Motility and Fertilizing Capacity of Rabbit Spermatozoa after Freezing in a Medium Containing Dimethyl Sulfoxide.

REsPONSE oF SPERMATOZOA to freezing varies according to species. Hoagland and Pincus observed that rabbit spermatozoa could scarcely revive following immersion in liquid nitrogen after various plasmolytic pretreatment, whereas human spermatozoa remained highly active after freezing and thawing. It has been reported that rabbit spermatozoa are easily damaged by hypertonic salt solutions at temperatures below zero, when the medium becomes concentrated due to separation of water as ice crystals.7 Rabbit sperm are also easily impaired by a sudden rise in concentration of glycerol in their suspending medium.8• 10 According to Smith and Polge, about 30% of rabbit spermatozoa survived when cooled slowly to -79° C. in a medium containing 10% glycerol, but only one fertilized egg was found among 53 eggs from the does inseminated with frozen semen. Gradual addition of glycerol to rabbit semen by dialysis seemed not to improve their fertilizing capacity after freezing and thawing.9 Fox recently reported that 6 rabbits conceived, and 3 litters were obtained out of 22 inseminations with semen stored for 24 hr. at -90° C. in a glycerol-yolk-citrate buffer. However, when we repeated this experiment and cooled rabbit semen to -79° C. instead of -90° C., the motility of the spermatozoa was depressed when medium was added before cooling began, and only a few sperm survived freezing and thawing. Since Lovelock and Bishop reported that dimethyl sulfoxide (though less effective than glycerol) protects bull spermatozoa during freezing, it was thought possible that this agent might better protect rabbit spermatozoa, which are seldom protected by glycerol against dam-