FEMALE REPRODUCTIVE SYSTEM: DESCRIPTION OF THE EGG CELL
From one perspective, the egg is a highly specialized cell with a single function—namely, the generation of a new individual. From another perspective, the egg is developmentally the least restricted cell in an animal because, if fertilized, it can give rise to every cell type in the organism. Although it is barely visible with the naked eye, the egg is very large when compared to a typical body (somatic) cell. For example, a human egg is approximately 0.13 mm in diameter, a size 10 times the diameter and 1000 times the volume of most other human body cells. The egg is large so that it can adequately nourish the embryo until a connection is made between it and the mother.
In addition to its size, there are a number of structural features specific to the egg cell. Whereas most cells have an external surface called the plasma membrane, the egg has a special structure called the outer egg coat or zona pellucida. Consisting of a jellylike extracellular matrix composed mostly of glycoprotein molecules, the outer egg coat protects the egg from mechanical damage. It is also the site of specific receptors that enable same-species sperm (and only same-species sperm) to recognize the egg and, in the presence of certain conditions, to interact with it.
When such interaction does occur—that is, when a sperm cell actually does penetrate an egg cell—cortical granules, a set of specialized secretory vesicles located in the outer area of the egg's cytoplasm, release their contents and, in so doing, immediately alter the egg's outer coat so that no other sperm will be able to fuse with the egg.
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