Ultrastructural Study of the Canine Zona Pellucida Surface DuringIn VitroMaturation

Effects of oxygen concentration on in vitro maturation of canine oocytes in a chemically defined serum-free medium

Canine oocytes require an extended period of culture (72 h) in vitro for nuclear maturation to the metaphase II stage, which also results in high degeneration. Canine cumulus oocyte complexes were isolated by slicing from ovaries collected after ovariohysterectomy and cultured in serum-free synthetic oviductal fluid incubated at low (5%) or high (20%) oxygen levels. Changes in oocyte nuclear maturation rates, H2O2 levels within the oocytes and mRNAs of reactive oxygen species inhibitory genes superoxide dismutase 1 and 2 (SOD1 and 2), glutathione reductase (GSR), glutathione peroxidase (GPX1), and catalase (CAT) were quantified. Higher meiotic resumption from germinal vesicle breakdown up to MII was observed in low O2 (41.8±13.1%) compared to high O2 (15.8±8.2%) (P=0.014) after 52 h of culture (n=112). Extension of the culture period up to 84 h at low O2 (n=457 oocytes) produced the highest meiotic resumption at 72 h (64.1±6.0%; P=0.008), compared with 52 h. Oocytes (n=110) cultured in high O2 contained higher levels of peroxidase measured using the 2′,7′-dichlorodihydrofluorescein diacetate fluorescence assay after 72 h of culture compared with low O2 (P=0.004). High O2-cultured oocytes also showed higher amounts of SOD1, SOD2, GSR, GPX1, and CAT mRNA. Vitamin E in high oxygen level was able to decrease degeneration (P=0.008) but had no improving effect on percentage of oocytes in MII. These results for the first time showed that low oxygen gas composition improves nuclear maturation rates and alleviates the oxidative stress for canine oocytes during in vitro maturation.

Quantitative analysis of the intensity of zona pellucida birefringence of oocytes during IVF cycles

To determine whether the age of the female patient, oocyte maturation stage, pronuclei, embryonic quality and/or cycle outcome are related to the intensity of the zona pellucida birefringence (ZPB) in oocytes during IVF procedures, a retrospective study was conducted on 214 IVF cycles using a polarisation imaging system. A negative correlation was found between ZPB intensity and the age of female patients (r = -0.44; P < 0.0001). For oocytes at different maturation stages, the ZPB score was higher at the germinal vesicle stage (20.77 ± 1.88) than at MI (10.99 ± 1.35; P < 0.001), and higher at MI than at MII (7.91 ± 0.65; P < 0.05). For the pronuclei (PN) of fertilised oocytes, the ZPB score of oocytes with 3PN was significantly higher than that of oocytes with 1PN or 0PN (7.94 ± 0.78 v. 5.57 ± 1.51 and 4.45 ± 0.85, respectively; P < 0.001). However, there were no differences in ZPB scores between oocytes with 2PN and 3PN. Neither embryo quality on day 3 nor the clinical results could be predicted on the basis of the ZPB score (P > 0.05).