Eyeworld

EW GLAUCOMA 56 February 2019 Presentation spotlight by Stefanie Petrou Binder, MD, EyeWorld Contributing Writer and understanding that complex- ity could yield answers to many questions. "There is a conventional view we have of the distal outflow system," he explained. "To exit the eye, the aqueous humor has to pass through the trabecular meshwork into the distal outflow tract, which is the drainage tract that lies within the sclera. It exits the meshwork into Schlemm's canal and into the collector channels, through the intrascleral plexus, which are large lakes of aqueous within the sclera, and onto the aqueous veins to exit the eye into the episcleral veins. But is it this simple?" he said. Improved imaging systems have added much to the knowledge of the aqueous outflow pathway; how- ever, little has been elucidated about this tissue on the cellular level. In his research lab, Dr. Tan performed two-photon excitation fluorescence microscopy, a technology that uses long wavelength laser that allows deeper penetration of tissues. This imaging modality allows very pre- cise optical sectioning and high res- New data demonstrates how the distal outflow tract in mouse eyes exhibits cells with a mixed lymphatic and blood vessel identity N ew imaging data describes the distal outflow tract as not just an inert series of tubes and openings with- in the sclera but a vascular system of its own type. Investigator James Tan, MD, Department of Ophthalmology, University of Cal- ifornia, Los Angeles, discussed his novel research findings on imaging aqueous outflow during a sympo- sium at the 36th Congress of the European Society of Cataract and Refractive Surgeons. "The distal outflow system is not static, but a dynamic system with cells that are able to self-reg- ulate," Dr. Tan said in his presen- tation. "It can regulate itself in re- sponse to all kinds of factors, which raises some questions, for instance about the effect a dynamic system might have on bypass outcomes or how the distal outflow tract might change and adapt to the new fluid dynamic coming in from the ante- rior chamber after MIGS. Finally, it is possible that this system might be targeted and manipulated pharma- cologically, just as we would treat arteries for systemic hypertension." Trabecular meshwork Understanding the aqueous out- flow tract is key in working toward solving high IOP and is the subject of Dr. Tan's research. The trabecular meshwork has been considered a tissue of high resistance that limits the exit of aqueous into the distal outflow system. Why then is it that MIGS, in bypassing the trabecular meshwork, does not reduce IOP to that of episcleral venous pressure? And why are outcomes of MIGS so varied? According to Dr. Tan, the aque- ous outflow system may be more complex than what was thought, olution to image cellular and even sub-cellular compartments with minimal phototoxicity. In eyes of transgenic reporter mice engineered to allow visualization of specific mouse proteins marked with a fluo- rescent marker, Dr. Tan was able to come closer to the true nature of the aqueous collecting pathways. New imaging of the distal outflow tract "The aqueous outflow system of the reporter mouse is remarkably similar to that of humans, only smaller. By using two-photon microscopy, we were able to see that the intras- cleral plexus in mice is lined by cells," Dr. Tan explained. He used special markers to visualize both cells and collagen in mouse eyes. "We wondered, if the distal outflow tract is lined by cells, is it a type of vascular system? A vascular system would be expected to be lined by endothelium. There are different types of endothelia, however, with each peculiar to vessel type such as blood vessels or lymphatic vessels. We needed to distinguish between them. We also wondered whether the distal tract lining had muscle cells, as are present in blood vessels. If so, muscle cells could provide this system with the capacity to contract, just like blood vessels," he said. The type of endothelium pres- ent in a tissue will tell you the type of fluid you are dealing with, such as blood, lymph, or even aqueous humor. Dr. Tan investigated the presence of a protein that is known to be specific to lymphatic tissue in humans, called Prox1. He was able to visualize Prox1 in the engineered mouse. Prox1 is present through- out the mouse's Schlemm's canal, but also in the downstream vessels including collector channels and intrascleral plexus. He went on to assess if Schlemm's canal expressed other classic markers of true lym- phatic vessels. Dr. Tan labeled the tissue for LYVE1, a protein found only in true lymphatic endothelial cells, and found that it was present in adjacent lymphatic vessels but not Schlemm's canal itself. He con- cluded that Schlemm's canal has a partial lymphatic identity due to the presence of Prox1 but it is not truly a lymphatic vessel. New imaging technique uses fluorescent markers to identify cell makeup of aqueous collecting system Two-photon image of collector channel. A significant proportion of the channel cross-sectional area comprises wall cells with filamentous actin (red), representing a contracted state. This leaves a lumen (black oval) that is smaller than expected relative to the scleral opening (sclera in cyan). Source: James Tan, MD Scleral opening CC lumen Contractile wall region Sclera

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