How does the sperm acrosome help the sperm enter an egg?

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Front view anatomical diagrams of the male and female reproductive systems, with labeled organs including penis, testis, prostate gland, uterus, ovary, and vagina. — human reproductive system Organs and structures of the male and female reproductive systems.

The acrosome is a specialized structure at the anterior tip of a spermatozoon that facilitates entry into an egg through the acrosome reaction.1

When a sperm contacts an egg, the acrosomal vesicle bursts, releasing enzymes such as lysin or hyaluronidase. These substances break down the protective barriers surrounding the egg, such as the vitelline coat or the corona radiata, allowing the sperm to penetrate.1

In many species, the reaction also triggers the formation of an acrosomal tubule, which extends to the egg surface and fuses with the egg's plasma membrane, creating a pathway for the sperm's nucleus to enter the egg. In mammals, the process is preceded by capacitation, a physiological change that enables the sperm to undergo the acrosome reaction and traverse the egg's outer layers, including the zona pellucida.1