Soluble sperm factors and Ca2+ release in eggs at fertilization

All eggs are activated at fertilization by an increase in intracellular free Ca2+. How the spermatozoa triggers the release of intracellular Ca2+ has not been established in any species. One hypothesis is that spermatozoa introduce a Ca(2+)-releasing factor into the cytoplasm after gamete membrane fusion. It has been suggested that spermatozoa may introduce Ca2+ itself, or a small molecular weight factor that triggers the release of Ca2+ from intracellular stores. However, these suggestions are not consistent with a number of experiments in vertebrate eggs. Here, I present the argument that spermatozoa cause intracellular Ca2+ oscillations in mammalian eggs by introducing a specific protein called an oscillogen. It is suggested that it does not mediate its effects by causing increased inositol, 1,4,5-trisphosphate (InsP3) production, but instead directly affects the ability of the stores to undergo Ca(2+)-induced Ca2+ release. The sperm oscillogen hypothesis may help explain the patterns of intracellular Ca2+ waves that occur during fertilization in hamster eggs, as well as the ability of an injection of live spermatozoa to activate development in human eggs.