HYDRODYNAMICS OF AQUEOUS HUMOR OUTFLOW

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Our broad goal is to determine the principal site of resistance to the outflow of aqueous humor from the normal eye, and to determine how resistance is changed in primary open-angle glaucoma (POAG). We have recently identified several new fundamental features of the aqueous outflow system that appear to have important hydrodynamic implications. Among these is a previously unrecognized hydrodynamic interaction, the "funneling" effect, between the pores of the inner wall endothelium of Schlemm's canal and the juxtacanalicular connective tissue (JCT), that may greatly increase the flow resistance of this region. We plan to determine the importance of this phenomenon by combining perfusion studies, morphometric examination of the region where the "funneling" effect is manifest, and engineering porous media analysis. A further finding of our group is that much of the protein in the aqueous humor enters via the root of the iris rather than, as conventionally thought, through the posterior chamber. The immediate proximity of this protein entry point to the trabecular meshwork (TM) raises the possibility that the concentration of protein within the inter-trabecular spaces may be much higher than previously suspected. Initial experimental studies in bovine eyes have demonstrated the hydrodynamic implications of this added protein load, and suggest that these proteins may contribute to the normal maintenance of bovine outflow resistance. The decrease in outflow resistance that occurs in non-human experimental perfusions ("wash-out") can now be explained and prevented. We plan to explore this finding in greater depth and to determine its relevance to human outflow physiology. Finally, as occurs in other connective tissues, we have demonstrated an age-related decrease of glycosaminoglycans in the TM, and postulate that this decrease may have important physicochemical hydrodynamic consequences including an increased tissue surface hydrophobicity perhaps leading to coalescence of collagen fibrils and adsorption of lipophilic proteins. Using morphological techniques, including fluorescent, hydrophobic probes, we will determine the extent of these changes in POAG; and, we will experimentally modify the aqueous outflow pathway to simulate these changes and determine their effect on aqueous outflow dynamics. Exploiting our combined expertise in the areas of modeling, hydrodynamics, ultrastructural and immunohistochemical methods, progress toward determining the locus of flow resistance in normal and glaucomatous eyes has become a realistic possibility. New histological probes (immunoaffinity markers and fluorescent physicochemical probes), in conjunction with powerful new morphological methods (quick freeze/deep etch) will allow us to visualize the outflow pathway in ways not previously possible, thus allowing us to greatly refine our models of the hydrodynamics of aqueous outflow. In turn, the models have and will continue to direct the nature of these morphological studies by suggesting where further investigation is best directed.