Relation of the Position of Bovine Ova and Zona Fracture in the Straw during Freezing and Thawing

Abstract

The influence of the ovum location in a plastic straw during freezing and thawing was investigated as to the morphology of zonae. A fertilized bovine ovum was in a plastic straw by successive aspiration located on cotton-plugs (group I), into the meniscus between two adjacent air babbles (group II) or in the center of 10% glycerol solution (group III) while the straws were kept vertically. In group IV the ovum was attached to the inner wall of the middle portion of 10% glycerol solution while the straws were held horizontally. The straws were cooled to -5°C from room temperature at the rate of 1°C/min. After 5 min the ova were seeded and kept at the seeding temperature for further 10 min, and then were cooled to -30°C at the rate of 0.3°C/min before being plunged into LN₂. The ova thus treated were thawed in the water bath at 37°C and examined for the occurrence of damaged zonae under a stereomicroscope.
The occurrence frequencies of ova with damaged zonae were 42.6% (86/202), 21.6% (22/102) and 22.0% (22/100) for group I, II and III respectively. In group IV the percentage of ova with damaged zonae was as low as 16.2 (44/271). A statistically significant difference was observed between group 1 and II, III and IV.