Lowering storage temperature during ovary transport is beneficial to the developmental competence of bovine oocytes used for somatic cell nuclear transfer

The objective of this study was to determine the effect of storage temperature during ovary transport on the developmental competence of bovine oocytes for use in somatic cell nuclear transfer (SCNT). Ovaries obtained from a slaughterhouse were stored in physiological saline for 3-4h at one of the three temperatures: 15 °C, 25 °C, or 35 °C. The developmental competence of oocytes used for SCNT was ascertained by cleavage and blastocyst formation rate, total cell number, apoptosis index, and the relative abundance of Bax and Hsp70.1 in day 7 blastocysts. Ovaries stored at 35 °C for 3-4h reduced the recovery rate of grade I and II oocytes compared with those stored at 25 °C or 15 °C (45.1±0.7% vs. 76.7±1.2% or 74.8±2.0%, P<0.05). The proportion of oocytes matured to the MII stage (maturation rate) for oocytes stored at 35 °C was significantly lower than those stored at 25 °C or 15 °C (51.3±0.9% vs. 75.1±1.4% or 71.7±1.3%, P<0.05). Cleavage rate (77.7±2.1%, 77.9±1.1% and 72.1±0.7% for 15 °C, 25 °C and 35 °C groups, respectively) and blastocyst formation rate (39.1±0.5%, 36.8±1.4% and 32.2±0.9% for 15 °C, 25 °C and 35 °C groups, respectively) following SCNT were not significantly different between treatments. Oocytes from ovaries stored at 15 °C, however, produced blastocysts with higher cell numbers (97.3±8.6 vs. 80.2±10.8 or 77.4±11.7; P<0.05) and lower apoptotic index (5.1±1.3 vs. 13.5±1.6 or 18.6±1.1, P<0.05) than those stored at 25 °C or 35 °C. The relative abundance of Bax and Hsp70.1 in day 7 blastocysts produced from oocytes derived from ovaries stored at 15 °C was lower than those stored at 25 °C or 35 °C (P<0.05). It was concluded that a storage temperature of 15 °C for a 3-4h period had a significant beneficial effect on the quality and developmental competence of oocytes used for SCNT due to the alleviation of stresses on the oocytes compared with those subjected to storage temperatures of 25 °C or 35 °C.