Cortical reaction

The cortical reaction and cortical granules were first observed by Ethel Brown Harvey in 1910 in sea urchins. The fertilization membrane had been previously defined by Derbès when he observed the fusion of sperm and eggs of sea urchins; however, scientists believed that seminal fluid or other chemicals that the sperm brought into the egg caused the hardening of the fertilization membrane. E. B. Harvey studied the mechanisms after sperm contact that lead to calcium release and formation of the fertilization membrane. Cortical granules were discovered in vertebrates first in hamster oocytes in 1956 by C. R. Austin using a phase contrast microscope. ==Echinoderms==

In the well-studied sea urchin 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 osmotic pressure 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. Most mammalian cortical granules are 0.2 - 0.6 um in diameter. Upon contact of the sperm with the oocyte membrane, a calcium wave is induced through the PIP2 pathway involving IP3 production. IP3 then binds to its receptor on the endoplasmic reticulum which triggers a release of calcium into the cytoplasm. The release of calcium triggers a change in the SNARE protein complex, and the conformation facilitates the fusion of the cortical granule with the oocyte membrane, releasing granules into the perivitelline space. The cortical reaction leads to a modification of the zona pellucida that blocks polyspermy; enzymes released by cortical granules digest sperm receptor glycoproteins ZP2 and ZP3 so that they can no longer bind spermatozoon. This is often referred to as zona hardening. ==See also==