Question

how is the entry of only 1 sperm and not many ensured into an ovum during fertilisation in humans?

Answer 1

Fusion of one more than one sperm, i.e., polyspermy is prevented in humans by bringng changes in the egg membranes.

Once the egg/ovum is activated ( binding of sperm to ovum), then first through various phyical and chemical changes, depolarisation of egg membrane occur. Secondly when sperm plasma membrane fuses with egg membrane , then cortical reaction, zona reaction occur that make the egg impervious to any second sperm. In this way polyspermy is prevented.

The cortical reaction may be defined as a massive exocytosis of cortical granules which are observed shortly sperm plasma membrane fuses with egg membrane. These cortical granules contains various enzymes such as several proteases which in turn diffuse into the zona pellucida after exocytosis from the egg. These proteases then changes the structure of zona pellucida resulting in the zona reaction.

The zona reaction may be defined as the process catalyzed by proteases from cortical granules which results in the alteration in the structure of the zona pellucida, Zona pellucida turns into hard covering which prevents entry of any other sperms inside the egg.