Zona pellucida

The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a glycoprotein layer surrounding the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The zona pellucida is surrounded by the cumulus oophorus. The cumulus is composed of cells that care for the egg when it is emitted from the ovary.First stages of segmentation of a mammalian ovumSection of vesicular ovarian follicle of cat, x 50The initial stages of human embryogenesis The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a glycoprotein layer surrounding the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The zona pellucida is surrounded by the cumulus oophorus. The cumulus is composed of cells that care for the egg when it is emitted from the ovary. 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. 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 growth and fertilization. The zona pellucida is a translucent matrix of glycoproteins that surrounds the mammalian oocyte, and its formation is critical to successful fertilization. In non-mammals it is called the vitelline membrane or vitelline envelope. The thick membrane of the zona pellucida functions to only allow species-specific fertilization; to prevent polyspermy, and enable the acrosome reaction for the successful adhesion and penetration of the sperm cell. The major glycoproteins of the egg coat responsible, are known as sperm-binding proteins. The four major sperm-binding proteins, or sperm-receptors, are ZP1, ZP2, ZP3, and ZP4. They bind to capacitated spermatozoa and induce the acrosome reaction. Successful fertilization depends on the ability of sperm to penetrate the extracellular matrix of the zona pellucida that surrounds the egg.In the mouse: