118 CRYOPRESERVATION OF CONVENTIONAL AND SEX-SORTED BULL SPERM USING A DIRECTIONAL FREEZING METHOD

Abstract

The objective of this study was to find an optimal parameter (liquid-ice interface velocity; velocity) of the directional freezing method (Arav et al. 2002 Reprod. Nutr. Dev. 42, 583–586) for conventionally processed bull sperm stored in 0.5-mL straws. We also evaluated sex-sorted bull sperm frozen using this method. Experiment 1: Each single ejaculate from two Holstein bulls was split and processed in egg yolk citrate extender (2 steps, 6% glycerol final; EYC) or egg yolk Tris extender (1 step, 7% glycerol final; EYT1). After cooling and loading in 0.5-mL plastic straws, each batch of semen was frozen using a directional 0.5-mL straw freezer (MTG 550; IMT, Ltd., Ness-Ziona, Israel) in 6 different velocities (v2.0, v2.2, v2.4, v2.6, v2.8, v3.0; mm s^-1). Ice seeding time was 45 s. Static freezing in liquid nitrogen vapor was used as the control. Sperm parameters were measured using computer-assisted semen analysis (CASA) at 0, 1, and 2 h after thawing. The thawed samples were also evaluated for sperm viability and acrosomal integrity by triple staining. The experiment was repeated 3 times. Data were analyzed by ANOVA. No significant differences in general and progressive motility were observed in each extender regardless of the freezing method. However, sperm viability and acrosomal integrity of sperm frozen in EYC were highest at v3.0 (73.2% and 84.7%, respectively) and were generally higher after MTG freezing (63.4 to 73.2% and 80.6 to 84.7%, respectively) than in the control (52.6% and 73.1%, respectively; P < 0.05 or 0.01) except for v2.4 (54.5% and 70.8%). Sperm viability (74.6 to 77.9% vs. 63.2%) and acrosomal integrity (84.8 to 88.9% vs. 79.3%) in EYT after MTG freezing were also higher than in the control (P < 0.01). Experiment 2: Two single ejaculates from a Holstein bull were flow cytometrically sex-sorted (Schenk et al. 1999 Theriogenology 52, 1375–1391) for X sperm. Each ejaculate was assigned to processing in EYC or egg yolk Tris extender (2 steps, 6% glycerol final; EYT2). Processed sperm was frozen using MTG 550 (velocity set at v3.0) or in liquid nitrogen vapor as control and analyzed as in Experiment 1. In the control, general (40.2% vs. 21.1%; P < 0.01) and progressive (16.3% vs. 4.3%; P < 0.05) motility in EYC were higher than in EYT2, respectively, at 0 h. In MTG freezing, motility (at 0 h, 42.0% vs. 28.3%; P < 0.05), viability (60.5% vs. 40.8%; P < 0.01), and acrosomal integrity (86.9% vs. 71.0%; P < 0.05) in EYC were higher than in EYT2, respectively. But in experiment 2, there were no differences in motility, progressive motility, viability, and acrosomal integrity between MTG freezing and control. The above results suggest that MTG freezing improves membrane and acrosomal quality of bull sperm frozen in 0.5-mL straws. Faster interface velocity (3.0 mm s^-1) seems optimal for the given condition. Egg yolk citrate extender seems to be beneficial for MTG freezing of sex-sorted bull sperm, but further studies using ejaculates from different bulls are required to confirm the apparent benefit.