Immunology | Immune system| Immunity

Fertilization

Fertilization involves the fusion of haploid male and female gamete to form diploid zygote.Fertilization in the human beings is internal and takes place in the proximal part of fallopian tube of the female. Fertilization involves following processes :

1. Approach of sperm to ovum:

During the copulation, male inserts its erectile penis in the vagina of female and releases about 3.5 ml of seminal fluid. This process is called ejaculation. Seminal fluid contains as many as 400 million sperms.This ensures the reaching of number of sperms.This ensures the reaching of a number sperms to the ovum as 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 only about 100 sperms reach the fallopian tube. The sperms swim in the seminal fluid by lashing movements of their tail at the rate of 1-4 mm per minute.The sperms swim towards the fallopian action of the uterus and peristaltic movements of the fallopian tube. Capacitation is the phenomenon of physiological maturation of sperms inside the female genital tract. It occurs in the presence of viscous fluid secreted from the secretory cells of epithelial lining of oviducal mucosa.It takes about 5-6 hours.
Ovum is released from the graafian follicle of ovary on the 15th day of menstrual cycle and the process is called ovulation (the expulsion of an ovum from the ovary). Ovum is trapped by the fimbrae of the ampulla of fallopian tube. Ovum moves in the tube towardsthe uterus by peristalsis and ciliary action. At the time of ovulation,egg is at secondary oocyte stage.
Fertilizability and viability of gametes are limited varying from few minutes to a few hours or days. The fertilizability of human sperms in the female genital tract is of 12 to 24 hours while its survival value is 3 days.Secondly the ovum is non motile and the energy contents of sperm must approach the egg as quickly as possible. Fertilizability period of ovum is only 24 hours though it can live for 72 hours.

2. Penetration of sperms:

Secretions of seminal vesicles,prostate gland and cowper's gland form the major part of seminal fluid. These secretion activate the sperms in fertilizing the egg and neutralise the acidity if vagina.
The sperm generally comes in contact with ovum in the animal pole while the opposite of the ovum is called as vegetal pole.Ovulation in  the human female occurs at secondary oocyte stage in which meiosis-I has been completed and first polar body has been released but second maturation is yet to complete. Penetration of sperm is chemical mechanism. In this acrosome of sperm undergoes acrosomal reaction and release certain sperm lysins which dissolves the egg envelopes locally and make the path for penetration of the sperm. Sperm lysins are acidic proteins.

3. Cortical reaction:

The penetration of sperm into the egg initiates a series of processes like:
Cortical granules appear in the egg cortex. The vitelline membrane starts lifting the cortical surface of the egg. The produces a perivitelline space within the vitelline membrane.Cortical granules are extruded in the peri vitelline space by exocytosis and some of these are attached along inner surface of the vitelline membrane which now thickens and becomes imperviuos to many other sperm entry. It is now called fertilization mambrane. It prevents the polyspermy.
4. Fusion of gametic nuclei:
The sperm entry stimulates the stimulates the secondary oocyte to undergo meiotic II division which produces ovum and second polar body.Inside the ovum,sperm nucleus takes definite path called copulation path. The centrioles of the middle piece of the sperm form a spindle. The nuclear membrane of the gametic nuclei degenerates and two sets of chromosomes initially lie on two poles of the spindle but later these sets of chromosome mix up and the process called ''amphimixis''. The fertilized egg is now called zygote while the zygote nucleus is called synkaryon.