structural:expansion
In fertilization

Ca 2+ influx during fertilization has been observed in many species as a trigger for development of the oocyte. These influxes may occur as a single increase in concentration as seen with fish and echinoderms, or may occur with the concentrations oscillating as observed in mammals. The triggers to these Ca 2+ influxes may differ. The influx have been observed to occur via membrane Ca 2+ conduits and Ca 2+ stores in the sperm. It has also been seen that sperm binds to membrane receptors that lead to a release in Ca 2+ from the ER. The sperm has also been observed to release a soluble factor that is specific to that species. This prevents cross species fertilization to occur. These soluble factors lead to activation of IP which causes a Ca 2+ release from the ER via IP receptors. It has also been seen that some model systems mix these methods such as seen with mammals. Once the Ca 2+ is released from the ER the egg starts the process of forming a fused pronucleus and the restart of the mitotic cell cycle. Ca 2+ release is also responsible for the activation of NAD + kinase which leads to membrane biosynthesis, and the exocytosis of the oocytes cortical granules which leads to the formation of the hyaline layer allowing for the slow block to polyspermy.