Formation of diploid zygote by fusion of sperm and egg, Fertilization

 

Fertilization in Human Beings: 

The fusion of haploid male and female gametes to form diploid zygote is termed as Fertilization. Fertilization in human beings is internal and takes place at the ampullary junction of the fallopian tube of the female genital tract.

To proceed the process of fertilization, Sperm or male gamete or spermatozoa  are released in the form of seminal fluid from the testes. Sperm are produced in testes through the process of spermatogenesis. Similarly, egg or ovum is produced in the ovary through oogenesis in females.

During the coitus ,  male inserts the seminal fluid  in the vagina. This process is called ejaculation. Seminal fluid contains as many as 200-300 million sperms. Male ejaculate millions of sperm in the female genital tract for better chances of fertilization but almost fifty percent of sperm are killed by the acidity of the female genital tract . Moreover, many sperms are engulfed by the phagocytes of the vaginal epithelium. 

The sperms swim in the seminal fluid by the movement of their tail. The sperms swim towards the fallopian tube .The peristaltic movements of the fallopian tube helps the sperms to approach the ova for fertilization.

Capacitation is  physiological maturation of sperms inside the female genital tract. It occurs in the presence of  fluid secreted from the Secretory cells of the fallopian tube . 

Ovum is released from the rupturing of the mature Graafian follicle from  the ovary on the fourteen or fifteen day of the menstrual cycle by the  process of  ovulation. Ovum is collected by  the  finger-like projection of the fallopian tube called fimbriae of the ampulla of the fallopian tube. Ovum moves in the tube towards the uterus by peristalsis and ciliary action. 

The capacity  of fertilization or  viability of gametes are  varied from a few minutes to a few hours or days. The  human sperm in the female genital tract remain viable or active upto  48 hours. The ovum is non-motile and the energy content in sperm is  very low so the sperm must approach the egg as quickly as possible.

Fertilizin—antifertilizin reaction

The ovum release a chemical substance, called fertilizin. This fertilizin  has various  spermophillic sites where the sperms may bind with the  antifertilizin site. This fertilizin—antifertilizin reaction is a highly specific reaction. Approach of sperm to ovum is achieved by this reaction. 

The secretions of seminal vesicles, prostate gland and Cowper’s glands form  seminal fluid along with sperm. This seminal fluid activates the  fertilizing  capacity of sperm  and neutralises the acidity in the vagina. 

Acrosomal reaction. 

A cap-like structure located on the head of sperm is called acrosome. The sperm undergoes an acrosomal reaction.  Under this reaction, Acrosome  releases Hyaluronidase enzymes or sperm lysin enzyme that  dissolve the egg envelopes  and make the path for the penetration of sperm. Sperm lysins are acidic proteins. These sperm lysins contain the hyaluronic acid that dissolves  the intercellular spaces which hold the granulosa cells of corona radiata. Acrosin or Zona lysin  dissolves the zona pellucida. After the dissolution of   the zona pellucida. Only the nucleus and middle piece of sperm make entry into  the ovum and The tail is lost in humans.

 Cortical reaction

As the sperm penetrates into egg,various event takes place.

1.Cortical granules appear in the egg cortex. 

2.The vitelline membrane is emerged from the cortical surface of the egg. This creates a  perivitelline space within the vitelline membrane. 

3.Cortical granules are protruded in the perivitelline space  and some of these are attached along with  inner surface of vitelline membrane. 

4.These cortical granules make  impervious  the egg to any other sperm entry. Now It is termed as   fertilization membrane. It prevents the polyspermy.

Fusion of nucleus of sperm and egg 

The entry  of sperm through copulation path, causes  the second  meiotic division to release the ova and second polar body. The nuclear membrane of the gametic nuclei degenerates and two sets of chromosomes are intermixed. This process is called amphimixis . The fertilized egg is now called zygote. while the zygote nucleus is called synkaryon having two sets of chromosomes.

Significance of fertilization:

It stimulates the secondary oocyte to undergo second maturation division to release second polar body and to form haploid ovum.  It restores the diploidy level in the zygote. Fertilization membrane prevents the polyspermy. Metabolic activities are increased as more mitochondria are available.  Fertilization combines the characters of two parents and introduces variations. So helps in evolution. Centrioles of sperm from the spindle to initiate the cleavage of zygote.