Recepter-mediated signal transduction and egg activation

Egg activation at fertilisation is composed of a complex, choreographed series of events, the initiation of which still is not understood. Two major hypotheses have emerged as explanations for the mechanism of egg activation (reviewed by Nuccitelli, 1991; Whitaker & Swann, 1993). The first holds that the sperm delivers an ‘activating factor’ (e.g. inositol trisphosphate, calcium, or a protein) that diffuses into the egg cytoplasm after gamete fusion. The second hypothesis holds that sperm bind to receptors spanning the egg plasma membrane which then transduce a signal to second messenger enzymes. We present here the evidence for receptor-mediated signal transduction in egg activation at fertilisation. By ‘receptor’ we mean only molecules that bind an extra-membrane ligand and which transduce a signal to molecules residing on the cytoplasmic side of the egg membrane. It is critical to be aware that several alternative activating mechanisms are not mutually exclusive and that species differences may exist. In fact, as more has been learnt about the molecules involved in gamete recognition and binding and of the signalling pathways in the egg, it seems likely that multiple pathways exist to trigger complete egg activation.