What is the protective glycoprotein layer surrounding an egg called?

The protective glycoprotein layer surrounding an egg is called the zona pellucida. This structure plays a critical role during fertilization by providing a barrier that sperm must penetrate to fertilize the egg. It is made primarily of glycoproteins, which are essential for protecting the egg and providing signals for sperm binding and fusion. The zona pellucida also helps prevent polyspermy, ensuring that only one sperm fertilizes the egg, which is crucial for proper embryonic development.

The cortex refers to the inner layer of the egg’s cytoplasm and is involved in various metabolic processes but does not serve as a protective layer in the same way as the zona pellucida. The follicle refers to the structure that contains the developing egg and surrounding granulosa cells in the ovary, and it is not directly related to the egg's outer protective layer. The vitelline layer, while it is a term sometimes used to refer to the layer surrounding the egg, particularly in some species, is less commonly recognized in humans than the zona pellucida, which is specifically named and functionally defined in mammalian biology.