Events of Fertilisation

Sperm deposited in the female reproductive tract are not immediately capable of fertilising an egg; they must first undergo capacitation, a biochemical conditioning process that primes them for the task ahead. To actually reach the egg, a sperm must swim through the corona radiata, the coat of follicular cells still surrounding the ovulated oocyte, which are held together by an adhesive substance called hyaluronic acid. Contact with this coat and the underlying zona pellucida triggers the acrosomal reaction: the sperm's acrosomal membrane breaks down, releasing the enzyme hyaluronidase, which digests a path first through the corona radiata and then through the zona pellucida, allowing the sperm to finally reach and fuse with the egg's plasma membrane. The instant this fusion succeeds, cortical granules stored in the egg's cytoplasm are released and act on the surrounding layers to build a barrier called the fertilisation membrane arou …