Fertilisation is the process whereby the haploid gametes (egg and sperm) fuse together to form a diploid zygote

• The egg cell is surrounded by an extracellular matrix called the zona pellucida (jelly coat) which protects against sperm penetration

• Successful fertilisation involves two key processes – the acrosome reaction and the cortical reaction

Acrosome Reaction:

When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat

• The sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida (jelly coat)

• The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix

• The sperm then pushes its way through the softened jelly coat and binds to exposed docking proteins on the egg membrane

• The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg

Cortical Reaction:

The cortical reaction occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy

• Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida 

• These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat

• This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid