Heat stress can potentially affect most aspects of reproduction in mammals. To our knowledge, no studies have ever been conducted for evaluating the influences of hot season on the developmental competence of ewe oocytes. In the present study, for the first time, we evaluated the effects of season (winter or summer), in vitro thermal stress, and their interaction on the ewe oocytes harvested from slaughterhouse ovaries. Cumulus-oocyte complexes (COCs) were either incubated at 39 °C for the entire length of IVM period or first incubated at 41 °C for 12 h and then at 39 °C. Evaluated endpoints included the ratios of total aspirated COCs/ovary and good-quality COCs/ovary, the apoptosis (Annexin V staining) and nuclear maturation of oocytes after 24-h IVM, and the developmental competence of oocytes after IVF. Our results showed that the number of aspirated oocytes per ovary was similar in both seasons, but the winter ovaries yielded significantly more oocytes with acceptable morphology in winter than in summer (2.1 ± 0.14 vs. 1.5 ± 0.09, P < 0.05). There was a significant interaction between season and thermal stress on the apoptosis, some nuclear maturation parameters, and blastocyst development of oocytes (P < 0.05). Although the winter oocytes were more developmentally competent than the summer oocytes, the winter oocytes were more sensitive to the thermal stress than summer oocytes. In conclusion, the developmental competence of ovine oocytes was lower in summer than in winter. However, it seemed that summer oocytes were more resistant to the in vitro thermal stress during IVM period compared with winter oocytes.
Keywords: Apoptosis; Embryo; Heat stress; Nuclear maturation; Oocyte.