Effect of Different Thawing Rates on Post-Thaw Viability, Kinematic Parameters and Chromatin Structure of Buffalo (Bubalus bubalis) Spermatozoa.

Objective: The aim of the present study was to evaluate three thawing rates on the post thaw motility, viability and chromatin structure of buffalo semen frozen in 0.5-ml straws.

Materials And Methods: In this experimental study semen was collected with artificial vagina (42℃) from four buffalo bulls.Split pooled ejaculates (n=4) were extended at 37℃ with a Bioxcell® extender. Semen was cooled to 4℃ within 2 hours, equilibrated at 4℃ for 4 hours, then filled in 0.5 ml French straws, and frozen in programmable cell freezer before plunging into liquid nitrogen. Straws were thawed at water bath temperatures of 37, 50 or 70℃ for 30, 15 and 6 seconds, respectively. Semen was incubated at 37℃ for 2 hours and evaluated for post thaw motility, viability, acrosomal and DNA integrity of spermatozoa. Analysis of variance (ANOVA) was used for comparisons of means. When the ANOVA test showed statistical differences, the mean of the treatments were compared using Duncan's multiple range tests.

Results: The initial postthaw motility (0 hour) averaged 62.7 ± 7.2%, 73.1 ± 9.77%, and 74.9 ± 8.58% for the three thaw rates, respectively. Kinematic parameters such as average path velocity, linearity and beat/cross frequency in the thaw rate of 70℃ for 6 seconds were superior to other rates studied (p<0.05). After 2 hours of incubation, proportions of progressive motility and Kinematic parameters decreased in all groups (p>0.05). A positive correlation was detected between sperm motility and thawing rate after two hours incubation times. The percentage of viable spermatozoa and spermatozoa with an intact acrosome and plasma membrane integrity were not different between the groups of samples thawed at different temperatures (p>0.05). The percentage of spermatozoa with chromatin dispersion forthe thaw rate of 70℃ for 6 seconds was significantly higher than for the to other rates studied (p< 0.05). In contrast with motility and viability, the DNA integrity of post thaw spermatozoa remained unaffected during 2 hours incubation.

Conclusion: The post thaw motility and kinematic parameters of buffalo spermatozoa were significantly improved immediately after thawing by increasing the thawing rate from 37℃ in 30 seconds to70℃ in 6 seconds. However, this relative advantage had disappeared after incubation in a water bath at 37℃ for two hours.A thaw rate of 70℃ for 6 seconds was associated with higher chromatin dispersion than the other thaw rates studied. Sperm thawing over at 50 degrees could be safely used to improve motility recovery after sperm cryopreservation in buffalo bulls.