Why does the zona pellucida layer block the entry of additional sperm?

Zona pellucida:

1. All mammalian eggs and pre-implantation embryos are covered in a thick extracellular layer known as zona pellucida.
2. Fertilization is the process of union of a sperm and an ovum.
3. When sperm interacts with the zona pellucida layer of the ovum during fertilization, alterations in the membrane prevent the entry of further sperms.
4. It makes sure that an ovum can only be fertilized by one sperm.
5. Sperm are assisted in crossing the zona pellucida and plasma membrane to enter the cytoplasm of the ovum by the secretions of the acrosome.
6. One sperm entering through the zona pellucida causes the layer to alter structurally and hinders the entry of further sperms.