Fertilization Reaction without Changes in Intracellular Ca2+ in Medaka Eggs-An Experiment with Acetone-Treated Eggs.

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To investigate whether or not causal relationship exists between the increase in intracellular Ca2+ and other cortical reactions at fertilization in the medaka, Oryzias latipes, intracellular Ca2+ was determined from luminescence of aequorin previously microinjected into cortical cytoplasm in acetone-treated eggs, when they were inseminated or activated by microinjection of Ca2+ . Neither an increase in cytoplasmic calcium nor exocytosis of cortical alveoli occurred in eggs treated with acetone, though other events of fertilization i.e. completion of meiosis, fusion of pronuclei, and accumulation of cortical cytoplasm with intact cortical alveoli in the animal pole region were observed in normal time sequence in these eggs. When denuded eggs were treated with acetone, contraction of the egg and slow resumption of meiosis (extrusion of polar body) were observed without insemination. When denuded eggs were inseminated immediately after acetone-treatment, the number of spermatozoa that penetrated into the egg was greater in the animal hemisphere than in the vegetal hemisphere. These results may indicate that acetone inactivates the egg plasma membrane or its adjacent cortical cytoplasm so that it cannot participate in a propagative increase in intracellular Ca2+ and exocytosis, while it also induces cytoplasmic activation leading to egg contraction, resumption of meiosis and formation of pronuclei. The present results suggest that sperm penetration, resumption of meiosis and ooplasmic segregation are regulated separately from the release of intracellular Ca2+ and exocytosis.