The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...

that surrounds the plasma membrane of mammalian

oocyte An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...

s. 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 follicle An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. At the time of puberty, women have approximately 200,000 to 300,000 follicles, each with th ...

s. The zona pellucida is surrounded by the

corona radiata In neuroanatomy, the corona radiata is a white matter sheet that continues inferiorly as the internal capsule and superiorly as the centrum semiovale. This sheet of both ascending and descending axons carries most of the neural traffic from and ...

. The corona is composed of cells that care for the egg when it is emitted from the ovary. This structure binds

spermatozoa A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, ...

, and is required to initiate the

acrosome reaction During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the female egg cell to fertilize it. Fusing to the egg cell usually causes little problem, whereas penetrating through the egg's hard shell or extracellular ...

. In the mouse (the best characterised mammalian system), the zona glycoprotein,

ZP3 Zona pellucida sperm-binding protein 3, also known as zona pellucida glycoprotein 3 (Zp-3) or the sperm receptor, is a ZP module-containing protein that in humans is encoded by the ''ZP3'' gene. ZP3 is the receptor in the zona pellucida which bin ...

, is responsible for sperm binding, adhering to

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

s 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 The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...

performs

zona hatching Implantation (nidation) is the stage in the embryonic development of mammals in which the blastocyst hatches as the embryo, adheres, and invades into the wall of the female's uterus. Implantation is the first stage of gestation, and when successf ...

; the zona pellucida degenerates and decomposes, to be replaced by the underlying layer of

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the pl ...

ic cells. The zona pellucida is essential for oocyte growth and

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

Structure

The zona pellucida is a translucent matrix of cross-linked

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

filaments that surrounds the mammalian

and is 6.5–20 μm thick depending on the species. Its formation, which depends on a conserved Zona pellucida-like (ZP) module that mediates the polymerization of egg coat components, is critical to successful fertilization. In non-mammals it is called the

vitelline membrane The vitelline membrane or vitelline envelope is a structure surrounding the outer surface of the plasma membrane of an ovum (the oolemma) or, in some animals (e.g., birds), the extracellular yolk and the oolemma. It is composed mostly of protein fi ...

or vitelline envelope.

Function

The thick membrane of the zona pellucida functions to only allow species-specific fertilization; to prevent

polyspermy In biology, polyspermy describes the fertilization of an egg by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or ...

, and enable the

for the successful adhesion and penetration by the

sperm cell Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

. Also allows correct

embryo development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

and size. 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 Zona pellucida sperm-binding protein 2 is a protein that in humans is encoded by the ''ZP2'' gene. Function The zona pellucida is an extracellular matrix that surrounds the oocyte and early embryo. It is composed primarily of three (mouse) or ...

, and ZP4. They bind to capacitated spermatozoa and induce the

. Successful

depends on the ability of sperm to penetrate the

of the zona pellucida that surrounds the egg. In the mouse: *

allows species-specific sperm binding *

mediates subsequent sperm binding * ZP1 cross-links ZP2 and ZP3. Data with native human protein are not currently available.

Immunocontraception

ZP module-containing glycoproteins ZP1,

and ZP4 are targets for

immunocontraception Immunocontraception is the use of an animal's immune system to prevent it from fertilizing offspring. Contraceptives of this type are not currently approved for human use. Typically immunocontraception involves the administration of a vaccine tha ...

in mammals. In non-mammals, the zona pellucida is called the

or envelope, and the

vitelline envelope The insect vitelline envelope is the outer proteinaceous layer outside the oocyte and egg. The vitelline envelope, not being a cellular structure, is commonly referred to as a membrane. However, this is a technical misnomer as the structure is com ...

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s, and plays an important role in preventing cross-breeding of different species, especially in species such as

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

that fertilize outside of the body. 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 An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...

. This effect can be temporary or permanent, depending on the method used. In

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

, immunocontraception using

porcine zona pellucida Porcine zona pellucida (PZP) is the zona pellucida extracted from the ovaries of pigs which is used as a source of antigens for immunocontraception. The zona pellucida is a thick membrane that surrounds the unfertilised eggs of mammals. In order fo ...

has been trialled for the control of

deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the reindeer ...

Additional images

File:Gray9.png, First stages of segmentation of a mammalian

ovum The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...

File:Gray1164.png, Section of vesicular ovarian follicle of cat, x 50 File:HumanEmbryogenesis.svg, The initial stages of

human embryogenesis Human embryonic development, or human embryogenesis, is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of de ...

References

External links