Question

how are cotical reaction?how is monospermy achieved?

Answer 1

Once the egg/ovum is activated ( binding of sperm to ovum), then first through various physical 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, monospermy ( fusion of single sperm with egg) is achieved.

In cortical reaction, the cortical granules are released from the cortex of egg and fuse with the plasma membrane of egg and then released outside. These corticle granules modify the extracellular material such as vitelline layer in case of most animals or Zona pellucida in case of mammals into such harder material which become impenetrable for next coming sperms.