Oocytes in vitro maturation were vitrified and warmed by commercial Kitazato vitrification and warming kit according to the manufacturer’s protocol. Vitrification procedures were performed at room temperature (25–27°C). The oocytes in vitro maturation were transferred from the culture medium into the ES medium (KitazatoBioPharma Co, JP) for 15 min and then VS for 90 s. The oocytes were aspirated and placed on the tip of the Cryotop (KitazatoBioPharma Co, JP) and the Cryotop sheet were plunged into liquid nitrogen immediately. Warming procedures were performed by placing the Cryotop in a warming solution (TS, 1 mol/L sucrose) for 50–60 s at 37°C and moving into a dilution solution (DS, 0.5 mol/L sucrose) for 3 min at room temperature. The oocytes were transferred onto the bottom of WS1 dish with small amount of DS and kept for 5 min in WS1 solution and were then transferred onto the surface of WS2 dish with minimum amount of WS1, then kept for 5 min in WS2 on a plate warmer (37°C).