255 PRONUCLEUS FORMATION IN RAT ZONA-FREE OOCYTES CO-CULTURED WITH HOMOLOGOUS POST-THAW SPERMATOZOA

Abstract

The aim of the present study was to develop an IVF system with frozen/thawed rat spermatozoa. We examined the effect of cooling rate to 5.0°C on post-thaw sperm motility and membrane integrity, and also investigated the ability of post-thaw spermatozoa to form pronuclei. Under room temperature, epididymal spermatozoa of Wistar rats were collected in 2.0 mL of egg yolk medium containing 8.0% (w/v) lactose monohydrate and 0.7% (v/v) Equex Stem. Samples were loaded into 0.25-mL straws and cooled to 5.0°C in the chamber of a programmed freezer. For cryopreservation, the samples were exposed to liquid nitrogen (LN) vapor for 10 min and then plunged into LN. Straws were thawed in a 37.0°C water bath for 10 s. Ovulated oocytes were collected and the zona pellucidae were removed with 0.1% pronase. One-hundred μL of thawed samples were put into a droplet of 400 μL R1ECM and pre-incubated for 1 h. R1ECM solution was added to the droplet to adjust to 0.5–1.5 × 10⁶ sperm mL⁻¹. The zona-free oocytes were then transferred into the droplet and co-cultured for 10 h. Oocytes were observed for pronuclei formation by means of an inverted phase contrast microscope. In Experiment I, the influence of sperm cooling rate to 5.0°C on sperm motility and membrane integrity was evaluated. Portions of samples were cooled at 54.0°C/min, 0.9°C/min, 0.5°C/min, and 0.3°C/min. The remainders were then frozen. The non-cooled samples were designated as controls. In Experiment II, we examined whether post-thaw spermatozoa have the ability to form pronuclei in vitro or not. All percentage data were arc-sine transformed and then analyzed by the Student's t-test. In Experiment I, the membrane integrity between the spermatozoa cooled at 0.5°C/min and the non-cooled spermatozoa was not different (38.1% vs. 37.2%; P > 0.05), but the integrity of these was higher than in spermatozoa cooled directly at 54.0°C/min (38.1% vs. 25.3%; P < 0.05). After culture for 1 h, the motility of spermatozoa cooled at 0.5°C/min was higher than that of those cooled at 54.0°C/min (61.3% vs. 53.3%; P < 0.05). At 2 h post-thaw the motility of spermatozoa cooled at 0.5°C/min was higher than that of spermatozoa cooled at 54.0°C/min and at 0.9°C/min (11.0% vs. 4.5%, 4.9%; P < 0.05). The membrane integrity of post-thaw spermatozoa cooled at 0.5°C/min was also higher compared to that of spermatozoa cooled at 54.0°C/min (22.5% vs. 8.4%; P < 0.01). In Experiment II, 28 (26.2%) of 107 oocytes had pronuclei when the post-thaw spermatozoa cooled at 0.5°C/min were used. The results indicated that the frozen/thawed spermatozoa cooled to 5.0°C at 0.5°C/min showed higher sperm motility and membrane integrity, and that spermatozoa can form pronuclei in homologous zona-free oocytes in vitro. Although in the rat sperm damage occurred during cooling to 5.0°C, and sperm motility and membrane integrity were also decreased by the cold shock, it is possible to decrease the damage by cooling slowly to 5.0°C at 0.5°C/min.