What is the primary function of the zona pellucida?

The primary function of the zona pellucida is to facilitate the binding of sperm to the egg during fertilization. This glycoprotein layer surrounds the oocyte and is crucial for the fertilization process. The zona pellucida contains specific receptors that interact with sperm, allowing the sperm to attach and initiate the acrosome reaction, which leads to the sperm penetrating the egg's outer layers.

This interaction is essential not only for successful fertilization but also plays a role in ensuring species-specific fertilization, as the receptors in the zona pellucida are tailored to recognize the proteins on the sperm of the same species. After fertilization occurs, the zona pellucida also helps prevent polyspermy, which is the fertilization of an egg by multiple sperm.

While protection of the embryo until implantation is a function associated with the zona pellucida, as it does provide a protective layer prior to implantation in the uterine wall, its most critical and primary role is the binding of sperm during the fertilization process. Hence, the binding aspect is central to the function of the zona pellucida in reproductive biology.