At fertilization, mammalian eggs show repetitive transient [Ca2+]i rises each of which is due to Ca2+ release from the endoplasmic reticulum through inositol 1,4,5-trisphosphate (IP3) receptors. During fertilization, a factor from the sperm, the sperm factor, is released into the oocyte and induces a long-lasting series of Ca2+ spikes (Ca2+ oscillations) that are required for egg activation. IP3-producing enzyme phospholipase C zeta (PLC) is a strong candidate for the sperm factor. The Ca2+ spikes initiate the extrusion of cortical granules that block the entry of other sperm. At the same time, maturation (M-phase) promoting factor (MPF) is inactivated by the Ca2+ oscillations, resulting in exit from metaphase II arrest. Meiosis resumes with formation of the second polar body and complete meiotic division, one-cell embryos with the male and female pronuclei attain the first cleavage division through nuclear envelope breakdown.
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Vol. 27 • No. 4