What is Fertilization ?
⇨ Contact and recognition between sperm and egg. These Consists of
following events:
i. Capacitation
A species Specific chemical substance (fertilizin) is produced by the
ripe egg and another substance (Anti-Fertilizin) by sperm.
The fertilizin acts as receptor for Anti-Fertilizin and makes them
capable of fertilizing the egg of the same species. This process is
called "capacitation".
ii. Fertilizin and Anti- Fertilizin Reaction
According to F.R Lillie (1919) Fertilizin is composed of Glycoprotein
or Mucopolysaccharide (Molecular Weight-82000 to 300000).
The protein consists of several amino acids and the polysaccharides
consists of glucose, fructose and galactose. The monosaccharides are
esterified by Sulphuric Acid.
Each fertilizin has more than one active groups and hence can react and
agglutinate more than one sperm.
iii. Fertilizin And Anti-Fertilizine
Ramaro and Garbens in 1985, isolated a chemotaxin called resact (A 14
amino acid long Peptide) from Sea Urchin egg jelly which attracts sperm
of it's own species. It also act as sperm-activating peptide.
A. Acrosomal Reaction And Sperm Penetration : It occurs in the
following steps:
a. The contact between sperm and egg jelly stimulate an exchange of
extracellular Ca++ with intracellular K+ by sperm plasma membrane leads
to increase in PH (>7.2) of intracellular region.
b. The localized fusion of outer acrosomal membrane with sperm's plasma
membrane.
c. Dissolution of acrosomal membrane and release of hydrolytic enzymes
of acrosome.
d. Penetration of acrosomal tubule gradually through the egg envelope
to reach the egg plasma membrane.
e. Fusion of membrane of the acrosomal tubule with the egg
membrane.
B. Prevention of Polyspermy : It is an two step process:
⇨ Prevention of Polyspermy
a. Fast Block- It occurs by a temporary electrical change in the plasma
membrane within 2-3 seconds and lasts for 60 seconds. The membrane
potential changes from -70 mv to +20 mv.
b. Slow Block-This step is stimulated by fast block and proceeds in the
following steps:
i. Mobilization of Ca++ from ER within the egg.
ii. They are released at the site of sperm entry first and than spreads
to other parts.
iii. This initiates cortical reaction leads to the formation
Fertilizing membrane.
⇨ Cortical Reaction
i. The Ca++ moves to the inner surface of the egg plasma membrane and
fuse with it.
ii. This fusion brings rupture of cortical granules to release their
content into the space between the plasma membrane and vitelline
membrane.
iii. The release of Proteases break the molecular bond of the vitelline
membrane and cut off the receptors in sperms.
iv. The mucopolysaccharides produces an osmotic gradient for entry of
water into the inside fertilization membrane and expands it.
v. The peroxidase enzymes harden the fertilization membrane.
vi. A hyaline Layer forms a coating below the fertilization membrane.
The Hyaline layer later support blastomeres during cleavage.
Activation of Egg
i. It begins with blocking of polyspermy and cortical reaction.
ii. The oxygen consumption in the egg increases by 3-5 folds.
iii. Activation of the enzyme NAD + Kinase, which converts NAD+ to
NADP+ leads to biosynthesis of membrane lipids, which required during
cleavage.
iv. Then influx of Na+ and efflux of H+ from the egg increases.
Formation & Migration of Pronuclei & AmphimixisIt occurs in the
following steps:
a. Entry Of sperm stimulates second maturation division in egg nucleus.
The egg nucleus lies in the peripheral cytoplasm in the form of several
vesicles (Karyomeres) which now fuse to form female perinucleus. Then it
also becomes vesicular and migrates towards the predetermined site of
fusion.
b. Amphimixis-It differs in different species. In Sea Urchin and
vertebrates, The nuclear membrane breaks at point of contact. There
content get surrounded by a common nuclear membrane forming the nucleus
of zygote.
Regulation Of Fertilization
The responsible factors are:
⇨ Hormones(Gammons)
⇨ Source
⇨ Function
⇨ Androgamone-I Sperm Conserves sperm activity
⇨ Androgamone-II Sperm Dissolves vitelline membrane & helps in
sperm entry.
⇨ Gynogamone-I Ovum Neutralizes Androgamone-I & increases sperm
activity.
⇨ Gynogamone-II Ovum Make the sperm head sticky to facilitate it's
attachment to egg surface.
Significance Of Fertilization
i. It Restores diploid chromosomal configuration.
ii. Fusion Of genetic material during fertilization results in new
genetic recombination ensuring better adaptation to changing
environment.
iii. Activation f ovum results in further cell division and development
by bringing about increase in metabolic activities and the changes
required for embryonic development.
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