Effects of Microinjected Cations on the Early Event of Fertilization in the Medaka Egg: (microinjection/divalent...

Effects of microinjected cations on the early events of fertilization were examined using eggs of Oryzias latipes. Microinjection of either Ca2+ , Ba2+ or Sr2+ into the thin cortical cytoplasm induced breakdown of cortical alveoli (vesicles) (CABD) under Ca-Mg-free conditions, but microinjection of Mg2+ , Mn2+ or Co2+ prevented CABD at the injected region when the eggs were inseminated in regular saline. Under Ca-Mg-free conditions, CABD could also be induced by microinjection of various solutions (NaCl, choline chloride, sucrose, pH buffer) without any divalent cations or ionophore A23187. Ca2+ microinjected into the cortical cytoplasm did not play a role in sperm penetration. Upon microinjection with either Ca2+ , Mg2+ or K+ , the resting membrane potential leakage was transiently observed. However, depolarization of the membrane followed by slow hyperpolarization was observed only upon microinjection of Ca2+ . From these experiments, it was inferred that microinjected divalent cations such as Ca2+ , Ba2+ or Sr2+ do not act directly upon the cortical alveolus membrane, but trigger the induction of CABD via depolarization of the membrne and increase in intracellular Ca2+ .