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| Description: (a) Acrosomal reaction. The contact of a sperm with an egg initiates a series of events that permits the head of the sperm to bind to the plasma membrane of the egg. This depolarizes the egg and blocks other sperm from entering, in the fast block to polyspermy. (b) Cortical reaction. Sperm fusion leads to an increased level of cytosolic Ca2+ that ultimately causes the vitelline layer of the egg to harden, creating the slow block to polyspermy. (c) Calcium wave in a sea urchin egg. The increase in cytosolic Ca2+ begins near the site of sperm entry and propagates throughout the egg. The highest concentration of cytosolic Ca2+ released from the endoplasmic reticulum is indicated with brightest colour.
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