Physiological and molecular changes of hamster egg surfaces upon sperm-egg interaction
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Temporal changes in the binding properties of hamster zona pellucida and egg plasma membrane upon sperm-egg interaction and the occurrence of the polyspermy block are investigated by an in vitro reinsemination method. The eggs are triggered by preinsemination with a low sperm concentration for various times followed by reinsemination with a higher sperm concentration. The physiological changes are detected by the deviation of the binding properties and the levels of polyspermy from those of the controls without preinsemination. The results indicate that temporal changes in the binding properties of hamster zona pellucida and egg plasma membrane correlate with the polyspermy block; both occur as early as 20 and 15 min following insemination of zona-intact and zona-free eggs, respectively. The time correlation of egg surface changes with the cortical reaction seems to indicate the involvement of the cortical reaction in the observed physiological changes. Despite the physiological changes, scanning electron microscopic analysis of gold-lectin labelling reveals no observable alteration of the terminal sugar residues on the zona pellucida. Plasma membrane shows a better correlation of the molecular and physiological changes. There is a slight reduction of terminal D-galactose and N-acetyl-D-glucosamine residues by 45 min after insemination of zona-free eggs. Decline of terminal mannose residues is observed as early as 20 min after insemination, and there is approximately 20 min after insemination, and there is approximately a 98% reduction by 45 min after insemination as shown by both gold-Con A and ('125)I-Con A labelling. This decline of the sugar residues may be a consequence of the cortical reaction. The surface topography of both zona pellucida and plasma membrane is also altered after insemination. The zona surface exhibits additional "depressions" or "channel-like" structures which may be the result of digestion by bound sperm. Distortion at certain regions of the zona surface by penetrating sperm is also seen. The plasma membrane surface has changed from microvillous to a ruffled surface by 45 min after insemination of zona-free eggs. The late plasma membrane surface changes may be a result of the completion of the cortical reaction or of other early development events.