Eyeworld

57 EW GLAUCOMA February 2019 Contact information Tan: oranghutan@aol.com Dr. Tan considered the possibil- ity that the distal tract also showed blood vessel characteristics. CD31 is a protein found in blood vessel en- dothelium but barely in lymphatics. He labeled the same mouse tissues for CD31 and found that whereas blood vessels expressed CD31 and lymphatics Prox1, Schlemm's canal endothelium expressed both CD31 and Prox1. The evidence suggests Schlemm's canal and the distal tract carry an identity somewhere between that of blood and lymphat- ic vessels. Dr. Tan searched for other sim- ilarities between the distal aqueous tract and blood vessels such as the presence of smooth muscle in vessel walls. The distal outflow tract was labeled for alpha smooth muscle actin, a smooth muscle marker. Dr. Tan established that the intrascleral plexus was lined by smooth muscle in a pattern resembling choroidal arteries from the same eye, indi- cating the wall organization of the intrascleral plexus resembles that of arteries. "In a high magnification image of a collector channel taken by our team, we visualized large scleral openings, which we tend to think are open conduits for the outflow of aqueous. In fact, they are packed with cells with a smooth muscle identity. These cells have the capac- ity to contract and in doing so alter the lumen size of collector channels. This could be deduced in our im- ages as the relatively small chan- nel lumen relative to surrounding lining cells in a contracted state," he explained. Based on Dr. Tan's imaging data it seems the distal outflow tract is anything but an inert series of draining pipes. "We are dealing with a vascular system of its own type," he said. "It has an endothelium a bit like lymphatics, but not exactly, and it does not have the full characteris- tics of blood vessels. We think it has its own type of endothelium and is its own type of vascular system. This is not that surprising given it carries a different kind of fluid. Aqueous humor is a unique fluid that is different from other fluids in the body. But the distal tract is similar to blood vessels in having a lining of smooth muscle with capacity to con- tract. If it contracts, you can imagine how it could constrict the lumen of this system, creating a lower outflow, high resistance system that could affect IOP. It certainly seems more dynamic than the system we had always envisioned," he said. EW Editors' note: Dr. Tan has no financial interests related to his comments.

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