Abstract
Abstract 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...
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