In sperm cells of many higher animals, the acrosome develops over the anterior half of its head. It is a cap-like structure derived from the Golgi apparatus. In Eutherian mammals, acrosome formation is completed during testicular maturation. The acrosome contains digestive enzymes (including hyaluronidase and acrosin) needed to penetrate the zona pellucida covering the ovum.
When the sperm comes in contact with ZP3 in the zona pellucida, this reaction takes place, during which actin polymerizes to form the acrosomal process; also, the acrosomal vesicle fuses with the plasma membrane of the egg cell, and digestive enzymes are released from the sperm to digest the zona pellucida and give access to the egg.