Fertilization - Definition, Types and Mechanism

 

Fertilization


It is the fusion of male and female gametes sperm and ova respectively in the oviduct or fallopian tube of the female. After fertilization a zygote is formed from which a fully grown organisms or individuals is formed. It is of two types- 1. External fertilization 2. Internal fertilization

1. External fertilization : It is the fusion of the sperm and ova take place outside the body of maternal parent called external fertilization. Ex. Fishes, Amphibians sperm and ova shed in to the surrounding water where fertilization take place.

2. Internal fertilization: If the fusion of ova and sperm is take place inside the oviduct (fallopian tube) of the female called internal fertilization. Here male introduce sperm into the female genital tract where the fusion takes place. They posses specialised sex organs for transmitting and receiving the sperm.



Mechanism of Fertilization:

Fertilization occur through following 5 steps.

1)      The Meeting of Gametes 2) Barrier penetration 3) Sperm & egg fusion (the acrosomal reaction) 4) Activations of the ovum. 5) Migration of pronuclei and amphimixis.

1.Meeting of Gametes: In the 1st step the spermatozoon encounter the ovum which brought about by the swimming movement of the spermatozoon. It is achieved by the following steps.

A) Agglutination B) Fertilization ( Antifertilization reaction. C) Capacitation

A) Agglutination: In most animals, spermatozoa adhere to the surface of the egg results in their clumping or Agglutination.

B) Fertilization ( Antifertilizing Reaction) : According to the Lillie the agglutination is brought about due to fertilizing antifertilizing reaction. The adhesion of spermatozoon to the surface of the egg  is brought about by the linking of fertilization molecules produced by egg with the antifertilization molecules present on the surface of the spermatozoon. This phenomenon established on initial bond which would later lead to the penetration of the sperm into the egg. This reaction is highly specific and it reduces the risk of polyspermy.

C) Capacitation: The fertilizin – antifertilizin reaction qualifies the spermatozoa to become capable of fertilizing an egg of the same species called capacitation.

2. Barrier Penetration: In order to penetrate through the protective layer of egg, the sperm contains lysine enzyme in the acrosome to dissolve the egg envelop and clean the path. In mammals sperm contains enzyme hyaluronidase which dissolves the hyaluronic acid present in the egg meeting surface thus enable the spermatozoon to penetrate the corona radiate.

3. Sperm & egg Fusion : (Acrosome reaction) As the spermatozoon makes its initial contract with the eggs envelope, the sperm p-lasma membrane and the acrosomal membrane rupture at the place of contact.

 The acrosomal membrane then join with plasma membrane around the margin of rupture, so that the acrosomal granule is exposed to the surface of the egg envelope and its lysine are extruded.

The central part of the acrosome elongates and become a long tube known as acrosomal tubule or filaments. The acrosomal tubulen transvers the egg envelops ( Jelly & vitelline membrane ) and finally touches the egg plasma membrane. These two membrane now fuses at the point of contact.

4. Activation of the ovum: It occurs in the following steps ..

a) Formation of fertilization core: Immediately often the fusion of the acrosomal tubule with the egg plasma membrane. The cytoplasm of the egg bulges upward at the point of contact as a conical projection called fertilization cone, which gradually engulfs the sperm nucleus, middle piexe and tail and carries the spermatozoon inward into the egg cytoplasm.

b) Cortical reaction and formation of fertilization membrane: The fertilization cone formation is followed by changes in the surface of the egg cytoplasm called cortical reaction. The  outer membrane called vitelline membrane is about 30A thick and inner plasma membrane is about 60A thick. A layer of cortical granules is found beneath it. The outer vitelline membrane become lifted off from the plasma membrane and undergoes expansion and gives rise to the outer layer of the fertilization membrane. The space between albumen and the surface of the egg is called pre vitelline space.

C) Metabolic Activity: These structure changes are followed by alteration in the physical properties of the egg substance. These includes- i) Changes in permeability of egg plasma membrane. ii) ionic changes iii) respiration changes iv) changes in the rate of proteins synthesis v) initiation of mitosis.

5. Migration of Pronuclei and amphimixes: (Union of haploid nuclei) The male nucleus starts swelling by absorbing fluid from the surrounding cytoplasm becomes vesicular and caleed male pronuclei with the production of polar body,the egg nucleus on female pronuclei is ready for union with the male pronuclei. The male pronucleus advancing along the penetration path move directly towards the female pronucleus.

   The fusion of male and female pronuclei is called amphimixia. In a few animals, the two pronuclei actually fuse together . in mammals and other vertebrate, they donot fuse instead each loses its membrane and its chromatin desolves into the haploid set of chromosome. The two set of chromosome then arranged themselves across the division spinole and results in zygote nucleus which is ready for first cleavage division.


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