same time, the study overturns a long-standing assumption about a mysterious structure in the eye that has puzzled scientists since the 17th century. Most animals supply neural tissue with oxygen through dense networks of tiny blood vessels. This is considered essential, as neurons have an exceptionally high energy demand. The retina, a highly specialized extension of the brain, is no exception—and in fact consumes more energy than any other tissue in the body.

Birds, however, present a paradox. Their retinas are avascular, meaning they lack blood vessels within the retinal tissue itself. This feature is thought to improve visual acuity, since blood vessels scatter light in its path to the photoreceptor. But how the retina survives without a blood supply has remained unknown. "Our starting point was simple," says biologist Christian Damsgaard, first author of the study and associate professor at Aarhus University in Denmark. "According to everything we know about physiology, this tissue should not be able to function." While the starting point may have been simple, the journey to the end point was anything but simple. It has taken Damsgaard and a growing team of researchers, mostly from Aarhus University, 8 years to produce the results, that are now finally published.

The pecten is a vascular structure within the vitreous humor of the eyes of birds with a previously unknown function. Credit: Aleksandrina Mitseva / Nature Read more

The study shows that the pecten supplies glucose to the retina at rates that far exceed the glucose supply rates to the brain, as illustrated on the autoradiography images, showing much higher glucose uptake (pink) in the retina compared to the brain. Credit: Christian Damsgaard and Morten Busk, Aarhus University / Nature Read more

Biologist Christian Damsgaard (right) began working on the mystery of the missing oxygen when he was a fellow at the Aarhus Institute of Advanced Studies (AIAS). Shortly afterwards, Professor at Dept of Clinical Medicine Jens Randel Nyengaard (left) joined the project, and since then more than 20 other Danish and German researchers from a wide range of disciplines have contributed. The chicken in the middle of the photo was not itself involved, but some of its conspecifics were—as model organisms in the project alongside the zebra finch, pigeon, and green anole. Credit: Peter F. Gammelby, Aarhus University Read more