Bird Retina Survives Without Oxygen Thanks to the Pecten Oculi

Here is a piece of scientific trivia: unlike the inner retina of most animals, including humans, the inner retina of birds operates without oxygen. Researchers led by a team at Aarhus University in Denmark have now established how this is possible.

In the retinas of nearly all vertebrates, red blood cells supply the oxygen needed to turn glucose into enough energy for cells to operate.

Birds are different. Their retinas contain no blood vessels, meaning oxygen can reach them only by diffusing in through the surface. As a result, the inner retina is anoxic, or oxygen-free.

Although cells can extract energy from glucose without oxygen, this method is inefficient and rapidly produces a toxic accumulation of waste products.

Fortunately, birds have evolved an answer: a plumbing-like system whose purpose has been debated by bird anatomists for centuries.

How the bird retina functions without oxygen

"Our study reveals an impressive anoxia tolerance in the inner bird retina," write the researchers in their published paper.

"Our findings are interesting, as neural tissues of warm-blooded animals are generally considered to be highly vulnerable in anoxia, rapidly leading to cellular dysfunction."

Central to this tolerance is the pecten oculi, a component of the bird eye first identified in the late 17th century. Positioned beside the retina, this structure is densely filled with blood vessels, though its precise function had remained uncertain until now.

By closely observing the eyes of living zebra finches (Taeniopygia guttata)-including measurements of oxygen levels, nutrient movement and gene activity-the team established that the inner retina does not use oxygen whatsoever.

Instead, retinal cells depend on anaerobic glycolysis. In this process, glucose yields small quantities of energy through a different series of reactions that does not need oxygen. However, it also produces lactic acid, which can harm tissue when it builds up to sufficiently high levels.

The pecten oculi supplies glucose and removes waste

This is where the pecten oculi comes in: it delivers large amounts of glucose and clears away lactic acid before it can damage the retinal cells.

Bird eyes may have developed this feature partly to avoid blood vessels that would obstruct vision, or possibly to enable birds to migrate at high altitudes, where oxygen is scarce.

For example, short-toed snake eagles (Circaetus gallicus) have retinas more than four times thicker than the oxygen-diffusion limit in mammalian retinas. This leaves a substantial part of the organ without oxygen, which may benefit these birds as they soar 500 metres above the ground for extended periods.

"Establishing the function of this enigmatic structure in the birds' eye is super cool," says biologist Coen Elemans, from the University of Southern Denmark.

"This pecten allows a snake eagle the incredible acuity of vision to spot a tiny still lizard from great heights, but also may have had a crucial role in allowing birds to migrate. That is wild!"

The finding may help guide related research into how cells survive anoxic conditions. Understanding the mechanisms used by bird eyes could ultimately contribute to treatments for strokes, for instance, another situation in which nerve cells are deprived of oxygen.

With scientists now having a far clearer understanding of the pecten oculi and its role, future work can examine in greater detail how the eye's essential glucose supply influences retinal performance. The system evidently needs considerable glucose to function correctly: the study reports that it takes up around 2.5 times as much as bird brains do.

The research paper was eight years in development and drew on contributions from specialists across numerous scientific disciplines, delivering another significant insight into the course of bird evolution over millions of years.

"This study is really a tour de force and a beautiful piece of work that combines the expertise and hard work from many people," says Elemans.

The research has been published in Nature.