Modifications of the lectin binding pattern in the rat zona pellucida after in vivo fertilization

The zona pellucida (ZP) is an extracellular matrix surrounding the mammalian oocyte. It is involved in the sperm-egg adhesion phenomenon, induces the acrosome reaction, and participates in the late blockage to polyspermy. Thus, during the process of fertilization the cortical reaction is induced and the biochemical and biological properties of the ZP are modified. Some of these changes have been suggested to prevent the polyspermy. However, the mechanisms behind most of these changes are not well understood. Carbohydrate residues of the ZP glycoproteins have been shown to play a key role in the early step of fertilization. In the present study, the changes produced in the terminal oligosaccharide sequences of the rat ZP glycoproteins after in vivo fertilization were investigated by means of lectin-gold cytochemistry. A comparative quantitative analysis of the density of labeling in the ZP before and after fertilization was carried out by automatic counting of gold particles. The ZP of fertilized and unfertilized eggs were labeled by a battery of lectins including PNA, LFA, MAA, AAA, DSA, RCA I, and WGA. For all lectin studied in both fertilized and unfertilized eggs the labeling was preferentially located in the inner region of the ZP. After fertilization, binding of PNA, LFA, MAA, AAA, and DSA decreased in both inner and outer regions of the ZP. Labeling of RCA I-binding sites only decreased in the inner ZP, whereas reactivity to WGA was increased in the inner ZP, whereas reactivity to WGA was increased in the inner area of the ZP. Digestion of the thin-sections with neuraminidase prior to labeling with WGA resulted in a decrease of labeling for WGA binding sites. However, the labeling density of WGA binding sites was similar in both unfertilized and fertilized eggs upon treatment with neuraminidase. The present results demonstrate that the oligosaccharide chains contained in the rat ZP are modified after fertilization of the oocyte. Cortical granules of the oocytes might be involved in these modifications by two mechanisms: 1) by hydrolysis of terminal carbohydrate residues of ZP glycoproteins by specific glycosidases contained in the granules; and 2) by addition of new glycoproteins to the ZP after the exocytosis of the cortical granules (cortical reaction).