MODIFICATIONS OF THE LECTIN-BINDING PATTERN IN THE RAT ZONA-PELLUCIDAAFTER IN-VIVO FERTILIZATION

The zona pellucida (ZP) is an extracellular matrix surrounding the mam malian oocyte. It is involved in the sperm-egg adhesion phenomenon, in duces the acrosome reaction, and participates in the late blockage to polyspermy. Thus, during the process of fertilization the cortical rea ction 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 a re 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 oligosacch aride sequences of the rat ZP glycoproteins after in vivo fertilizatio n were investigated by means of lectin-gold cytochemistry. A comparati ve quantitative analysis of the density of labeling in the ZP before a nd after fertilization was carried out by automatic counting of gold p articles. 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 l abeling was preferentially located in the inner region of the ZP. Afte r fertilization, binding of PNA, LFA, MAA, AAA, and DSA decreased in b oth inner and outer regions of the ZP. Labeling of RCA I-binding sites only decreased in the inner ZP, whereas reactivity to WGA was increas ed in the inner area of the ZP. Digestion of the thin-sections with ne uraminidase prior to labeling with WGA resulted in a decrease of label ing for WGA binding sites. However, the labeling density of WGA bindin g sites was similar in both unfertilized and fertilized eggs upon trea tment with neuraminidase. The present results demonstrate that the oli gosaccharide chains contained in the rat ZP are modified after fertili zation of the oocyte. Cortical granules of the oocytes might be involv ed in these modifications by two mechanisms: 1) by hydrolysis of termi nal 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 reacti on). (C) 1996 Wiley-Liss, Inc.