MOLECULAR MECHANISMS OF SPERM-EGG INTERACTIONS AND EGG ACTIVATION

The binding of acrosome reacted mammalian sperm to the egg plasma memb rane initiates a series of signaling events in the egg, termed ''egg a ctivation'', which lead to the completion of meiosis II and the initia tion of a mitotic cell cycle. Many of these signaling events have char acteristics of classical signal transduction events in somatic cells. Currently, there are two hypotheses for how sperm-induced egg activati on is initiated. In the ''receptor'' hypothesis, the fertilizing sperm interacts with a specific egg surface receptor, and this interaction leads to signal transduction and effector activation. In the ''fusion' ' hypothesis it is postulated that following fusion of the sperm and e gg plasma membranes a soluble sperm-derived factor enters the egg's cy toplasm and activates pathways leading to egg activation. This chapter will provide an overview of the processes of cell-cell interaction an d signal transduction leading to mammalian egg activation. It will con centrate on specific molecules proposed to be involved in sperm-egg in teraction, signal transduction and effector mechanisms involved in egg activation, and a discussion of sperm-associated factors that have be en implicated in egg activation.