**4.5. Sperm-oocyte fusion**

A link exists between enhanced ROS levels and increased sperm-oocyte fusion. High rates of sperm-oocyte fusion are correlated with increased expression of phosphorylated tyrosine proteins [6], suggesting that sperm-oocyte fusion is related to the events of capacitation and AR. Both H2 O2 and O2 ●− contribute to the increase in fertilization rates as revealed by the fact that the addition of catalase or SOD significantly decreased the fusogenicity, whereas the addition of H2 O2 or O2 ●− significantly increased the fusogenicity [53, 64].

Ultimately, ROS are thought to increase membrane fluidity using two mechanisms: (1) deesterification of membrane phospholipids and (2) activation of phospholipase A2 (PLA2) [65].

Once the zona pellucida and corona radiata are penetrated by the sperm cell, the oocyte prevents eventual polyspermy by turning the vitelline layer into a hard envelope. o,o-Dityrosine crosslinks catalyzed by ovoperoxidase lead to the formation of a single macromolecular structure acting as the envelope [66]. H2 O2 serves as the substrate to ovoperoxidase to provide for the envelope formation. With our understanding of ROS and their spermicidal effect, H<sup>2</sup> O2 proves to be an effective spermicide agent against polyspermy [66, 67].
