Developing follicles of the spotted ray Torpedo marmorata express different glycoside residues in relation to granulosa differentiation and vitelline envelope formation

Lectins constitute a class of proteins/glycoproteins that specifically bind to terminal glycoside residues. The present investigation aimed to identify lectin-binding sites in developing follicles of Torpedo marmorata. Using eleven lectins (WGA, GSI-A4, GSI-B4, PSA, UEA-I, PNA, MPA, Con-A, DBA, LCA, BPA, SBA), we demonstrated that the biochemical nature and the distribution of carbohydrate residues significantly change during oogenesis in the granulosa cells and the vitelline envelope. In fact, a progressive appearance of surface glycoproteins bearing terminated ss-GlcNAc O-linked side chains was observed in the granulosa during the differentiation of pyriform-like cells from the small ones via intermediate cells simultaneously with a significant reduction of the D-Gal chains present in their nucleus. Glycoproteins bearing ss-GlcNAc O-linked side chains were first evident on the surface of small cells in contact with the oocyte, then on the intermediate ones, and finally on pyriform-like cells. The distribution pattern of such glycoproteins over the differentiated granulosa cells remained unchanged during the subsequent stages of the oocyte growth so granulosa cells preserved the same sugar distribution pattern. Furthermore, a progressive loss of D-Gal residues was evident in the nucleus of granulosa cells. In fact, staining for D-Gal was intense in the nucleus of small follicle cells and progressively reduced till disappearing in differentiated pyriform-like cells. Conversely, the small follicle cells located under the basal lamina were devoid of ss-GlcNAc residues, and the nuclear content in D-Gal remained unchanged. This finding strongly suggests that surface glycoproteins containing ss-GlcNAc residues, and the nuclear content in D-Gal might be related to the differentiation of pyriform-like cells. The present investigation also demonstrates that the content of the sugar residues of the vitelline envelope (VE) changes during oocyte growth, suggesting that pyriform-like cells may contribute to its formation.