Localization of sialic acid moieties in the endothelial lining of Schlemm's canal in normal and glaucomatous eyes.

The distribution of sialic acid moieties in the trabecular wall of Schlemm's canal in normal human and monkey eyes, as well as in limited tissue samples from patients with primary open-angle glaucoma (POAG), was investigated by using polycationic ferritin (PCF) at varying pH and colloidal iron at pH 1.8. At pH 1.8-1.9, both PCF and colloidal iron labeled the luminal surface of the endothelial cells of Schlemm's canal in normotensive monkey and human eyes, whereas binding was sparse and random on the basal surface. At a pH of 3.3 and higher, PCF showed increased binding that was confirmed exclusively to the luminal surface of the cells. In trabeculectomy tissue samples from eyes with POAG, PCF at pH 1.9 showed a greater density of labeling on both the basal and luminal cell surfaces, as well as in the subendothelial region, than was observed in normotensive eyes. Given the fact that sialic acid is a moiety which stabilizes cell membranes, we suggest that in normal eyes the absence of sialated molecules on the basal cell surface leaves 'receptor' sites for the initiation of the macrovacuolation process which allows the bulk outflow of aqueous humor across the endothelial barrier of Schlemm's canal. Our preliminary observations in glaucomatous eyes indicate a loss of such receptor sites, which may account for the depletion of macrovacuolar configurations in cases of advanced POAG. Our discussion relates these findings to the pathogenesis of POAG and to the possible role for a defective transmembrane interaction of the surface glycoproteins (fibronectin and laminin) with the cytoskeleton of the endothelial cells lining Schlemm's canal and the cells in the pericanalicular tissue.