Cortical reaction
Encyclopedia
The cortical reaction occurs when a sperm
cell unites with the egg
's plasma membrane, (zona reaction) modification of the zona pellucida
that blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.
The cortical reaction is exocytosis
of the egg's cortical granules. Cortical granules are secretory vesicles that reside just below the egg's plasma membrane. When the fertilizing sperm contacts the egg plasma membrane, it causes calcium to be released from storage sites in the egg, raising the intracellular free calcium concentration. This triggers fusion of the cortical granule membranes with the egg plasma membrane, liberating the contents of the granules to the extracellular space. Fusion begins near the site of sperm contact, and then as the wave of calcium release sweeps around the egg, a wave of cortical granule fusion results. The contents of the granules vary with the species, and are not fully understood.
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 fertilisation envelope". Some of the former granule contents adhere to the fertilisation envelope, and it is extensively modified and cross-linked. As the fertilisation envelope elevates, non-fertilizing sperm are lifted away from the egg plasma membrane, and as they are not able to pass through the fertilisation envelope, they are prevented from entering the egg. Therefore, the cortical reaction prevents polyspermic
fertilisation, 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.
This is considered the slow block to multiple fertilisation at an animal egg cell
Spermatozoon
A spermatozoon 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...
cell unites with the egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
's plasma membrane, (zona reaction) modification of the zona pellucida
Zona pellucida
The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...
that blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.
The cortical reaction is exocytosis
Exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...
of the egg's cortical granules. Cortical granules are secretory vesicles that reside just below the egg's plasma membrane. When the fertilizing sperm contacts the egg plasma membrane, it causes calcium to be released from storage sites in the egg, raising the intracellular free calcium concentration. This triggers fusion of the cortical granule membranes with the egg plasma membrane, liberating the contents of the granules to the extracellular space. Fusion begins near the site of sperm contact, and then as the wave of calcium release sweeps around the egg, a wave of cortical granule fusion results. The contents of the granules vary with the species, and are not fully understood.
In the well-studied sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
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
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
osmotic pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
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 fertilisation envelope". Some of the former granule contents adhere to the fertilisation envelope, and it is extensively modified and cross-linked. As the fertilisation envelope elevates, non-fertilizing sperm are lifted away from the egg plasma membrane, and as they are not able to pass through the fertilisation envelope, they are prevented from entering the egg. Therefore, the cortical reaction prevents polyspermic
Polyspermy
In biology, polyspermy describes an egg that has been fertilized 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 more copies of each chromosome -- one from the...
fertilisation, 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.
This is considered the slow block to multiple fertilisation at an animal egg cell