Figure 6. Putrescine delayed the postovulatory aging of MII oocytes by regulating PDK4 and mitochondrial activity. In the present study, we explored three key mechanisms in the oocytes during postovulatory aging, including oxidative stress, apoptosis, and mitochondrial autophagy. Putrescine exerts a protective role during the postovulatory aging process by regulating mitochondrial function in addition to providing an antioxidant effect. Oxidant stress, showing as the increased ROS, is main mechanism of the aging of postovulatory oocytes. The level of PDK4, which is a key factor of TCA in the mitochondria of the aging oocytes, was significantly upregulated by putrescine. The effects of putrescine were partially blocked by the downregulation of PDK4. In conclusion, putrescine delays the aging of oocytes by regulating PDK4 expression, antioxidation and mitochondrial activity.