Ca²⁺ homeostasis and regulation of ER Ca²⁺ in mammalian oocytes/eggs

The activation of the developmental program in mammalian eggs relies on the initiation at the time of fertilization of repeated rises in the intracellular concentration of free calcium ([Ca²⁺]_i), also known as [Ca²⁺]_i oscillations. The ability to mount the full complement of oscillations is only achieved at the end of oocyte maturation, at the metaphase stage of meiosis II (MII). Over the last decades research has focused on addressing the mechanisms by which the sperm initiates the oscillations and identification of the channels that mediate intracellular Ca²⁺ release. This review will describe the up-to-date knowledge of other aspects of Ca²⁺ homeostasis in mouse oocytes, such as the mechanisms that transport Ca²⁺ out of the cytosol into the endoplasmic reticulum (ER), the Ca²⁺ store of the oocyte/egg, into other organelles and also those that extrude Ca²⁺. Evidence pointing to channels in the plasma membrane that mediate Ca²⁺ entry from the extracellular milieu, which is required for the persistence of the oscillations, is also discussed, along with the modifications that these mechanisms undergo during maturation. Lastly, we highlight areas where additional research is needed to obtain a better understating of the molecules and mechanisms that regulate Ca²⁺ homeostasis in this unique Ca²⁺ signaling system.