What is "zona pellucida"

Zona \Zo"na\, n.; pl. Zon[ae]. [L., a girdle. See Zone.] A zone or band; a layer. Zona pellucida. [NL.] (Biol.)

1. The outer transparent layer, or envelope, of the ovum. It is a more or less elastic membrane with radiating stri[ae], and corresponds to the cell wall of an ordinary cell. See Ovum, and Illust. of Microscope.

2. The zona radiata.

   Zona radiata [NL.] (Biol.), a radiately striated membrane situated next the yolk of an ovum, or separated from it by a very delicate membrane only.

n. (context anatomy English) A glycoprotein membrane surrounding the plasma membrane of an oocyte

n. thick membrane around the mammalian ovum; can be penetrated by one sperm in the fertilization process; usually remains around the fertilized egg until it is implanted in the wall of the uterus

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 follicular cells. 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 (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).