The effect of inhibitors of glycoconjugate synthesis on optic cup formation in the chick embryo

Abstract

Invagination of the optic primordium in the chick embryo coincides with the appearance of material at the apical cell surface that precipitates cetylpyridinium chloride. Since this precipitate is soluble only at high salt concentrations, it is suspected of being an acidic glycoconjugate. In order to test whether glycoconjugates play a role in optic cup formation, embryos were treated with drugs that interfere with glycoconjugate synthesis. 6-Diazo-5-oxo- l-norleucine was selected as a general inhibitor of glycoconjugates and tunicamycin as a specific inhibitor of asparagine-linked oligosaccharides. Both inhibitors prevented invagination of the optic vesicle and resulted in absence of a cetylpyridinium chloride precipitate at the apical cell surfaces. In addition to the effect within the cavity of the optic vesicle, both inhibitors also affected the attachment between the lens epithelium and the retinal disc. During normal development, the basal laminae of the lateral surface of the optic vesicle and the overlying ectoderm fuse to form a thickened basement membrane at the margin of the future retinal disc. As the retinal disc and lens epithelium invaginate, additional attachments form toward the center of the two folding layers. These attachments are perturbed by both diazo-oxo-norleucine and tunicamycin. However, the lens epithelium forms a vesicle even if the optic cup does not form. Since both inhibitors have been reported to affect processes in addition to glycoconjugate synthesis, the effect of tunicamycin on sugar, amino acid, and sulfate incorporation was studied. The inhibitor had a significant effect only on glucosamine incorporation and appeared to selectively inhibit an increase in incorporation that began in control heads at the time of invagination.