How do sperm and egg cells join during fertilization?

Fertilization is the complex process by which male and female gametes unite to form a zygote. In mammals, spermatozoa must first undergo capacitation within the female reproductive tract to become capable of fertilization.1

The process involves several key stages:

• Penetration: Spermatozoa use the enzyme hyaluronidase to disperse the outer follicle cells of the egg. Upon reaching the zona pellucida, the sperm undergoes the acrosome reaction, releasing substances that help it traverse the egg's protective layers.21
• Fusion: Once a sperm contacts the egg surface, their plasma membranes fuse. In mammals, this typically occurs along the lateral surface of the sperm. This fusion triggers surface changes in the egg that render its membrane impermeable to other sperm.12
• Union: The successful sperm's nucleus enters the egg cytoplasm, becoming the male pronucleus. The egg completes its final maturation, forming the female pronucleus. These two pronuclei meet, lose their membranes, and combine their chromosomes to create a new, unique individual.2

This interaction is highly species-specific, regulated by the biochemistry of the egg's outer coatings.3