Question

There are hundreds of sperm that reaches the ovary.Then, why only one sperm fertilises and forms a zygote? Can there be chances of other sperms also to fuse?

Answer 1

Although hundreds of sperm reaches the ovary but only one fertilizes the ovum because when one sperm enters into the egg it causes some changes in the zona pellucida layer of egg by increasing the Ca²⁺ ion concentration which blocks the entry of other sperms. Thus inhibiting polyspermic fertilization. Generally only one sperm binds to an egg but in rare cases more than one sperm can fuse with egg leading to monozygotic or dizygotic twins.