Corneal glycoconjugates: an ultrastructural lectin-gold study

The oligosaccharide chains of cell surface and extracellular matrix glycoconjugates are essential for the biological properties of these molecules. We have, therefore, investigated carbohydrate residues in the rat cornea using biotinylated lectin-gold probes. Fixed corneas were removed and embedded in Lowicryl HM20 or LR White. Ultrathin sections were incubated in one of the lectins: Triticum vulgare (WGA), Canavalia ensiformis (Con A), Griffonia simplicifolia (GS-1), Limax flavus (LFA) and Allomyrina dichotoma (Allo A), followed by streptavidin-gold, or the sections were incubated in cationic colloidal gold. Semi-quantification of gold labelling was determined for corneal endothelium, Descemet's membrane, stroma and epithelium from electron micrographs. WGA and Con A binding sites were expressed either moderately or strongly throughout the cornea, suggesting a preponderance of alpha-mannose and N-acetylglucosamine residues. A particular concentration of these sugars was found in Descemet's membrane. In contrast, GS-1 (specific for alpha-galactose) and Allo A (specific for beta-galactose) labelled all regions weakly. Sialic acid residues, as defined by LFA labelling and the expression of neuraminidase-sensitive cationic colloidal gold binding sites, were sparsely distributed throughout the stroma, Descemet's membrane and endothelium. In contrast, sialoglycoconjugates were found in significant concentrations in the epithelium. Electron microscopy proved useful in providing new information on the cellular and subcellular localization of these lectin binding sites.