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S. J. Folz, S. M. Roberts, D. R. Overby; Whole-Eye Mapping of Segmental Outflow Patterns in Human Trabecular Meshwork. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3293.
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Aqueous humor outflow is non-uniform or "segmental" such that only a fraction of trabecular meshwork is active in hydrodynamic filtration at any given instant. The goals of this project are to determine the filtration-active fraction of human trabecular meshwork and whether these filtration patterns are sensitive to IOP.
Our experimental design uses a two-color fluorescent tracer perfusion, where one tracer color is perfused at baseline followed by a second color after changing IOP. Importantly, this allows direct comparison of pre- and post-stimulus tracer patterns within individual specimens. Four pair of enucleated human eyes were perfused at high IOP (30 mmHg) with Dulbecco’s PBS + 5.5 mM glucose (DBG). Outflow facility was recorded throughout the experiment. Following baseline DBG perfusion at 30 mmHg, anterior chamber contents were exchanged and perfused with DBG containing green fluorescent microspheres (0.2 µm, 0.002% v/v, COOH). Tracer was then removed by exchange and perfusion with DBG alone. IOP was then reduced to 7 mmHg in the experimental eye, while the contralateral control eye was maintained at 30 mmHg. Anterior chamber contents were then exchanged and perfused with red fluorescent microspheres. Red tracer was removed by exchange and perfusion with DBG alone, followed by exchange and perfusion with 4% paraformaldehyde. Anterior segments were quadrisected and processed for epifluorescence microscopy. All 4 quadrants were imaged in entirety to quantify the distribution and intensity of both tracers.
Large spatial heterogeneity in tracer labeling patterns was observed with no more than 43-54% of the meshwork appearing filtration-active. Variability in tracer labeling between quadrants (CV = 37±13%, mean±SD, N = 8) was similar to variability within quadrants (CV = 31±13%, N = 32), with nasal quadrants tending to receive the most tracer. Changing IOP from 30 to 7mmHg had a relatively minor effect upon tracer patterns, as judged by co-localization of the two tracer colors. Perfusion with the first tracer led to a 19±3% (mean±SEM) decrease in facility with an additional 11±3% decrease with the second tracer.
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