How did the team find the pathway?

The researchers combined several advanced imaging methods in mice: magnetic resonance imaging (MRI), near-infrared fluorescence imaging, and microscopic examination. They placed fluorescent tracer molecules into a narrow space at the rear of the eye, then tracked where the fluid traveled as the tracers moved.

The results revealed tiny lymphatic vessels in the choroid — a thin, blood vessel-rich layer beneath the retina that nourishes the retina's outer layers. Scientists had not previously established that lymphatic vessels exist in this region.

The team watched fluid move from the back of the eye into the surrounding orbital tissues, which fill the space around the eyeball. Within minutes, the tracers reached nearby lymph nodes, connecting the eye's newly identified drainage route to the body's broader lymphatic network — the system of vessels and tissues that collects excess fluid, transports waste products, and helps coordinate immune responses.

"Because lymphatic vessels in the choroid were thought not to exist, the team used multiple techniques to demonstrate both their presence and function," said Dr. Yeni Yücel, professor and director of ophthalmic pathology at the University of Toronto and a pathologist-scientist at St. Michael's Hospital. "We were surprised to see such a direct route for fluid to leave the eye and connect with the lymphatic system. It suggests the back of the eye has an active clearance pathway, which could play an important role in removing fluid, proteins and inflammatory material that build up in disease."

Does this overturn older research?

Yes — and it builds on earlier work by the same researchers. In 2009, Dr. Gupta and Dr. Yücel identified a lymphatic-related drainage route in the front of the eye, which they named the "uveolymphatic" pathway. That finding hinted the eye had unrecognized connections to the body's fluid clearance mechanisms.

Even so, the back of the eye remained largely unexplored — a significant gap, because the retina and its surrounding tissues are where many of the most serious vision-loss diseases occur.

"The retina is responsible for vision and it's also where many of the most serious vision-loss diseases occur," Dr. Yücel said. "Understanding how this part of the eye maintains a balanced environment and flow of materials is critical."

Could the eye's cleanup system help prevent blindness?

Eventually, perhaps — but important caveats apply. All experiments were conducted in mice, and further studies must establish whether the POLO pathway functions the same way in humans. Researchers also need to determine whether changes in this drainage system actually contribute to eye disease, and whether its activity can be safely modified.

If those questions find answers, the pathway could support several future approaches:

• Therapies that improve fluid clearance, potentially reducing the buildup of substances linked to inflammation and tissue damage.
• Better drug delivery to structures deep within the eye, using knowledge of the drainage route.
• Sharper insight into how pressure, inflammation, and fluid movement interact in glaucoma and age-related macular degeneration.

These applications remain potential future developments rather than established treatments. Still, the discovery hands scientists a new anatomical pathway to study and a possible explanation for processes that have puzzled them for decades.

"This is a foundational discovery that shows the eye is not as closed a system as we previously thought," Dr. Gupta said. "It gives us a new map, a new mechanism and a new set of questions to explore."

The study's first author was Babishaa Sauntharrajan, with co-authors including Xun Zhou, Ekim Gümeler, Greg Stanisz, Wilfred W. Lam, Margaret Koletar, and Yeni H. Yücel. The research received support from the Canadian Institutes of Health Research, Glaucoma Research Society of Canada, Canadian Space Agency, Canada Foundation of Innovation, and several named research funds and chairs.