The influence of cooling on the properties of the zona pellucida of the mouse oocyte

Mouse oocytes were exposed to a variety of cooling regimes priorto insemination in vitro. Exposure to 4°C, but not to 25°C,was associated with a reduced fertilization rate, but developmentto the blastocyst stage of those oocytes that fertilized wasnot consistently different from that of non-cooled controls.The reduced fertilization rate seems to result from an effectof cooling on the zona pellucida, since it was not observedif the zona was removed prior to insemination, and since coolingrendered the zona pellucida resistant to the action of chymotrypsin.Using chymotrypsin resistance as an assay, the nature of thecooling-induced effect on the zona was investigated. It is suggestedthat rapid cooling to 4°C may promote release of corticalgranules and a premature zona reaction.