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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 tough outer covering of the ovum.
When the sperm comes in contact with that covering (the egg jelly), the acrosome 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 egg jelly and give access to the egg.
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