Zona pellucida

The zona pellucida (plural zonae pellucidae) is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it. The zona pellucida first appears in multilaminar primary oocytes.

This structure binds spermatozoa, and is required to initiate the acrosome reaction. In the mouse (the best characterised mammalian system) the zona glycoprotein, ZP3, is responsible for sperm binding, adhering to proteins on the sperm plasma membrane (GalT). ZP3 is then involved in the induction of the acrosome reaction, whereby a spermatozoon releases the contents of the acrosomal vesicle. The exact characterisation of what occurs in other species has become more complicated as further zona proteins have been identified.

In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes to be replaced by the underlying layer of trophoblastic cells.

The zona pellucida is essential for oocyte death and fertilization.

In some older texts, it has also been called zona striata and stratum lucidum (not to be confused with the stratum lucidum of the skin).

Immunocontraception
Glycoproteins in ZP1, 2, 3 and 4 are targets for immunocontraception in the human.

In non-mammalian animals, the zona pellucida (called vitelline layer) plays an important role in preventing cross-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".

Zona pellucida glycoproteins
There are four major zona pellucida glycoproteins, termed ZP1-4. ZP1, ZP3 and ZP4 bind to capacitated spermatozoa and induce the acrosome reaction.Successful fertilization depends on the ability of sperm to penetrate extracellular matrix surrounding eggs. In the mouse:
 * ZP3 allows species-specific sperm binding
 * ZP2 mediates subsequent sperm binding
 * ZP1 cross-links ZP2 and ZP3.

Data with native human protein is not currently available.