Question

How is polyspermy checked by the zona pellucida of the ovum?

Answer 1

once sperm comes in contact with the fertilization cone that lead to depolarization of plasma membrane  depolarization act as fast block to polyspermy .

after that cortical reaction and zona reaction will act as slow block to poly spermy , depolarization triggers intra cellular release of calcium ions which causes exocytosis of cortical granules due to this cortical granules releases cortical enzymes into the perivetaline space  these enzymes destroy remaining ZP₃  receptors and make zonapellucida very hard so that new sperm cannot digest zona pellucida.

Answer 2

ANSWER :-

Polyspermy is checked by the zona pellucida of the ovum by the process of a lock and key reaction which works as follows-

• The surface of Zona pellucida has ZP3 receptors that act as locks.
• The surface of sperms has acidic proteins that are known as "Bindin proteins" and these proteins act as a key to the lock that a ZP3 receptor is.
• The process is carried out by gamete agglutination as when the binding proteins of the first sperm attach to a ZP3 receptor, "Zona lysins" are released from the acrosome of the sperm after acrosomal reaction.
• The sperm strikes the surface of the oocyte after the zona lysins dissolve the corona radiata and zona pellucida and this leads to the activation of an enzyme named adenylate cyclase which is present at the inner side of the plasma membrane of the oocyte.
• The activation of adenylate cyclase leads to the conversion of ATP into cyclic AMP and this cyclic AMP causes the influx of calcium ions into the oocyte.
• During the influx of calcium ions, the cortical granules (modified Golgi bodies) filled with mucopolysaccharides and some enzymes, present at the inner side of the oocyte's plasma membrane get ruptured and their content flows out into the perivitelline space and forms a mucoplysaccharide membrane and the "some enzymes" cause degradation of the left out ZP3 receptors.
• Even if any Zp3 receptor remains not degraded by the enzymes by chance and gets stimulated by another sperm, there is no way that the new sperm is going to pass through the mucopolysaccharide membrane to enter into the oocyte as sperms don't have any enzymes that are capable of degrading this membrane.