For more than a century, the eye was considered one of the few organs without its own lymphatic system. That assumption is now being challenged: researchers at the University of British Columbia and the University of Toronto report finding a previously unknown drainage route at the back of the eye through which fluid and metabolic waste leave the retina.
The team calls the route the posterior ocular lymphatic outflow, or POLO, pathway. It answers a long-standing question: where does the waste from the eye's most hard-working tissue go? "The retina is one of the most metabolically active parts of the body and constantly generates byproducts that need to be cleared," says Neeru Gupta, who works at both universities.
To make the hidden pathway visible, the group worked with mice and combined several imaging methods – magnetic resonance imaging, near-infrared fluorescence and microscopic analysis. Glowing tracer molecules injected into the back of the eye, which bind only to lymphatic tissue, were carried along with the tissue fluid. That allowed the researchers to follow in real time how the fluid drained through fine lymphatic vessels in the choroid, passed into the surrounding orbital tissue and reached nearby lymph nodes within minutes. The smallest of these channels measure only about 50 micrometres.
The finding is not entirely a surprise: Gupta and her colleague Yeni Yücel had already described a lymphatic outflow at the front of the eye back in 2009. But the rear of the eye, where many serious eye diseases originate, had remained a blank spot.
That is exactly where the significance lies. Conditions such as glaucoma and age-related macular degeneration involve fluid build-up, deposits and inflammation at the back of the eye. Macular degeneration alone affects around 2.5 million people in Canada, according to UBC. If a natural cleaning system exists, it might be possible to strengthen it deliberately. "This gives us a whole new framework for understanding these diseases – and a potential target for therapeutics," says Gupta. Turning that into treatments remains a long way off; but the animal-model results, published in the journal Translational Vision Science & Technology, mark a start.