Factors affecting the pores of the inner wall endothelium of Schlemm's canal

Purpose. A linear relationship between the density of pores in the inner wall of Schlemm's canal and aqueous outflow facility has been reported previously in a study in which investigators examined only eyes fixed at constant pressure, so that fixative flow rates differed from eye to eye. Because pores may form as a function of flow rate, the purpose in the current study was to verify the previous findings, using constant flow perfusions. Methods. Outflow facility was measured in enucleated human eyes. Eyes were fixed under either constant flow or constant pressure conditions, microdissected to expose the inner wall of Schlemm's canal, and prepared for scanning electron microscopy. The density and diameter of pores in the inner wall were measured. Results. Statistical analysis showed no correlation between outflow facility and either the density or the diameter of pores. Pore density decreased significantly during the hours after death. Examining only eyes for which experimentation was started within 20 hours of death, we found that pore density increased significantly with the volume of fixative that had been perfused through the outflow pathway. Conclusions. The correlation found by Allingham et al between outflow facility and pore density in the inner wall endothelium was not confirmed. However, the relationship between pore density and volume of fixative perfused is consistent with and may be responsible for the finding in the previous study. Because fixation conditions can influence the apparent pore density in the inner wall endothelium significantly, the conclusion reached previously, that pores contribute only 10% of the aqueous outflow resistance, may require reevaluation.