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 prior to insemination in vitro. Exposure to 4°C, but not to 25°C, was associated with a reduced fertilization rate, but development to the blastocyst stage of those oocytes that fertilized was not consistently different from that of non-cooled controls. The reduced fertilization rate seems to result from an effect of cooling on the zona pellucida, since it was not observed if the zona was removed prior to insemination, and since cooling rendered the zona pellucida resistant to the action of chymotrypsin. Using chymotrypsin resistance as an assay, the nature of the cooling-induced effect on the zona was investigated. It is suggested that rapid cooling to 4°C may promote release of cortical granules and a premature zona reaction.