MI arrest and MI pause in starfish oocytes

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In many animals, oocytes arrest at metaphase of the first or second meiotic division (MI or MII) prior to fertilization. Meiosis reinitiation occurs after fertilization. Starfish oocytes were usually isolated from the ovary without stimulation, placed in seawater, and then treated with 1-MA in most studies. In this in vitro oocyte maturation, meiotic division of maturing oocytes proceeds without arrest, followed by formation of two polar bodies. However, when 1-MA is injected into the body cavity of the animal to induce in vivo oocyte maturation, GVBD occurs but polar bodies are not formed in the ovary, indicating that ovarian oocytes arrest at MI.In this study using starfish Asterina pectinifera, the duration of MI after spawning was~20 min and~30 min in fertilized and unfertilized oocytes, respectively. This prolongation of MI in unfertilized oocytes, referred to as the MI pause, was maintained by MAPK as well as low intracellular pH (~7.0). MI pause was released by intracellulat pH increase by Na^+/H^+ exchanger. MI arrest was not maintained by MAPK, since it was inactive in the oocytes arrested at MI in the ovary and activated immediately after spawning. Also cyclin B was not degraded at pH 6.7 in the cell-free preparation without MAPK activity, while it was degraded at pH 7.0, suggesting that MI arrest was solely maintained by lower pH (<7.0). Normal development occurred when the spawned oocytes were fertilized before the first polar body formation, while fertilization after the first polar body formation increased the rate of abnormal development. Thus, due to MI pause and MI arrest, the probability to be fertilized before the polar body formation could be increased, leading to normal development.