145 STAGE-SPECIFIC EFFECT OF OXIDATIVE STRESS ON DEVELOPMENTAL COMPETENCE, ROS GENERATION AND DNA DAMAGE OF PORCINE PARTHENOGENETIC EMBRYOS

Abstract

We investigated the effect of oxidative stress on stage specific developmental ability, reactive oxygen species (ROS) generation and DNA damage of parthenogenetically activated porcine embryos. Cumulus-oocyte complexes (COCs) were aspirated from follicles on the surface of ovaries. The COCs were matured in NCSU-23 containing 10% (vol/vol) porcine follicular fluid and 10 IU/mL hCG during the first 22 h followed by an extra 22 h of culture in the hormone free NCSU-23. After 44 h of maturation, oocytes were denuded of cumulus cells and used for activation. Oocytes were activated by a 100-¼sec pulse of 1.5 kV/cm DC with 1-mm electrodes in 0.3 m mannitol, 0.1 mm MgSO₄, and 0.05 mm CaCl₂. Activated oocytes were then cultured for 5 h in NCSU-23 containing 5 mg/mL BSA, 10 ¼g/mL EGF and 7.5 ¼g/mL cytochalasin B. Embryos were then cultured for 6 days in PZM-5. In Experiment 1, after parthenogenetic activation, embryos were cultured at 38.5°C under 5% O₂, 5% CO₂ and 90% N₂ (defined as 5% O₂) or 5% CO₂ in air (20% O₂). The oxygen concentration for embryo culture was changed from 5% to 20% on day 1, 2, 3, 4, and 5 post-activation, respectively. Embryos were also cultured throughout 6 days in 5 and 20% O₂. About 100 embryos were used in each experiment. The number of embryos cleaved and developed to blastocyst stage was observed on day 2 and 6, respectively. In Experiment 2, 10 to 20 embryos cultured in 5 and 20% O₂ were collected on Days 2, 4, and 6 for the detection of ROS, intracellular glutathione (GSH) levels and DNA damage. Intracellular ROS and GSH levels, were measured with fluorescent dyes (22,72-dichlorodihydrofluorescein diacetate for ROS and Cell Tracker^" Blue for GSH). DNA damage of individual embryos was detected with a comet assay. DNA damage was quantified by measuring the length of the streak of DNA comet tail between the edge of the zona pellucida and the end of the visible comet tail by image analysis software. The rate of migrated DNA area per total DNA was also quantified. In Exp. 1, the rate of blastocyst formation was significantly decreased (P < 0.001) when embryos were cultured for 6 days under 20% O₂ (17.8 ± 4%) than 5% O₂ (38.5 ± 5%). The rates of blastocyst formation were significantly decreased (P < 0.05) when O₂ concentration was changed from 5 to 20% before Day 3. After Day 4, high O₂ concentration did not affect the development. In Exp. 2, relative ROS levels were significantly higher (P < 0.05) on Day 2 (1.5 ± 0.03) and Day 4 (1.4 ± 0.06) in embryos cultured under 20% O₂ than in those cultured under 5% O₂ (1.0). No difference was observed in GSH level. DNA damage was significantly increased (P < 0.05) in Day 2 embryos cultured under 20% O₂ (161 ± 54 ¼m) than 5% O₂ (65 ± 8.8 ¼m). These results indicate that the oxidative stress to embryo development by high O₂ concentration is stage specific, that embryos are more sensitive in early stages, and that the oxidative stress has correlation with the increase of intracellular ROS and DNA damage.