33development. This was followed by the identification of the signaling toolkit that is
used in the generation of the Ca2+ signal. In nonmammalian vertebrates a single
transient elevation is observed at fertilization, while in mammals the sperm induces
a series of low-frequency oscillations in the egg’s cytosolic Ca2+ level. The source
of Ca2+ is intracellular and it is believed to be the smooth endoplasmic reticulum.
The factor that causes activation is best characterized in mammals. It is now widely
accepted that the Ca2+-mobilizing molecule is a sperm-specific PLCζ, which, upon
sperm- egg fusion, diffuses into the ooplasm and hydrolyzes PIP 2. This leads to the
generation of IP 3 that causes the discharge of stored Ca2+ from the endoplasmic
reticulum. PLCζ isoforms have been identified in nonmammalian species, but the
evidence that these are used during fertilization has yet to be reported. Additional
sperm-resident proteins such as PAWP might also have a role in the fertilization
process. The Ca2+ elevation usually takes the form of a Ca2+ wave that originates at
the site of sperm entry and traverses across the entire cytoplasm. Although Ca2+
originates from the intracellular store, a Ca2+ influx across the plasma membrane is
essential to sustain the train of Ca2+ spikes. The Ca2+ entry might be operated by the
filling status of the stores, although this needs further verifications. A better under-
standing of the mechanism that regulates egg activation is essential for the improve-
ment of the fertilization process in vivo and for enhancing the efficiency of a great
variety of assisted reproductive technologies.
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1 Egg Activation at Fertilization