2 PORE TYPES IN THE INNER-WALL ENDOTHELIUM OF SCHLEMMS CANAL

PURPOSE. It has been reported that fixation conditions significantly i nfluence the apparent pore density in the inner-wall endothelium of Sc hlemm's canal. In the present study, the manner in which fixation cond itions affect the two subtypes of inner-wall pores, intracellular pore s and intercellular (or border) pores, was investigated. METHODS. Outf low facility was measured in enucleated human eves. Eyes were fixed un der ''constant flow'' Or constant pressure conditions, microdissected to expose the inner wall of Schlemm's canal, and prepared fur scanning electron microscopy. The density and diameter of the two subtypes of pores in the inner wall were measured. RESULTS. Intracellular pore den sity decreased with increasing postmortem time (P < 0.001) and increas ed with increasing volume of fixative passed through the outflow pathw ay (P < 0.001), whereas border pore density showed no dependence on th ese parameters (P > 0.25 and P > 0.15, respectively). Border pore dens ity increased with increasing fixation pressure (P < 0.005), even thou gh intracellular pore density showed no such dependence (P > 0.4). No correlation was found between outflow facility and the predictions of Poiseuille's law, Sampson's law, or the funneling theory for the hydra ulic conductivity of the intracellular pores (P > 0.35) or the border pores (P > 0.1). CONCLUSIONS. The intracellular and border pores form two morphologically and functionally distinct populations in the inner wall of Schlemm's canal. The dependence of intracellular pore density on postmortem time and on volume of fixative passed through the outfl ow pathway suggests that these pores are artifacts of tissue fixation or processing conditions. That border pores do not depend on such cond itions and that their presence is correlative with perfusion pressure suggests that this population may be nonartifactual. New histologic te chniques for examining the inner wall of Schlemm's canal are necessary to determine the in vivo state of inner-wall pores and how they influ ence outflow facility.