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 amphibian fertilization. The Ca2 wave seems to be mediated by IP3-receptors on the endoplasmic reticulum and by IP3 produced by hydrolysis of PLC activated by a Src-related protein tyrosine kinase (Xyk) in Xenopus eggs. We have proposed three different hypotheses for initiation of egg activation in amphibian eggs: the Ca2 -influx model, the membrane receptor model, and the soluble factor model. The membrane receptor model and the soluble factor model seems to be applied to the monospermic Xenopus fertilization and the physiologically polyspermic Cynops fertilization, respectively. The Ca2 wave at egg activation induces a positive fertilization potential which prevents entry of a second sperm in fertilization of monospermic species. In physiologically polyspermic urodele eggs, several sperm enter the egg at normal fertilization, but only one sperm nucleus with a centrosome participates in the embryonic development. The degeneration of accessory sperm nuclei is closely involved in differential distributions of both γ-tubulin and cyclin B in the egg cytoplasm, which causes developing a larger sperm aster and earlier entry into M phase in a zygote nucleus, respectively. We have discussed the molecular mechanisms of egg activation and polyspermy blocks in amphibians and make some comparisons with other vertebrates, such as fishes and mammals.
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