Cytoplasmic maturation of mammalian oocytes: development of a mechanism responsible for sperm-induced Ca2+ oscillations

The oocytes of most mammalian species, including mouse and human, are fertilized in metaphase of the second meiotic division. A fertilizing spermatozoon introduces an oocyte-activating factor, phospholipase C zeta, triggering oscillations of the cytoplasmic concentration of free calcium ions ([Ca(2+)](i)) in the oocyte. [Ca(2+)](i) oscillations are essential for the activation of the embryonic development. They trigger processes such as resumption and completion of meiosis, establishment of the block to polyspermy and recruitment of maternal mRNAs necessary for the activation of the embryo genome. Moreover, it has been recently shown that [Ca(2+)](i) oscillations may also influence the development of the embryo. The ability to generate [Ca(2+)](i) oscillations develops in mammalian oocytes during meiotic maturation and requires several cytoplasmic changes, including: 1/ reorganization of endoplasmic reticulum, the main stockpile of calcium in the oocyte, 2/ increase in the number of 1,4,5-inositol triphosphate (IP(3)) receptors, 3/ changes in their biochemical properties (e.g.: sensitivity to IP3), and possibly both 4/ an increase in the concentration of Ca(2+) ions stored in endoplasmic reticulum (ER) and 5/ redistribution of Ca(2+)-binding ER proteins. The aim of this review is to present the state of current knowledge about these processes.