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.