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The zona pellucida (ZP) is an extracellular glycoprotein coat surrounding the oocyte, composed of four sulfated glycoproteins — ZP-1, ZP-2, ZP-3, and ZP-4. It serves the following key functions:

ZP-3 and ZP-4 act as primary sperm-binding receptors, while ZP-1 and ZP-2 serve as secondary binding proteins interacting with the equatorial segment of the sperm head. This binding is species-specific and ensures that only compatible spermatozoa can initiate fertilization. — Histology: A Text and Atlas (Pawlina), p. 2197

After a capacitated spermatozoon binds to the zona pellucida, ZP-1, ZP-3, and ZP-4 trigger the acrosome reaction in the sperm. This releases acrosomal enzymes (notably acrosin, a proteolytic enzyme, along with esterase and neuraminidase) that lyse the zona pellucida and allow the sperm to penetrate through to the oocyte. — The Developing Human (Moore & Persaud), p. 122; Langman's Medical Embryology, p. 872

Once a single spermatozoon fuses with the oocyte plasma membrane, cortical granules release lysosomal enzymes into the perivitelline space. These enzymes (metalloendoproteases) cleave zona pellucida glycoproteins, making them unrecognizable for binding — a process called the zona reaction. This renders the zona pellucida impermeable to additional spermatozoa, preventing polyspermy. — Histology: A Text and Atlas, p. 2197; The Developing Human, p. 122

The zona pellucida acts as a physical protective barrier around the oocyte and early embryo (morula stage), shielding it from mechanical damage and immune attack as it travels through the uterine tube toward the uterus.

By maintaining the early embryo as a compact unit enclosed within the zona pellucida, it prevents premature implantation in the fallopian tube. The zona must be shed (hatching) before the blastocyst can implant in the endometrium.