The characteristics of the calcium signals that activate mammalian eggs at fertilization

Gamete membrane fusion in mammals brings the paternal genome into the cytoplasm of the egg. It also enables signals to pass from the sperm into the egg to trigger the completion of meiosis and the start of embryo development. The essential signal to activate development in all mammals studied, consists of a series of transient increases in the cytosolic Ca²⁺ concentration driven by cycles of InsP₃ production. This review focusses on the characteristics of these sperm-induced Ca²⁺ signals. I consider how some specific features of sperm-derived phospholipase C-zeta (PLCζ), along with the known properties of the type 1 InsP₃ receptor, provide a basis for understanding the mechanisms of the dynamic changes in Ca²⁺ observed in fertilizing eggs. I describe how the PLCζ targeting of cytoplasmic vesicles in the egg cytoplasm, that contain PI(4,5)P₂, is necessary to explain the rapid waves associated with the rising phase of each Ca²⁺ transient. I also discuss the importance of the repetitive Ca²⁺ rises for egg activation and the way mitochondrial ATP production may modulate Ca²⁺ release in eggs. Finally, I consider the role that a sperm-induced ATP increase may play in the egg activation process.