Polyspermia

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Polyspermia (from Greek : πολύς = polys = a lot and sperm ) describes the penetration of several sperm into an egg cell during conception . Since this process leads to abnormal development of the fertilized egg, there are various mechanisms that prevent polyspermia (so-called polyspermia block). In most cases, when the first sperm penetrates and the sperm cap ( acrosome ) bursts, reactions are triggered in the egg that no longer allow another sperm to penetrate. Such reactions are, for example, the change in the electrical charge of the female egg cell from negative to positive or the formation of a calcium shell around the egg cell after the first contact with a sperm.

fast polyspermia block

The sperm binding causes an influx of sodium [Na +], this lasts 1–3 seconds. This increases the electrical potential of the egg cell membrane from −70 mV to +20 mV. However, this condition only lasts for about a minute, but prevents further sperm from penetrating the egg cell until the slow polyspermia block occurs.

slow polyspermia block

The release of calcium about 6 seconds after the first contact between the egg and the sperm, initiates the slow block against polyspermia, also known as the cortical reaction. The cortical granules release their ingredients, such as enzymes and proteoglycans, through exocytosis . This hardens (in mammals) the zona pellucida and further penetration of sperm is no longer possible. In amphibians, similar substances are released through the cortical granules. Here a lifting of the fertilization membrane by mucopolysaccharides can be determined (water influx between plasma and vitelline membrane). In addition, bindin receptors (binding site of the sperm) are split off from the surface of the egg in amphibians (by proteolytic enzymes).