Fertilization :
Fertilization is the fusion of male gamete (spermatozoa) and female gamete ( egg ) resulting in the formation of zygote.The combination of the paternal and maternal hereditary character in the new organism is known as ‘Amphimixis’.
According to the site, fertilization can be of two types i.e.one is external fertilization and another one is internal fertilization. In external fertilization the sperm and ova are generally released into water for encounter the fertilization ( e.g. some fishes, amphibians, corals and sea anemone ) whereas internal fertilization takes place inside the female body in the genital tract ( certain lizards and snakes, some fishes and scorpion, marsupial, man ).
Steps in Fertilization :
The process of fertilization in animals is completed in five steps;
1) Meeting of gametes.
2) Barrier penetration.
3) Sperm and egg fusion; acrosome reaction.
4) Activation of ovum.
5) Migration of pronuclei and amphimixis.
1) Meeting of gametes :
It is the first step of fertilization in which sperms are move randomly towards the ovum and when it reaches to the ovum it adhere to the egg surface and the process is called agglutination/clumping. Then the fertilizin-antifertilizin reaction takesplace. Fertilizin is the gylcoprotein molecules released by the ovum to the surrounding medium and attract the sperm because the counterpart antifertilizin ( acid protein) molecules found on the surface of the sperm. The chemicals fertilizin and antifertilizin are specific in therfore one species sperm cannot fertilize other species egg, and prevent cross-breeding.
Fertilization is the fusion of male gamete (spermatozoa) and female gamete ( egg ) resulting in the formation of zygote.The combination of the paternal and maternal hereditary character in the new organism is known as ‘Amphimixis’.
According to the site, fertilization can be of two types i.e.one is external fertilization and another one is internal fertilization. In external fertilization the sperm and ova are generally released into water for encounter the fertilization ( e.g. some fishes, amphibians, corals and sea anemone ) whereas internal fertilization takes place inside the female body in the genital tract ( certain lizards and snakes, some fishes and scorpion, marsupial, man ).
Steps in Fertilization :
The process of fertilization in animals is completed in five steps;
1) Meeting of gametes.
2) Barrier penetration.
3) Sperm and egg fusion; acrosome reaction.
4) Activation of ovum.
5) Migration of pronuclei and amphimixis.
1) Meeting of gametes :
It is the first step of fertilization in which sperms are move randomly towards the ovum and when it reaches to the ovum it adhere to the egg surface and the process is called agglutination/clumping. Then the fertilizin-antifertilizin reaction takesplace. Fertilizin is the gylcoprotein molecules released by the ovum to the surrounding medium and attract the sperm because the counterpart antifertilizin ( acid protein) molecules found on the surface of the sperm. The chemicals fertilizin and antifertilizin are specific in therfore one species sperm cannot fertilize other species egg, and prevent cross-breeding.
2) Barrier penetration :
A mature egg is surrounded by different layers of cells. To cross this layer of barriers the sperm undergoes a serial chemical process; incase of mammals (human) the outer layer corona radiata is dissolved first by the enzyme called hyaluronidase secreted by acrosome of sperm. The corona radiata cells are held together by cement substance known as hyaluronic acid. Another chemical secrete by the acrosome called sperm lysin which breake the egg membrane.
A mature egg is surrounded by different layers of cells. To cross this layer of barriers the sperm undergoes a serial chemical process; incase of mammals (human) the outer layer corona radiata is dissolved first by the enzyme called hyaluronidase secreted by acrosome of sperm. The corona radiata cells are held together by cement substance known as hyaluronic acid. Another chemical secrete by the acrosome called sperm lysin which breake the egg membrane.
3) Sperm and egg fusion ( acrosome reaction) :
When the sperm come in contact with egg, there is a radical changes occurs within sperm plasma membrane and acrosomal membrane at the point of touch of both membrane. Both the membrane burst at the point of touch. The central part of the acrosome get elongated called acrosomal tubule as a result of which the acrosomal granule exposed to egg envelop. By disintegration of cell envelop by lysin enzyme the tubules traversed the egg envelop and touches the egg membrane.
When the sperm come in contact with egg, there is a radical changes occurs within sperm plasma membrane and acrosomal membrane at the point of touch of both membrane. Both the membrane burst at the point of touch. The central part of the acrosome get elongated called acrosomal tubule as a result of which the acrosomal granule exposed to egg envelop. By disintegration of cell envelop by lysin enzyme the tubules traversed the egg envelop and touches the egg membrane.
4) Activation of the ovum :
This activation is caused by contact of acrosomal tubule with the plasma membrane of ovum and a sequence of reaction occurs which makes the egg capable to develop into diploid cells and undergoes multiple cleavage to develop into a embryo. The activation of ovum consist of ;
a) Formation of fertilization cone.
This activation is caused by contact of acrosomal tubule with the plasma membrane of ovum and a sequence of reaction occurs which makes the egg capable to develop into diploid cells and undergoes multiple cleavage to develop into a embryo. The activation of ovum consist of ;
a) Formation of fertilization cone.
b) Cortical reaction and formation of fertilization membrane.
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| Credit- www.sciencedirect.com |
a) Formation of fertilization cone :
After the fusion of acrosomal tubule with the egg’s plasma membrane, the egg’s cytoplasm swell upwards at the point of contact like a conical protuberance known as fertilization cone. The fertilization cone slowly slowly engulf the sperm nucleus, middle piece and tails and guide the sperm into the cytoplasm.
b) Cortical reaction and formation of fertilization membrane :
After the formation of fertilization cone, there is changes observed in egg’s cytoplasm called cortical reaction. Based on the presence and absence of cortical granules the cortical reaction and changes occured in plasma membrane can be studied. Let’s have the discussion of this cortical reaction in different eggs.
I ) Cortical reaction in sea urchin eggs :
a) The color of the sea urchin’s egg surfaced changes from yellow to white at the point of connection of sperm egg’s surface and slowly spreads the changes all over the surface of egg.
b) Sea urchin’s egg cortex is surrounded by two membranes from outside it is called vitelline membrane and from inside it is called plasma membrane. The cortical granules layer is found below the plasma membrane.
c) The outer vitelline membrane undergoes expansion and turned into outer layer of fertilization membrane. The space between vitelline membrane and plasma membrane is called perivitelline space. The question comes how it expand into this ? The fusion of plasma membrane of sperm and egg causes the release of calcium ions from cortical smooth endoplasmic reticulum which further cause the exocytosis of cortical granules; the components released by the cortical granules contain numerous protein and enzymes. This enzymes detached the vitelline membrane and keep it away from plasma membrane, and make the fertilization membrane. Therefore fertilization membrane is nothing but expansion of vitelline membrane, the extracellular components of cortical granules such as matrix protein including the hyalin protein deposited on the egg surface and formed hyaline layer. Another liquid material composed of mucopolysaccharides present in cortical granules absorb the water and become liquified. This liquid get discharge into perivitelline space and add to the perivitelline fluid.
II) Cortical reaction in mammals :
Cortical granules are not found in eggs of all mammals but are found in eggs of human,hamster and rabbit. These are released into the perivitelline space which get dissolved slowly so there is no new membrane is formed around the egg at the time of fertilization.
III) Cortical reaction in guinea pig and rat :
These animals do not possess cortical granules. There is no such morphological changes found in this animal as compared to sea urchin.
5) Migration of pronuclei and amphimixis :
Sperm nucleus through the fertilization cone proceeds towards female pronucleus. Along with pronucleus the centriole,middle piece and tail also followed the path. Rotation of centrioles and pronucleus, the centriole occupy the front of sperm nucleus. The middle piece tail get dissolved in cytoplasm. Now the sperm nucleus called male pronucleus. The male pronucleus before much closer to the female pronucleus the sperm followed a path called penetration path.
Amphimixis is the process of fusion of male and female gamete in sexually reproductive organism. Generally in mammals and other vertebrates the male and female pronuclei do not fuse rather each pronuclei loose it’s membrane and chromatids to resolve into haploid set of chromosomes. The two set of chromosome arrange themselves across the division spindle and supposed to go for cleavage.