2. Defination of fertilization
• Also called syngamy because it is a complex process
which involves the fusion of male & female gametes
accompained by the fusion of their cytoplasm &
nuclei.
• Acc. To E.B.wilson , fertilization involves the union
of sperm nucleus of paternal origin with the egg
nucleus of maternal origin to form the primary
nucleus of the embryo.
• Acc to Albert Taylor , fertilization includes all the
processes from the approach of spermatozoon to the
fusion of pronuclei of gametes.
3. Process of fertilization has two independent
functions
1) ACTIVATION :- it involves the penetration of
sperm in the egg followed by a series of
changes in egg cortex.
2) AMPHIMIXIS :- it involves the fusion of male
& female pronuclei. It restores the DIPLOIDY &
INDUCE GENETIC VARIATIONS IN THE SPECIES.
5. Acc. To the PLACE & NATURE OF FLUID MEDIUM
FOR THE MOVEMENT OF SPERMS fertilization is
of two types
EXTENAL FERTILIZATION :- In this type of fertilization the
gametes can fuse OUTSIDE THE BODY OF THE FEMALE. This
type of fertilization usually occurs in the liquid medium ,
outside the body in MARINE & FRESH-WATER ORGANISMS.
Eg.. echinoderms ,nereis, many bony fishes & frog
INTERNAL FERTILIZATION :- in this type of fertilization the
fusion of male & female gametes occurs INSIDE THE BODY
OF THE FEMALE In this type of fertilization the sperms are
shed inside the female genital tract by intromittent organ of
the male
6. On the basis of NATURE OF FUSING
GAMETES
ISOGAMY:- when two fusing gametes are
morphologically or physiologically SIMILAR. Eg
Monocystis
ANISOGAMY:- when two fusing gametes are
morphologically & physiologically DIFFERENT.
EG. Frog, humans etc.
7. On the basis of NUMBER OF SPERMS ENTERING INTO THE
OVUM
MONOSPERMY:- when only one sperm penetrates
the ovum. Found in most of the animals.
POLYSPERMY:- when many sperms penetrates the
ovum. It may be PATHOLOGICAL POLYSPERMY(
due to over-ripening of egg) or PHYSIOLOGICAL
POLYSPERMY ( natural entry of many sperms like
in the insects , urodeles , birds. BUT always only
one sperm fuses with the ovum, while other male
pronuclei degenerate before the initiation of
cleavage.
8. Acc. To the basis of SOURCES OF
GAMETES
ENDOGAMY / SELF-FERTILIZATION:- when two
fusing gametes are derived from the SAME
PARENT so sexual reproduction is UNIPARENTAL .
Eg Taenia, Fasciola.
EXOGAMY / CROSSFERTILIZATION:- when two
fusing gametes are derived from DIFFERENT
PARENTS. Sperm is derived from male parent &
ovum is derived from the female parent so sexual
reproduction is generally BIPARENTAL. Eg Found in
all unisexual forms.
10. Process of fertilization has five main steps.
• Approach of spermatozoan to ovum.
• Capacitation & contact of sperm & ovum.
• Acrosomal reaction.
• Activation of ovum.
• amphimixis
11. 1) APPROACH OF SPERMATOZOAN TO OVUM:- In sexual
reproduction the male & female gametes are brought together by
fertilization. The viability of gametes are limited varying from a few
minutes to few hours. The approximation of spermatozoa & ova occurs
by two different ways
EXTERNAL FERTILIZATION :- In marine animals like Asterias,
Sea Urchin etc, the males & females discharge their eggs &
sperms in sea water near each other. It is followed by mass
fertilization. As the life span of the gametes in sea water is
varying from few minutes to few hours so their discharge at
the same time necessary.
In FRESH-WATER organisms the life span of sperms & ova is
very short so sperms are discharged directly on ova. These
animals show Amplexus or pseudo-copulation.Eg frog
12. INTERNAL FERTILIZATION
• This type of fertilization is mostly takes place in
terrestrial animals.
• In this fertilization the spermatic fluid is injected
in the female genital tract by an intromittant
organ which is called penis in mammals ,
hemipenis in lizards, claspers in cartilage fishes .
This processs is called INSEMINATION. The
insemination occurs during sexual contact called
copulation or coitus.
13. • In some annelids, gastropods , amphibians the sperms are
received in specialized structures after copulation.
• In birds the sperms are transferred by cloacal apposition .
• In humans during copulation male releases about 3.5 Ml of
seminal fluid , called INSEMINATION OR EJACULATION & it
contains 200-300 MILLION SPERMS. It is so as many sperms are
eaten by phagocytes & many sperms are killed by vaginal
secretions. So only 100 sperms reach the ovum. The sperm swim
in seminal fluid by lashing movements of their tail at the rate of 1-
4 mm per minute aided by the ASPIRATORY ACTION of uterus &
PERSTALTIC MOMENTS of the fallopian tubes.
• To ensure fertilization, ovum & sperm must reach AMPULLARY-
ISTHMATIC junction simultaneously.
14. CHEMOTAXIS
in most of the cases, approach of a sperm to ovum is purely a
chance process but there are certain cases in which
CHEMOTAXIS of sperm towards the eggs of same species has
reported
• In the eggs of insects & fishes, chemicals present
near the micropyle of chorion attract the sperms.
• In the egg of SEA-URCHIN, there is a sperm
respiratory activating enzyme called RESACT which
attracts the sperms.
• The animal pole of jellyfish has protein containing
CUPULE which attracts the sperms.
15. 2) CAPACITATION & CONTACT OF
SPERMATOZOA & OVA
• It is a process which makes spermatozoon capable of
fertilizing the egg. So involves physiological
maturation of sperm.
• It occurs in the female genital tract in the presence of
viscous fluid secreted from the secretory cells of the
epithelial cells of ovIducal mucosa.
• It takes about 5-6 hours.
• This process is a result of some chemical reactions like
LOSS OF DECAPACITATION FACTORS, CALCIUM INFLUX
INTO THE SPERM, CHANGE OF UNDULATORY
MOVEMENTS OF FLAGELLUM INTO WHIPLASH TYPE.
16. SPECIFICITY OF FERTILIZATION
the eggs of a particular species are fertilized by the
spermatozoa of the same species which is called specificity of
fertilization.
• It is due to certain chemicals secreted by the
ova & the sperms response to that.
• In those animals where fertilization in
external, after release of mature ova , the
spermatozoa of same species get attached to
the egg on lateral side this process is called
CLUMPING or AGGLUTINATION.
17. FERTILIZIN OR ANTI-FERTILIZIN SPECIES-SPECIFIC
REACTION
F.R. LILLIE explained the process of agglutination. Two species specific
chemical substances called FERTILIZIN produced by the mature ovum &
ANTI-FERTILIZIN produced by the sperms. Both the chemical substances
are species specific & form a chemical lock.
Each molecule of FERTILIZIN may have more than one SPERMOPHILLIC
SITE. So one fertilizin molecule may attract more than one
ANTIFERTILIZINS. As a result AGGLUTINATION TAKES PLACE.
18. SIGNIFICANCE OF THIS REACTION
• It makes the ova receptive to sperms of the
same species.
• It makes the sperms to adhere to the surface
of eggs.
• Due to it clumping or agglutination takes
place.
• It leads to capacitation of sperms
19. 3) ACROSOMAL REACTION &
PENETRATION OF SPERM
• The entry of sperms into the eggs occurs by different methods in different animals.
• THROUGH MICROPYLES :- the eggs of gastropods, pelecypods, echinoderms, fishes have
pores called micropyles in surrounding membranes. There may be present one pore or
many pores for the entry of the sperm.
• BY THE ACTION OF LYSINS:- in sea urchins, the egg jelly is dissolved by the acidity of lysins
secreted by the spermatozoa.
• The lytic substances are secreted by the ACROSOME to dissolve membranes present
around the egg. In mammals CORONA RADIATA Of the egg is formed of a single layer of
follicular cells cemented togeather by a called hyaluronic acid. It is digested by the enzyme
hyaluronidase stored in acrosome. It is released after the acrosomal reaction.
• In humans ACROSOMAL REACTION is initiated by an INFLUX OF CALCIUM ION.
• During sperm penetration into ovum only nucleus & middle piece enter the ovum while tail
is lost.
20. In human beings the penetration of sperm into ovum is a CHEMICAL MECHANISM. In
this the acrosome of the sperm undergoes acrosomal reaction & releases certain
SPERM LYSINS when acrosome come in contact with the zona pellucida of ovum in
the animal pole. The sperm lysins dissolve the egg envelop locally & make the path
for the sperm entry.
• These sperm lysins are acidic proteins & includes:-
• HYALURONIDASE:- it is a lysing enzyme which dissolve the
hyaluronic acid polymers in the intercellular spaces which hold the
granulosal cells of corona radiata togeather.
• CORONA-PENETRATING ENZYME:- it dissolves the corona radiata.
• ACROSIN or ZONALYSIN :- it dissolve the zona pellucida.
21. Acrosomal reaction
the activated spermatozoan, on reaching the plasma membrane of the egg
undergoes a number of changes in acrosomal region. All these changes are called
acrosomal reaction.
The acrosomal membrane & plasma membrane of sperm dissolve in the apical region to
from an APICAL PORE through which the contents of the acrosomal vesicle are passed into
the egg.
• The contents of the acrosomal vesicle containing lytic enzymes come in contact with the
egg envelops & form depression there
• From acrosomal membrane ,an ACROSOMAL TUBULE develops & extend in the egg
envelopes.
• The acrosomal tubule penetrates the egg envelop & fuses with the egg membrane.
• The fertilization cone comes out of the egg membrane & engulfs the acrosomal tubule.
• The nucleus of the spermatozoon comes near the fertilization cone . The acrosomal tubule
dissolves, the NUCLEUS , MIDDLE PIECE of th e sperm comes into the egg cytoplasm.
22. 4) ACTIVATION OF OVUM
as the acrosomal tubule comes in contact with the plasma membrane of the egg,
certain reactions occur in the egg cytoplasm collectivelly known as ACTIVATION OF
OVUM
• A) FERTILIZATION CONE FORMATION :- as the acrosomal
tubule touches the egg plasma membrane the cytoplasm of egg bulges
forward at the point of contact & forms a hyaline projection called
fertilization cone , cone of reception.
• FUNCTION OF FERTILIZATION CONE :- the fertilization cone may have
several irregular pseudopodia or maybe it is in the form of a cytoplasmic
cylinder which gradually engulfs the spermatozoan either completely or
a part of that
• B) CORTICAL REACTION :- before the formation of a
fertilization membrane & the entry of the spermatozoon into the egg ,
some PHYSIO-CHEMICAL changes occur in egg cortex called cortical
reactions.
23. FORMATION OF FERTILIZATION MEMBRANE
• In cortical reactions the first step is the formation of
fertilization membrane outside the egg plasma membrane.
• During cortical reaction, vitelline membrane is lifted from
the egg cortex resulting in the formation of PERIVITELLINE
SPACE within the vitelline membrane.
• Cortical granules are extruded in the perivitelline space by
exo-cytosis & some of these are attached on the inner
surface of the vitelline membrane which now thickens,
becomes impervious to any other sperm. Now it is called
FERTILIZATION MEMBRANE. Which blocks the entry of other
spermatozoa into the egg. Thus prevent POLYSPERMY .
24. METABOLIC ACTIVATION
The unfertilized egg undergoes degeneration if not fertilized. The entry of sperm
initiates many metabolic activities. Most of the reactions in fertilized egg are
initiated by 3’-5’,Cyclic AMP formed by the hydrolysis of ATP by an enzyme ADENYL
CYCLASE activated by the sperm entry.
• THE ACTIVITIES OCCURING IN METABOLIC ACTIVATION ARE :-
• The permeability of plama membrane is increased for water ,
ethylene, glycol phosphate & potassium ions.
• Due to change in the permeability, conc. Of sodium, potassium,
calcium changed & the membrane becomes DEPOLARIZED. This is
called FAST BLOCK & prevents POLYSPERMY.
• During fertilization, proteolytic enzymes remove inhibitors of
protein synthesis & activates protein synthesis
• In most of the animals, eggs complete meiosis after the entry of
the sperms.
25. KARYOGAMY / AMPHIMIXIS ( union of male &
female chromosomal sets )
• Before amphimixis, the pronucleus of the spermatozoan
migrates towards the female pronucleus where it rotates
at 180 degree so that its mitochondria & centriole come to
lie in forward position. The nucleus swells up. The
vesicular nucleus is called MALE PRONUCLEUS.In the
mean while the centriole is surrounded by astral rays.
• The nuclear membrane of the gametic nuclei degenerates
& two sets of the chromosomes initially lie on the two
poles of the spindle but latter these sets of chromosomes
mix up & process is called AMPHIMIXIS
26. • The male pronucleus & centrioles shifts towards the site of
amphimixis. The movement is accompained by the
movement of cortical & sub-cortical cytoplasm.
• In frog, due to pigmented sub-cortical cytoplasm a path like
structure is formed which is known as PENETRATION PATH. It
is towards the centre but then the male pronucleus become
inclined & takes copulation path toward the female
pronucleus located toward the animal pole.
• In sea-urchins & vertebrates, the nuclear membranes of both
pronuclei are broken down at the site of contact. Their
contents are surrounded by a common nuclear membrane
forming the zygote nucleus.
27. SIGNIFICANCE OF FERTILIZATION
• It stimulates the secondary oocyte to complete its maturation.
• Diploidy is restored by the fusion of male & female pronuclei.
• It brings together the genes from the two parents & results in
genetic recombinations.
• Due to fertilization sex of other generation is determined.
• It makes zygote more active as more mitochondria are available.
• Fertilization membrane prevents polyspermy.
