Human sperm is a microscopic structure consisting of a head, neck, middle piece, and tail . The head contains an elongated haploid nucleus, and its anterior portion is covered by a cap-like structure known as the acrosome . The acrosome is filled with enzymes (often called sperm lysins) that are essential for the fertilisation of the ovum . Specifically, these secretions help the sperm enter the cytoplasm of the ovum by allowing it to penetrate through the zona pellucida and the plasma membrane .