Early events in mammalian egg activation

The fertilizing spermatozoon initiates a series of events in the mammalian egg, referred to as ‘egg activation’. These biochemical and morphological events include a transient rise in intracellular Ca2+ concentration ([Ca2+]i) that leads to the cortical reaction (CR) and the establishment of the block to polyspermy on the one hand, and to the resumption of meiosis and later embryonic mitotic divisions on the other. The initial increase in [Ca2+]i appears to be critical for the initiation of egg activation. However, another second messenger, protein kinase C (PKC), was also suggested as a possible inducer of some aspects of egg activation. The review focuses on early events during mammalian sperm–egg interaction and discusses possible roles of Ca2+ and PKC in egg activation.