fertilization

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1. FERTILIZATION

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.

4. TYPES OF FERTILIZATION

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.

9. MECHANISM OF FERTILIZATION

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 .

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