Egg Activation at Fertilization: Where It All Begins

AbstractA centrally important factor in initiating egg activation at fertilization is a rise in free Ca2+ in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca2+ rise occurs as a result of inositol trisphosphate-mediated release of Ca2+ from the endoplasmic reticulum. The release of Ca2+ at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)γ. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLCγ appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm–egg contact or sperm–egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca2+ release by the same pathway as fertilization