Within minutes of being placed at the back of a mouse’s eye, fluorescent tracers turned up in nearby lymph nodes. That simple observation points to a drainage route that textbooks said did not exist.
Scientists at the University of British Columbia (UBC) and the University of Toronto have described the pathway. They call it the posterior ocular lymphatic outflow pathway, or POLO. According to the UBC release, it carries fluid and waste from behind the retina into the body’s lymphatic system. The work was done in mice and was published in Translational Vision Science & Technology, as reported by The News International.
A century-old assumption
The lymphatic system helps regulate fluid, clear waste and support immune function in almost every organ. For more than a century, the eye was thought to be an exception, according to the university’s account.
That view began to shift in 2009. Dr. Neeru Gupta of UBC and Dr. Yeni Yücel of the University of Toronto described a lymphatic-related route at the front of the eye, which they named the “uveolymphatic” pathway. The back of the eye, where many blinding diseases begin, remained largely unexplored.
How the team traced it
The researchers combined MRI, near-infrared fluorescence imaging and microscopy. They injected tracers into a space at the back of the eye and followed where the fluid went.
The tracers reached lymph nodes. The team also found small lymphatic vessels in the choroid, the blood-vessel-rich tissue beneath the retina. No lymphatic vessels had been known there before.
The retina is a heavy energy user. It turns light into signals the brain can read, and that work produces byproducts. “The retina is one of the most metabolically active parts of the body, constantly generating byproducts that need to be cleared,” Gupta said in the UBC statement.
Why it matters, and what is not yet known
Glaucoma, age-related macular degeneration and other major causes of permanent vision loss are linked to a buildup of fluid, metabolic waste and inflammatory material at the back of the eye. Researchers have struggled to explain how that region clears such material. The POLO pathway is a candidate answer.
The idea is that if drainage fails, pressure, inflammation and harmful proteins could accumulate. That is a hypothesis about disease. This study did not test it, and it does not show that any treatment works.
Several limits apply. The findings come from mice, and the sources do not say whether the same pathway works the same way in human eyes. The researchers describe the discovery as opening the door to new treatments, not as a treatment itself. Any clinical use would need further animal work and human studies first.
Still, the finding gives eye researchers a new place to look. Yücel said that understanding how the retina’s surroundings keep a balanced environment and flow of materials is critical, since the retina is both essential for vision and the site of many of the most serious vision-loss diseases.
For now, the practical result is a revised map of the eye. A route once assumed absent appears to have been there all along, and it was simply hard to see.