54 CRYOPRESERVATION OF DOMESTIC CAT EPIDIDYMAL SPERM IN A DEFINED EXTENDER WITHOUT ANIMAL OR PLANT PROTEINS

Abstract

Previously, we have shown that survival of cat sperm is maintained in both non-egg yolk, semi-defined extenders and in extenders with greatly reduced levels of egg yolk (2%). Usually, cryoprotectant is added to extended samples after gradual cooling to 4°C, but recent reports have shown that satisfactory sperm survival can be obtained after addition at 22°C. Here, our objectives were to examine sperm survival after (1) cryopreservation from 22°C vs after gradually cooling to 4°C or (2) cryopreservation in a completely defined extender without animal or plant proteins vs extender + 2% egg yolk. Epididymides from local veterinary clinics were dissected in HEPES 199 medium (He199). The sperm suspension was filtered (40 μ), layered onto a density gradient column and centrifuged at 650 × g for 20 min. Then, the sperm pellet was resuspended in 1 mL of He199 and centrifuged for 5 min at 800 × g and the subsequent pellet was extended in TEST Buffer with either 0% (0% EY) or 2% egg yolk (2% EY). Next, 0% EY samples were further split into 2 groups—either gradually cooled to 4°C before 12% glycerol (1:1) was added (4C-0%EY) or 12% glycerol (1:1) was added at 22°C without cooling (22C-0%EY). Control samples extended in 2% EY were cooled to 4°C before addition of 12% glycerol (1:1) (4C-2%EY). Samples were loaded into 0.25-mL straws and placed in a –80°C freezer for 20 min before storage in LN2. Sperm samples were thawed in air (22°C) for 5 s and immersed in a 60°C water bath for 5 s. After a 7-step addition of He199, samples were centrifuged at 800 × g for 5 min and pellets resuspended in He199. Sperm samples were evaluated for motility (Mot; computer-assisted semen analysis, 37°C) at 0 h (initial assessment), after cooling to 4°C (PC) and at 0-h (0-PT) and 3-h post-thaw (3-PT) incubation at 37°C. Membrane integrity (MI; SYBR 14-PI) and acrosomal status (AS; FITC-PNA) were analysed at the initial assessment, 0-PT and 3-PT. Results are shown in Table 1. At 4°C (PC), sperm extended in 0% EY and 2% EY maintained 92 and 91%, respectively, of their initial motility (66%). At 0-PT and 3-PT, motility in the 3 groups had decreased by >50% and >70%, respectively. Motility at 3-PT in the 22C-0%EY treatment was less than the other 2 treatments (P < 0.05; 1-way ANOVA). At 0-PT, samples in the 4C-2%EY group had a higher membrane integrity value (P < 0.05) than did the 22C-0%EY group, whereas that of the 4C-0%EY group was not different from the other 2 groups. However, at 3-PT, both groups cooled to 4°C before cryopreservation had higher membrane integrity values (P < 0.05) than the group cryopreserved at 22°C. At 0-PT and 3-PT, the percentage of sperm with intact acrosomes ranged from 69% (4C-2%EY) to 59% (22C-0%EY) and from 55% (4C-2%EY) to 43% (22C-0%EY) of the initial value (89%), respectively. In summary, we demonstrated that cat epididymal sperm could be frozen successfully in a completely defined TEST-buffered extender. Furthermore, we confirmed that addition of cryoprotectant (i.e. glycerol) after gradual cooling to 4°C is beneficial to post-thaw survival.

Table 1.  Motility (Mot), membrane integrity (MI) and acrosomal status (AS) of cat epididymal sperm before and after cryostorage