Calcium influx in mammalian eggs

Abstract

Calcium (Ca2+) 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 Ca2+ is released through specialized channels in the membrane of the endoplasmic reticulum to elevate the free Ca2+ concentration in the cytoplasm. Extracellular Ca2+ is also important, indicated by the fact that the mobilization of luminal Ca 2+ is typically followed by Ca2+ entry across the plasma membrane. The transmembrane Ca2+ flux replenishes the endoplasmic reticulum, and thus, it is essential to sustain prolonged Ca2+ 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 Ca2+ 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 Ca2+ 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. © 2013 Society for Reproduction and Fertility.