Effect of egg yolk on cryopreservation of rhesus monkey ejaculated and epididymal sperm

Sperm cryopreservation in rhesus monkeys has not been successful when applied in standard, intravaginal artificial insemination; thus, there is a need for substantial improvement in current cryopreservation protocols. The present study was part of our systematic approach to optimize the cryopreservation procedure. Specifically, we tested whether modification of the concentration of egg yolk, the dilution method, and the time delay between ejaculation and adding egg yolk would result in significant improvement of postthaw motility for ejaculated sperm. We also tested the effects of presence and absence of egg yolk on cryopreservation of ejaculated and epididymal sperm. Our findings indicated that the concentration of egg yolk (2%-50%, vol/vol), the dilution method, and the delay (1-5 hours) in addition of egg yolk had no significant effect on postthaw motility of ejaculated rhesus monkey sperm. The presence of egg yolk yielded significantly higher motility after thawing than samples without egg yolk for ejaculated and epididymal sperm. The present study suggests that as long as egg yolk is present in the extender, details such as the amount of egg yolk, as well as when and how to add the egg yolk, have little impact on the ultimate freezing outcomes for ejaculated rhesus monkey sperm. We also discuss the possible mechanism of the protective role of egg yolk in sperm cryopreservation.