The zona pellucida (plural zonae pellucidae, also egg coat) is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the oocyte, external but of essential importance to it. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the follicular cells.
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).