Biological Systems MCAT Practice Exam

What is the protective layer surrounding the egg called after ovulation?

1. Zona pellucida

2. Graafian follicle

3. Amnion

4. Allantois

The correct answer is: Zona pellucida

The protective layer surrounding the egg after ovulation is referred to as the zona pellucida. This glycoprotein layer is crucial for several reasons. First, it serves as a barrier to sperm, facilitating species-specific fertilization by allowing only sperm from the same species to penetrate. Once a sperm successfully penetrates the zona pellucida, it triggers biochemical changes that prevent other sperm from entering, ensuring that the egg is fertilized by a single sperm. In addition to its role in fertilization, the zona pellucida also provides structural support to the developing embryo until implantation occurs in the uterus. After fertilization, as the embryo develops and undergoes cleavage, it is initially protected and encased by this layer. The other options represent different structures or layers within reproductive and embryonic biology. The Graafian follicle refers to the mature ovarian follicle that releases the egg during ovulation, while the amnion is a membrane that forms a protective amniotic sac around a developing embryo in mammals. The allantois is involved in waste disposal and gas exchange for the embryo but is not directly related to the egg or its immediate protective layers post-ovulation.