Fertilization:

     The fusion of a haploid male gamete (spermatozoan) and a haploid female gamete (ovum) to form a diploid zygote is called fertilization.

     The main idea of fertilization was cleared by Leuwenhoek (1682).

     In human female, fertilization is internal and it takes place in the ampulla of the fallopian tube.

     During copulation, the semen is discharged into vagina which contain about 200-300 millions of sperms.

Fertilization includes four steps.

     i. Approach of sperm to ovum

     ii. Penetration of sperm into ovum

     iii. Activation of ovum

     iv. Fusion of gametic nuclei

i. Approach of sperm to ovum:

1)     After discharge of sperms into genital duct of female, sperms swim by flagellar movement toward the fallopian tube.

2)    Many sperms are killed by the acidity of female genital tract and many sperms are engulfed by the phagocytes of the vaginal epithelium, so that about 100 sperms reach the fallopian tube through the uterus.

3)    The sperms swim in the seminal fluid by the lashing movement of their tail at the rate of 1-4 mm/per minute.

4)    A sperm takes 2-3 hours to reach the egg from lower part of uterus by peristalsis and ciliary action.

5)    The viability of sperm is about 24 hours.

ii. Penetration of sperm into the ovum:

1)     Before the entry of sperm into the ovum for the act of fertilization they undergo a maturing process called capacitation.

2)    The decapacitation factors produced from testis and epididymis are present on the head of sperm.

3)    In female, genital tract, estrogen removes this decapacitation factor and sperm is called capacitated sperm.

4)    Penetration of sperm is chemical mechanism. Acrosome of sperm release lysin enzymes called hyaluronidase, Acrosin and Zona lysin.

a. Hyaluronidase and Acrosin neutralize the acidic effect of hyaluronic acid of corona radiata and penetrate corona cells.

b. Zona lysin penetrate zona pellucida.

     This makes the way for the entry of sperm at animal pole called penetration path.

     The nucleus and centrioles enter inside the ovum while tail remains outside which is degenerated.

iii. Activation of Ovum:

1)     After entry of sperm the vitelline membrane is converted into fertilization membrane which prevents the entry of other sperm.

2)    Sperm entry stimulates the ovum to undergo second-maturation division as a result it releases IInd polar body. The ovum become mature called ootid.

3)    The egg becomes activated and first of all changes occur in the cortex (surface) of the egg called cortical reaction.

4)    The cortical granules appear in egg cortex.

5)    The vitelline membrane gets lifted.

6)    The perivitelline space is developed.

7)    The cortical granules come in perivitelline space, absorb water, swell rapidly, become liquefied and attached to inner surface of vitelline membrane.

8)    The vitelline membrane becomes thick and is called fertilization membrane.

9)    It prevents the polyspermy.

10) After entry of sperm into ovum it also induces following metabolic activities.

a.   Increases permeability of plasma membrane

b.   Increases rate of protein synthesis

c.   Increases the rate of respiration so O2 consumption is increased.

iv. Fusion of gametic nuclei:

1)     After penetration, the nucleus of sperm and nucleus of ovum swell up.

2)    These nuclei are called male pronucleus and female pronucleus.

3)    The male pronucleus takes a definite path towards the female pronucleus by crossing 180° angle called as copulation path.

4)    Centrioles move in opposite direction and forms spindle apparatus.

5)    The nuclear membrane of gametic nuclei degenerates and two sets of chromosomes initially lie on two poles of spindle but later these sets of chromosome mix up and the process is called Amphimixis.

6)    The fertilized egg is now called zygote.

Significance of fertilization:

1)     It stimulates-the secondary oocyte to undergo second maturation division to release IInd  polar body & to form haploid ovum.

2)    It restores, diploid nature in the zygote (2n).

3)    Fertilization membrane prevents the polyspermy.     ,

4)    Metabolic activities are increased as more mitochondria are available.

5)    It combines the character of two parents and introduces variation in offspring, which helps in evolution.

6)    Centrioles of sperm from the spindle to initiate the cleavage of zygote.

7)    Copulation path sets axis of division.

8)    Sex chromosome of sperm is either X or Y and help in sex determination.

9)    Fertilization stimulates the zygote to undergo further development.

 

1)       What is fertilization?

2)      What is the nature and site of fertilization in humans?

3)      What is insemination?

4)      How sperms are made capable of fertilization?

5)      How sperms are activated?

6)      How much time is required for fertilization?

7)      Which material is secreted by ovum over its surface?

8)      Which material present over surface of sperm?

9)      Why sperm adheres to ovum?

10)   How hyaluronic acid over ovum is removed?

11)    What is the role of zona lysine or acrosome?

12)   How fertilization membrane is produced?

13)   What is the role of fertilization membrane?

14)   What is the significance of entry of sperm into the ovum?

15)   What does entry of sperm indicate>?

16)   What are pronuclei?

17)   What is karyogamy or amphimixis?

18)   What is the term applied for zygote & zygotic nucleus?

19)   Which is the first cell of new life?

20)  When the female is said to be conceived?

21)   Name the process which restores diploid nature of organisms?

22)  Which process determines the sex of young one?

23)  Name the structure missing in ovum and introduced by sperm?