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Prevention of Polyspermy

Polyspermy is fertilization of the egg by more than one sperm. Polyspermy result in non viable development and therefore several methods have been evolved to counteract fertilization of the egg by more than one sperm.

(1) Fertilzin is released into the environment by the egg coat and agglutinates excess of sperm.
(2) In addition to fertilization the egg also contains antifertilizin. At fertilization the antifertilizin may bind with the fertilizin of the egg. This is prevents attachment of additional sperms to the egg, because fertilizin binding sites are no longer available for the antifertilization of the sperm. Thus polyspermy is prevented.
(3) Immediately after penetration of the sperm the cortical granules explode and contribute to the formation of the fertilization membrane. This thick membrane prevents the entry of other sperms.
(4) The zona reaction. In mammals no fertilization membrane is formed. After penetration of the sperm, the zona pellucida surrounding the mammalian egg undergoes a change called the zona reaction. This change, which prevents the entry of other sperms, starts from the point of sperm entry and proceeds to the opposite pole. It is possible for a second sperm to enter the opposite pole of the egg immediately after entry of the first sperm, resulting in polyspermy.

Functions of the Fertilzin-antifertilizin interaction

(1) Assuming that fertilizin is located on the cell surface the initial attachment of the sperm to the egg is because of linkage of fertilizin and antifertilizin.
(2) Fertilizin from the jelly coat may diffuse into the environment and cause agglutination of sperms. Because of this, comparatively few sperms reach the egg surface. This may help the prevent polyspermy.

(3) Because of its high specificity the fertilization-antifertilizin complex ensures that fertilization between different species does not take place.

(4) Sperms contain lytic substances which can break down the egg coat. By holding together many sperms on the surface, the fertilization antifertilization reaction ensures sufficient quantities of lytic enzymes to dissolve the egg covering.
(5) Fertilizin has the property of activating sperms. If a jelly cost extract of fertilizing is added to a group of immotile sperms they at once start moving. Experimental evidence suggests that there are two types of fertilizin, an activation fertilizin suggests that there are two types of fertilizin, an activation fertilizin which activates the sperms, and a receptor fertilizin which agglutinates them.

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