The aim of this study was to compare two ethylene glycol (EG) equilibration procedures for the quick freezing of in vitro-produced bovine embryos. Cumulus-oocyte complexes (COCs) were collected from slaughterhouse ovaries. COCs were matured in TCM199 containing 10% of bovine fetal serum, LH, FSH and E2, and fertilized. Presumptive zygotes were co-cultured in TCM199 with a granulosa cell monolayer, at 39°C in humidified atmosphere of 5% CO2 in air. Grade 1, expanded blastocysts (n=761) were selected 7 and 9 days after insemination and randomly distributed to one of eight treatment groups. In Equilibration Procedure 1, embryos were exposed to 10% EG for 5 min, and then to 17%, 22% or 28% EG for 60s (respectively referred to as EG 17, EG 22 and EG 28). In Equilibration Procedure 2, embryos were exposed to the same EG solutions as in Equilibration Procedure 1, but the period of exposure was 10min to 10% EG and 30 s to EG 17, EG 22 and EG 28. In Equilibration Procedure 3 (slow-freezing controls), embryos were exposed to 10% EG for either 5 or 10min and then cryopreserved by slow-freezing method at 1.2°C/min. In all treatment groups, EG solutions were prepared in PBS+0.2% BSA, and embryos were exposed to EG solutions at 22°C. Embryos were loaded into 0.25mL straws and heat-sealed. Straws were cooled in liquid nitrogen vapor for 2min, and then plunged into and stored in liquid nitrogen. Straws were thawed in room temperature air for 10s, and then in 25°C water for 20s. Thawed embryos were diluted by transferring them into 0.5ml of PBS+0.2% BSA+0.3M sucrose for 3min, and then 0.5mL of PBS+0.2% BSA for 3min. Embryos were co-cultured on granulosa cell monolayer in TCM199 and evaluated after 24h for blastocyst re-expansion (EXP), and again at 48, 72 and 96h for hatching (HAT). A total of 724 in vitro-produced bovine blastocysts were used as controls to determine hatching rates. The results are presented in the table. Embryos exposed to 10% EG for 10min (Equilibration Procedure 1) yielded significantly higher rates of blastocyst re-expansion and hatching when compared to embryos exposed for 5min (Equilibration Procedure 2, P<0.05). These results suggest that quick freezing of in vitro-derived bovine embryos may be an alternative to vitrification; however, additional studies are needed to optimize cryopreservation protocols and increase post-thaw survival. This project was supported by FAPESP (01/11266-4)
Table 1
Effect of equilibration procedure on in vitro re-expansion and hatching rates of embryos cryopreserved by slow and quick freezing methods
113COMPARISON OF TWO ETHYLENE GLYCOL EQUILIBRATION TREATMENTS FOR THE
QUICK FREEZING OF IN VITRO-PRODUCED BOVINE EMBRYOS