Heat shock may affect different aspects of oocyte maturation and its subsequent development to the blastocyst stage. A series of in vitro experiments was performed to determine whether physiologically heat shock (41°C) disrupts the progression of the ovine oocytes through meiosis, activation and blastocyst formation. Cumulus-oocyte complexes (COCs) were aspirated from 2-6-mm follicles and cultured at 38.5°C (control) or 41°C (heat shock) for the first 12 h of maturation. The oocytes were incubated at 38.5°C during the last 10 h of maturation and 8 days after activation. Results showed that most of the oocytes matured under heat-shock conditions remained at the germinal vesicle breakdown (GVBD) stage and they showed an aberrant chromatin configuration. After heat shock, oocyte diameter and time spent for zona pellucida dissolution increased (P < 0.05). The heat-shocked group had a higher percentage of oocytes with incomplete migration of cortical granules (P < 0.05). The heat-shock condition decreased (P < 0.05) cleavage rates (56.19 versus 89.28%) and morula formation (26.85 versus 37.81%). However, there was no significant difference in blastocyst formation and percentage of hatched blastocysts. At 12 h, heat shock had an adverse effect on embryo quality and reduced inner cell mass number (P < 0.05). Quantitative gene expression analysis showed greater transcripts (P < 0.05) for Na/K-ATPase mRNA in heat-shocked oocytes. To sum up, heat shock has disruptive effects on ovine oocyte maturation and can impair cellular and molecular factors that are important for embryo development.
Keywords: Blastocyst; Embryo; Heat shock; Nuclear maturation; Ovine oocyte.