Effects of temperature gradients on in vitro maturation of bovine oocytes

The effect of temperature gradients on in vitro maturation of bovine oocytes was examined in this study. Six treatment groups were made by combining 3 different maturation periods (0 to 10 h, 10 to 18 h and 18 to 24 h) with 2 different culture temperatures (37.0 degrees C and 38.5 degrees C). The frequency of oocytes matured to the metaphase II stage was apparently gradually increased as the culture temperature was increased from 37.0 degrees C to 38.5 degrees C at 0, 10 and 18 h after the onset of culture (75.2 vs 80.5, 82.3 and 84.3%, respectively), but this difference was not significant. Neither was the minor decrease in the proportion of oocytes reaching metaphase II when the temperature was decreased from 38.5 degrees C to 37.0 degrees C at 10 and at 18 h after the onset of maturation (84.3 vs 82.4 and 78.0%, respectively). However, more oocytes cleaved (79.2%; P = 0.0653) and developed to morulae (43.6%; P = 0.0019) and blastocysts (27.4%; P = 0.1568) when they were in vitro matured at 38.5 degrees C between 0 and 10 h, and then at 37.0 degrees C from 10 to 24 h. Although only the morula group was statistically different, cleavage- (79.2 vs 69.8, 72.5, 74.2, 76.3, 74.3%, respectively) and blastocyst formation (27.4 vs 23.2, 24.6, 25.2, 19.6, 21.9%, respectively) from this group was the highest among the 6 treatments.