Holding immature bovine oocytes in a commercial embryo holding medium: High developmental competence for up to 10Â h at room temperature

Bovine in vitro embryo production (IVP) following Ovum Pick Up (OPU) is all too often hampered by a large time gap between the harvest of oocytes of the first and last OPU session of the day. Immediately after retrieval, oocyte maturation is initiated, resulting in oocytes maturing at different time points which necessitates laborious scheduling of the IVP process. In this study, the potential of a commercial embryo holding medium (EHM; Syngro, Bioniche Inc.) to hold immature bovine oocytes was validated. We assessed the effect of holding time and temperature on (1) oocytes' maturation; (2) blastocyst development and quality at day 8 post insemination; and (3) blastocyst yield in small groups of oocytes/zygotes simulating OPU settings. Oocytes, harvested from slaughterhouse ovaries, were held for 6 h (either at 4 °C, room temperature [RT; 22-25 °C], or 38.5 °C), for 10 h (at 4 °C or RT), and for 14 h (only at RT) in 1 mL sterile glass osmometer tubes filled with EHM prior to standard maturation (22 h at 38.5 °C) and subsequent IVP. Results were compared with controls in which no prior holding was applied. Differences between the treated and control groups were assessed by generalized mixed-effects models and considered significant at P < 0.05. Generally, oocytes held up to 14 h in EHM at different temperatures remained at the germinal vesicle stage. Holding immature oocytes in EHM for 6 h at 38.5 °C and for 10 h at 4 °C significantly decreased maturation (57.1 ± 4.1% VS 80.9 ± 3.2% and 68.6 ± 3.5% VS 80.7 ± 2.9%; respectively), and development (11.0 ± 1.8% VS 36.2 ± 2.8% and 20.1 ± 3.3% VS 40.6 ± 4.6%) (P < 0.05). However, holding in EHM for both 6 and 10 h at RT, did not affect the maturation rates (83.2 ± 2.9% and 78.9 ± 3.2%) nor day 8 blastocyst rates (35.2 ± 2.7% and 40.2 ± 4.5%). Prolonging holding time to 14 h in RT decreased maturation and day 8 blastocyst yield (71.9 ± 3.5% VS 84.5 ± 2.7% and 25.7 ± 2.5% VS 39.5 ± 2.8%, respectively) (P < 0.05). Holding oocytes in EHM did not significantly affect embryonic quality as assessed by differential apoptotic staining in any of the time points. To simulate OPU-settings, small groups of 10 oocytes were held in EHM for 6 or 10 h at RT. When subsequently matured, fertilized and cultured per 8 zygotes, day 8 blastocyst rate was not affected (19.8 ± 3.5% VS 20.6 ± 3.6% and 18.8 ± 3.6% VS 18.3 ± 3.4%). In conclusion, immature bovine oocytes can be successfully conserved in EHM at RT for up to 10 h without compromising their embryonic developmental competence nor quality.