Mechanisms of egg activation and polyspermy block in amphibians and comparative aspects with fertilization in other vertebrates

For precise temporal activation of the egg during amphibian fertilization, the sperm must provide a signal for egg activation at the time of membrane binding or fusion between sperm and eggs. A fertilizing sperm causes a Ca2 wave which is both necessary and sufficient for egg activation at amphibia n fertilization. The Ca2+ wave seems to be mediated by IP3-receptors on the endoplasmic reticulum and by IP3 produced by hydrolysis of PLC activated b y a Src-related protein tyrosine kinase (Xyk) in Xenopus eggs. We have prop osed three different hypotheses for initiation of egg activation in amphibi an eggs: the Ca2+-influx model, the membrane receptor model, and the solubl e factor model. The membrane receptor model and the soluble factor model se ems to be applied to the monospermic Xenopus fertilization and the physiolo gically polyspermic Cynops fertilization, respectively. The Ca(2+)wave at e gg activation induces a positive fertilization potential which prevents ent ry of a second sperm in fertilization of monospermic species. In physiologi cally polyspermic urodele eggs, several sperm enter the egg at normal ferti lization, but only one sperm nucleus with a centrosome participates in the embryonic development. The degeneration of accessory sperm nuclei is closel y involved in differential distributions of both gamma-tubulin and cyclin B in the egg cytoplasm, which causes developing a larger sperm aster and ear lier entry into M phase in a zygote nucleus, respectively. We have discusse d the molecular mechanisms of egg activation and polyspermy blocks in amphi bians and make some comparisons with other vertebrates, such as fishes and mammals.