Fertility: Store-Operated Ca 2+ Entry in Germ Cells: Role in Egg Activation.

At the time of fertilization, the sperm activates the egg and induces embryonic development by triggering an elevation in the egg's intracellular free Ca2+ concentration. In mammals the initial Ca2+ rise is followed by a series of repetitive Ca2+ transients (known as oscillations) that last for several hours. Although the source of Ca2+ during the signaling process is primarily the egg's smooth endoplasmic reticulum, the oscillations stop in the absence of extracellular Ca2+ indicating that a Ca2+ influx across the plasma membrane is essential to sustain them. Depletion of the intracellular stores using specific inhibitors generates a Ca2+ entry across the plasma membrane of eggs of various species, and a continuous influx of Ca2+ has been linked to the sperm-induced Ca2+ oscillations in the mouse; these data indicate that store-operated Ca2+ entry (SOCE) operates in eggs and may be the mechanism that maintains the long-lasting Ca2+ signal at fertilization. Recent findings suggest that the signaling proteins STIM1 and Orai1 are present in eggs; they are responsible for mediating SOCE, and their functions are essential for proper Ca2+ signaling at fertilization to support normal embryo development.