Britannica Sources used in this answer

What are the zona pellucida and corona radiata around a human egg cell?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Detailed cross-sectional illustration of a human ovary showing the stages of follicle development, ovulation, and corpus luteum formation, with labeled structures including primordial, primary, secondary, and mature follicles, a ruptured follicle releasing an ovum, and corpus luteum and albicans. An inset at the bottom shows the ovary's location in the female reproductive system. — ovary The steps of ovulation, beginning with a dormant primordial follicle that grows and matures and is eventually released from the ovary into the fallopian tube.

The human egg is protected by two distinct layers that a spermatozoon must navigate during fertilization:

Corona radiata: This is the outermost envelope, consisting of multiple layers of follicle cells that adhere to the oocyte before it is released from the ovarian follicle. During fertilization, spermatozoa release the enzyme hyaluronidase to break down the hyaluronic acid that binds these cells together, allowing the sperm to pass through.12
Zona pellucida: Located beneath the corona radiata, this is a thick, transparent, and elastic noncellular layer composed of glycoproteins. It serves as a critical barrier that the sperm must penetrate to reach the egg's plasma membrane. Spermatozoa typically create an oblique slit through this coat to gain entry.312

These envelopes are essential for regulating sperm-egg interaction, as contact with them can trigger the acrosome reaction in the sperm, which is necessary for successful fertilization.21