The role of carbohydrates in sperm-egg interaction in rats

The first step in fertilization is the interaction of the capacitated sperm with the zona pellucida. It has been proposed that the initial interaction, as in other types of cell adherence, is due to complementary interacting sites on the opposing surfaces of the gametes. This work intended to investigate the role of carbohydrates in sperm-egg binding in rats. Ejaculated sperm was collected from uterine horns of mated females. The sperm was suspended in Rat Fertilization Medium at final concentrations of 3-7 times 10(5) sperm/ml. After 5 1/2 h of sperm incubation, eggs were added to the sperm suspensions concomitantly with various carbohydrates to achieve a final concentration of up to 50 mM. The eggs were separated after 30 min, and the number of sperm bound to the zona pellucida was counted. Among a variety of monosaccharides tested at 50 mM concentration, it was found that alpha-methyl-mannoside was the most potent inhibitor (producing 80% inhibition); less potent was D-mannose and even less, L-fucose. A combination of alpha-methyl-mannoside and L-fucose showed a synergistic effect. Mannan was not more effective as an inhibitor than the monosugar mannose, while fucoidin was extremely potent, causing over 90% inhibition of binding at 0.1%. We assume the presence of macromolecules containing sugars on the zona pellucida because inhibition of sperm binding to this layer was observed: a) after preincubation of mannan or fucoidin with sperm, but not with the eggs; and b) after pretreatment of the egg with specific enzymes. The results obtained in this study in the rat are consistent with the hypothesis that carbohydrates are critical for the sperm-egg interaction.