Dilauroylphosphatidylcholine liposome effects on the acrosome reaction and in vitro penetration of zona‐free hamster eggs by bull sperm: II. A fertility assay for frozen‐thawed semen

Abstract Frozen‐thawed sperm from five bulls with fertility rates ranging from 48% to 77% were treated with seven concentrations of dilauroylphosphatidylcholine (PC12) liposomes to induce an acrosome reaction (AR) that enabled sperm to penetrate eggs. Treated sperm were incubated with liposomes for 7 min prior to insemination of zona‐free hamster eggs in vitro. Sperm and eggs were incubated 3 hr at 39°C prior to fixation, staining, and examination for sperm penetration and nuclear decondensation. The percentage of motile sperm immediately after thawing as well as after treatment with liposomes had a low correlation with sire fertility (r = .39 and ⩽.63, respectively). The percentage of sperm exhibiting an AR was more highly correlated with fertility (r ⩽ −.85). Similar correlations were found between fertility and the penetration rates of zona‐free hamster eggs or the total number of penetrating sperm. When data for two high and for two lower fertility buils were each grouped to increase information per data point the correlation between the PC12 concentration giving the maximum proportion of eggs penetrated and fertility was r = .92 (P ≤ .05). The correlation between the PC12 concentration producing the most total sperm penetrating the eggs and fertility r = .97 (P ≤ .05). It was concluded that PC12 liposomes induced an AR in bull sperm frozen‐thawed in egg yolk extender. Frozen‐thawed sperm from low fertility bulls require less PC12 to induce the AR and to penetrate zona‐free hamster eggs than do sperm from higher fertility bulls. These differences in lipid requirements may help to provide a quick, direct laboratory assay method to estimate the fertility of frozen bull semen.