The zona pellucida is a glycoprotein membrane surrounding an egg cell that binds sperm and is essential for fertilization. It contains three glycoproteins, with ZP3 being responsible for species-specific sperm binding. During fertilization, the acrosome reaction allows the sperm to break through the membrane and fertilize the egg. Male infertility can be caused by a failure of the acrosome reaction. After fertilization, the zona pellucida dies and is expelled. It can be used for population control in some animals.
The zona pellucida, formerly called the striatum, is a membrane surrounding an oocyte, or egg cell, made up of glycoprotein. It binds sperm and is essential for the acrosome reaction, which helps a sperm enter the egg cell during fertilization. It contains a special glycoprotein, ZP3, which is responsible for these biological functions.
An egg cell, like all animal cells, is surrounded by a plasma membrane, which protects intracellular components and regulates what materials enter or leave the cell. The zona pellucida lies just outside the egg cell’s plasma membrane and is a major part of it. It is composed of three glycoproteins, ZP1, ZP2 and ZP3. ZP3 allows for species-specific sperm binding, preventing sperm of a different species from fertilizing the egg. ZP3 is especially important for animals that have external fertilization, such as fish. ZP2 mediates sperm binding and ZP1 is important for the structural integrity of the zona pellucida.
During fertilization, the zona pellucida binds sperm by adhering to its plasma membrane proteins. When this happens, the acrosome, an organelle at the tip of the sperm head, opens and releases enzymes and antigens allowing it to break through the tactile plasma membrane of the oocyte. When a sperm undergoes the acrosome reaction and breaks through the plasma membrane of the egg, fertilization takes place.
Male infertility is sometimes due to sperm cells failing to undergo the acrosome reaction. Conversely, in some species, some sperm shed the contents of their acrosome before reaching the egg, eliminating their own chance of fertilizing the egg, but increasing the motility of other sperm. Premature acrosome reaction is more common in species where females have many sexual partners, resulting in high competition for sperm.
After fertilization, the zona pellucida dies and is expelled in a phenomenon called zona hatching. It is also necessary the death of egg cells in cases where there is no fertilization.
This membrane is sometimes used for population control in some animals. When animals are injected with the zona pellucida of another species, they are rendered sterile. For example, the zona pellucida of pigs has been used to sterilize the deer population in New Jersey.
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