The cortical reaction is a process initiated during

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 ...

that prevents

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 ...

, the fusion of multiple sperm with one egg. In contrast to the fast block of polyspermy which immediately but temporarily blocks additional sperm from fertilizing the egg, the cortical reaction gradually establishes a permanent barrier to sperm entry and functions as the main part of the slow block of polyspermy in many animals. To create this barrier,

cortical granule Cortical granules are regulatory secretory organelles (ranging from 0.2 um to 0.6 um in diameter) found within oocytes and are most associated with polyspermy prevention after the event of fertilization. Cortical granules are found among ...

s, specialized

secretory vesicles 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...

located within the egg's

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

(the region directly below the plasma membrane), are fused with the egg's plasma membrane. This releases the contents of the cortical granules outside the cell, where they modify an existing extracellular matrix to make it impenetrable to sperm entry. The cortical granules contain

proteases A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the for ...

that clip perivitelline tether proteins,

peroxidases Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides. Functionality Peroxidases typically ca ...

that harden the vitelline envelope, and

glycosaminoglycans Glycosaminoglycans (GAGs) or mucopolysaccharides are long, linear polysaccharides consisting of repeating disaccharide units (i.e. two-sugar units). The repeating two-sugar unit consists of a uronic sugar and an amino sugar, except in the case ...

that attract water into the

perivitelline space The perivitelline space is the space between the zona pellucida and the cell membrane of an oocyte or fertilized ovum. In the slow block to polyspermy, the cortical granules released from the ovum are deposited in the perivitelline space. Polysac ...

, causing it to expand and form the

hyaline layer Hyalin is a protein released from the cortical granules of a fertilized animal egg. The released hyalin modifies the extracellular matrix of the fertilized egg to block other sperm from binding to the egg, and is known as the slow-block to polys ...

. The trigger for the cortical granules to exocytose is the release of calcium ions from cortical

smooth endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

in response to sperm binding to the egg. In most animals, the extracellular matrix present around the egg is 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 comp ...

which becomes the fertilization membrane following the cortical reaction. In mammals, however, the extracellular matrix modified by the cortical reaction is the

zona pellucida The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first ...

. This modification of the zona pellucida is known as the zona reaction. Although highly conserved across the animal kingdom, the cortical reaction shows great diversity between species. While much has been learned about the identity and function of the contents of the cortical granules in the highly accessible sea urchin, little is known about the contents of cortical granules in mammals. The cortical reaction within the egg is analogous to the

acrosomal 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 extracellula ...

within the sperm, where the acrosome, a specialized secretory vesicle that is homologous to cortical granules, is fused with the plasma membrane of the sperm cell to release its contents which degrade the egg's tough coating and allow the sperm to bind to and fuse with the egg.

Echinoderms

In the well-studied

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

model system, the granule contents modify a protein coat on the outside of the plasma membrane (the vitelline layer) so that it is released from the membrane. The released cortical granule proteins exert a

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...

causing water to enter the space between the plasma membrane and the vitelline layer, and the vitelline layer expands away from the egg surface. This is easily visible through a microscope and is known as "elevation of the fertilization envelope". Some of the former granule contents adhere to the fertilization envelope, and it is extensively modified and cross-linked. As the fertilization envelope elevates, non-fertilizing sperm are lifted away from the egg plasma membrane, and as they are not able to pass through the fertilization envelope, they are prevented from entering the egg. Therefore, the cortical reaction prevents polyspermic fertilization, a lethal event. Another cortical granule component, polysaccharide-rich hyalin, remains adherent to the outer surface of the plasma membrane, and becomes part of the hyaline layer.

Mammals

Although various mammals have been studied, mice represent the best studied animal models for understanding the cortical reaction in mammals. In mammals the cortical reaction leads to a modification of the

that blocks polyspermy; enzymes released by cortical granules digest sperm receptor glycoproteins

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

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 ...

so that they can no longer bind spermatozoon.

References

*Haley SA, Wessel GM. Sea urchin cortical granules regulated proteolysis by cortical granule serine protease 1 at fertilisation. Mol Biol Cell. 2004 May;15(5):2084-92. *Sadler TW. Langman's Medical Embryology. Baltimore: Lippincott Williams and Wilkins, 2006.

External links

Regulated Proteolysis by Cortical Granule Serine Protease 1 at Fertilization
Germ cells Reproductive system Developmental biology

Echinoderms

Mammals

See also

References

External links