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The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but not extraneous to it. This structure binds spermatozoa, and is required to initiate the acrosome reaction. The zona glycoprotein, ZP3, is responsible for sperm binding, adhering to proteins on the sperm plasma membrane. There is evidence that ZP3 is then involved in the induction of the acrosome reaction, whereby a spermatozoon releases the contents of the acrosomal vesicle. In humans, five days after the fertilization the membrane degenerates and decomposes to be replaced by the underlying layer of trophoblastic cells. The zona pellucida is essential for oocyte survival and fertilization. Glycoproteins in ZP1, 2, and 3 are targets for immunocontraception. In non-mammalian animals, the zona pellucida plays an important role in preventing breeding of different species, especially in species that fertilize outside of the body (e.g. fish). The zona pellucida is commonly used to control wildlife population problems by immunocontraception. When the zona pellucida of one animal species is injected into the bloodstream of another, it results in sterility of the second species due to immune response. This effect can be temporary or permanent, depending on the method used. In New Jersey, porcine zona pellucida is used to keep deer populations low, and this process is commonly referred to as "spay-vac".
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