Mammalian cortical granule exocytosis during fertilization. Cortical granules are secretory vesicles located in the cortex of unfertilized oocytes (shown in circles in grey). Upon gamete fusion, cortical granules undergo exocytosis and release their contents into the extracellular space to establish a block of polyspermy. For clarity purpose, cellular structures are not illustrated in scale. CC = cumulus cells; CG = cortical granules; ECM = extracellular matrix; HA = hyaluronic acid; 1oPB = primary polar body; 2oPB = secondary polar body; PVS = perivitelline space; ZP = zona pellucid.