Calcium influx in mammalian eggs

Calcium (Ca(2)(+)) signals are involved in the regulation of oocyte maturation and play a critical role during fertilization. In the egg, Ca(2)(+) is stored in the lumen of the endoplasmic reticulum and a signal is generated when the stored Ca(2)(+) is released through specialized channels in the membrane of the endoplasmic reticulum to elevate the free Ca(2)(+) concentration in the cytoplasm. Extracellular Ca(2)(+) is also important, indicated by the fact that the mobilization of luminal Ca(2)(+) is typically followed by Ca(2)(+) entry across the plasma membrane. The transmembrane Ca(2)(+) flux replenishes the endoplasmic reticulum, and thus, it is essential to sustain prolonged Ca(2)(+) signals. It also seems to be responsible for the stimulation of important signaling cascades required for complete egg activation. Characterization of the pathway that mediates Ca(2)(+) entry implies that its major components include STIM1, a protein that senses the filling status of the stores, and ORAI1, a channel protein located in the plasma membrane. Defining the mechanism and functions of Ca(2)(+) entry will not only lead to a better understanding of egg physiology but may also help improving the efficiency of a number of assisted reproductive technologies.