Effect of Ovarian Storage Temperature and Time on Post Thaw Viability and Maturation Rate of Vitrified Immature Oocytes in Sheep

Abstract

BACKGROUND: Vitrification of oocytes as a method of cryopreservation is quite successful, although it is still being standardized because of structural and molecular sensitivity of oocytes to the cooling and freezing process. OBJECTIVE: To investigate the effect of ovarian storage temperature and time on post thaw viability and maturation rate of vitrified immature oocytes in sheep. MATERIALS AND METHODS: The work consisted of oocyte collection from ovaries of abattoir sheep stored at various temperature (0 °C , 4 °C and 25 °C) and time (0 h, 6 h, 12 h and 24 h) combinations and post thaw viability and in vitro maturation rate evaluation. Vitrification was done in 30% vitrification solution, using ethylene glycol and DMSO, with post vitrification evaluation after 1 week 's storage. RESULTS: Significantly higher post thaw viability was observed after storage at 0 °C for 6 h (95.3%) followed by 12 h (85%), with lowest value at 24 h (66.7%). However at 4 °C and 25 °C, values were nonsignificantly higher after 6 h (96.5 and 100% respectively) followed by 12 h (93 and 100%), with significantly lower values after 24 h (85.7 and 90.7%). At storage temperatures of 25 °C and 4 °C, a significantly higher percentage of mature oocytes was observed after 6 h (40 and 39.1%), 12 h (37.3 and 38.1%) and 24 h (34.6 and 36.4%) storage times compared to that at 0 °C (20.3% at 6 h, 14.2% at 12 h and only 13.8% at 24 h). However, at all storage temperatures, there was a tendency for the level of mature oocytes to decrease with storage time, and the levels were significantly lower than the control. CONCLUSION : Acceptable post thaw viability and in vitro maturation rates for oocytes is maintained up to 24 h in ovaries stored at 4 °C and 25 °C compared to at 0 °C, and these conditions may be used for the storage of ovaries meant for oocyte preservation.