2. Fertilization: A complex process involving the
fusion of male and female gametes.
✓ Dicovered by Leeuwenhock in 1683.
✓ Second essential event of sexual reproduction after
ametogenesis
✓ Mechanism includes all the processes from the approach of
spermatozoa to the fusion of pronuclei of gametes.
3. TYPES OF FERTILIZATION:
(A) With regard to the source of gametes:
2) Self fertilization / Endogamy:
-fusing gametes derived from
same parent i.e. uniparental.
-e.g: Taenia, Fasciola etc.
1) Cross fertilization/ Exogamy:
-combines gametes from two
different individuals.
-e.g: frog, human beings etc.
-found in all unisexual/
dioecious organisms.
-Some bisexual organisms also show cross fertilization. e.g: Protandrous
condition in Leeches and Pheretima; Protogynous condition in Scypha.
4. TYPES OF FERTILIZATION:
(B) With regard to the site of gametic fusion:
2) Internal fertilization:
-fertilization occurs inside
the female body.
-sperms are released inside
the female genital tract by
the copulatory organ of
male.
1) External Fertilization:
-gametes fuse outside the
female body.
-e.g In water in organisms
like Echinoderms, Frog etc.
5. TYPES OF FERTILIZATION:
C) on the basis of nature of fusing gametes:
1. Isogamy: fusing gametes are morphologically and
physiologically similar. e.g. Monocystis.
2.Anisogamy: fusing gametes are morphologically or
physiologically different. e.g. Frog, human beings etc.
6. Mechanism of Fertilization:
5 main steps:
1. Approach of Spermatozoa to ovum
2. Capacitation and contact of sperm and ovum
3. Acrosomal reaction and entry of sperm
4. Activation of ovum
5. Amphimixis
7. 1 .Approach of Spermatozoa to ovum
Approximation of gametes occurs by 2 phenomena:
A .Activation of Sperm Motility
B. Attraction of sperms to egg.
1. Activation of Sperm Motility:.
(a) In Sea Urchin:
-Sperm motility is controlled by their intercellular
pH.
-Testicular pH of 7.2 prevents respiration and
motility.
-At pH 7.6 in sea water sperms become motile.
-Jelly coat of the egg contains sperm-activating
peptide speract which assist in increasing the
sperm's internal pH.
-pH change activates dynein in flagellar axoneme.
(b) In mammals: Contraction of female reproductive tract help conduct spermatozoa
to the oviduct.
8. Spermatozoa locate the egg through Chemotaxis.
(a) In insects and fishes (e.g Sturgeon):
-Chorion layer around egg have a micropyle.
-Chemicals present near the micropyle attract the
sperms.
-Sperms are taken by cytoplasmic extension of
the egg.
B. Attraction of Sperms to the egg:
(b)In Sea Urchin: jelly coat of the eggs contains
a respiratory activating peptide- resact that
attracts the spermatozoa.
(c) Jellyfish sperms are attracted to a protein
present in a cupule at the animal pole of the
egg.
9. 2 .Capacitation and contact of spermatozoa and ova
Capacitation is the physiological maturation of sperm.
Significance
(i) Makes sperms capable to fertilize the egg.
(ii) If sperms are active earlier, their hydrolytic enzymes can destroy the
male genital tract.
-Occurs in the female genital tract
-takes about 5-6 hours.
-Sperm undergoes loss of decapacitation factors
-Calcium influx into the sperms changes electrical potential.
-Removal of cholesterol from its plasma membrane
-Change of undulatory movements of flagellum to whiplash type.
-Sperm become hyperactive.
10. 3. Acrosomal reaction and penetration of sperm
Activated spermatozoan on reaching the
vicinity of the egg undergoes a number
of changes in its acrosomal region.
The sperm lysins are acidic proteins and include:
(i) Hyaluronidase: dissolves hyaluronic acid
polymers holding corona radiata cells
(ii) Corona-penetrating enzyme: dissolves corona
radiata cells
(iii) Acrosin or zonalysin: dissolves zona pellucida.
-The outer acrosomal membrane and plasma
membrane breakdown and releases Lysins.
-The inner acrosomal membrane becomes
plasma membrane in the acrosomal region.
-The lytic substances dissolve egg
envelopes
11. Acrosome reaction in most marine invertebrates doesnot require capacitation.
-Sperms contact the jelly coat of an egg, the plasma membrane and the acrosomal membrane
break.
-Lysins form depression in egg envelopes.
-From Acrosomal membrane an acrosomal tube develops and extends in the egg envelopes.
-Growing acrosomal process is coated with protein Bindin. Bindin associates with a sperm
receptor in the egg vitelline membrane and ensures species-specific fertilization.
-Fertilization cone engulfs acrosomal tube.
In sea Urchins (an Echinoderm):
12. 4. Activation of Ovum
As the spermatozoa comes in contact
with plasma membrane of the egg after
penetrating the egg coats, certain
reactions occur in the egg cytoplasm
collectively known as activation of ovum.
-Cytoplasm of egg bulges forward
around the sperm nucleus and forms a
hyaline projection called fertilization
cone or cone of reception.
-gradually engulfs the spermatozoa
either completely or a part of it.
(i) Formation of fertilization cone:
13. Some physico-chemical changes occur in egg cortex known as cortical reactions.
Leads to the formation of fertilization membrane around the egg.
(ii) Cortical reactions (Fertilization membrane formation):
-Vitelline membrane is lifted up.
-Cortical granules are extruded in the
perivitelline space by exocytosis and
some of these are attached on the
inner surface of vitelline membrane
which now thickens.
-becomes impervious to any other
sperm. It is now called fertilization
membrane.
-500-900 A° in thickness.
-prevents polyspermy
14. 5. Karyogamy and Amphimixis:
Fertilization is only completed by the fusion of sperm and egg pronuclei.
-Egg completes its 2nd meiotic division and its nucleus is now called
female pronucleus, lies nearly at the centre of the egg.
-Sperm nucleus absorb water and swells up, its nuclear envelope
breaks down and its chromatin decondenses. It is now called male
pronucleus and lies in the cortex of the egg.
-Astral rays radiate from sperm centrioles pushes the male
pronucleus towards the centre of the egg.
-Male and female pronuclei finally meet together and a common
nuclear envelope is laid down around it, forming a zygote nucleus or
synkaryon. The egg is now called zygote.
(i) In Sea Urchin:
15. -Synkaryon is not formed.
-Sperm enters the ooplasm and rotates at an angle of 180°.
-The nucleus swells up forming male pronucleus.
-Spindle is formed between the centrioles of male and female pronucleus.
-Intermingeling of chromosomes occur in metaphase plate- Amphimixis
-Nuclear envelope is not formed and undergoes first cleavage division.
(ii) In Mammals:
16. 1. Provides stimulus for the egg to complete second meiotic
division.
2.Activates the egg to develop into a new individual by
cleavage divisions.
3. Restores diploidy
4. Introduce centrioles which may be lacking in the mature egg.
5.Introduces variations and contribute to evolution.
6. Determines the sex of the young one.
7. Fertilization membrane prevents polyspermy.
SIGNIFICANCE OF FERTILIZATION:
