Question

How does zona pellucida of ovum help in preventing polyspermy?


NCERT Class XII
Biology - Main Course Book

Chapter 3. Human Reproduction

Answer 1

1.) During fertilization, when a sperm comes in contact with the zona pellucida layer of the ovum a cortical reaction takes place which leads to a modification of the zona pellucida that blocks polyspermy .
2.) The acrosomal enzymes released by the head of the sperm begin to dissolve and the actin filament comes into contact with the egg's plasma membrane causes the release of the calcium from the storage site in the egg and raises the intracellular calcium concentration .It trigers the fusion of cortical granules membrane with the egg plasma membrane liberating the contents of the granules to the extra cellular space and establishes a permanent barrier to the prevent polyspermy

Answer 2

ANSWER :-

Zona pellucida of ovum helps in preventing polyspermy 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.