Fertilization in Human

Fertilization:

The process in which the union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum takes place thus diploid zygote is formed, is called fertilization. Fertilization stimulates the secondary oocyte cell, allowing the division to be completed. It usually happens when sperm and egg connect in the upper oviduct (ampulla-isthmic junction). During fertilization, a sperm contacts the ovum (zona pellucida layer) and causes modifications in the membrane that prevent additional sperms from entering. As a result, it assures that just one sperm can fertilize an ovum. The secretions of acrosome help the sperm to enter into the ovum through zona pellucida and the plasma membrane and thus secondary oocyte completes meiosis II and results in the formation of a second polar body and haploid ovum. The haploid nucleus of the sperm and ovum fuse together to form a zygote which develops into new individual. 

Polyspermy is the entry of more than one sperm nucleus into an ovum during fertilization. When the acrosome of a spermatozoa meets the surface of an egg, the cytoplasm of the egg bulges forward, generating a receptive cone or fertilization cone (a region where sperms enter the egg).

Passage of sperm to egg:  

During ejaculation, the male releases 80-100 million per ml of sperms in the vagina which are deposited at the top of the vagina close to the cervix. For fertilization to occur, sperms have to pass via the cervix, uterus, and oviduct. Cervix normally has very thick mucus. It prevents sperm motility. This mucus becomes thinner in the ovulatory part of the menstrual cycle, allowing the entry of sperms into the uterus and then into the oviduct. Movement of sperms is assisted by the contraction of smooth muscles of the uterine tract (under influence of prostaglandins from semen). Sperm can survive in the female is for 1 to 3 days but highly fertile for 12-24 hours only. Only a few hundred thousand complete their journey to the egg.

Mechanism of fertilization: 

1. Movement of sperms towards the secondary oocyte. 
2. Penetration of corona radiata by the hyaluronidase enzyme.
3. Adherence of sperm to the ZP3 receptors/region on zona pellucida, the glycoprotein layer surrounding the oocyte. 
4. Sperm bind to a sperm receptor on the zona and this leads to the initiation of the acrosomal reaction. Various enzymes are released. e.g., Acrosin (Zona lysin). 
5. Acrosin facilitates the penetration of sperm through zona pellucida.
6. Fusion of sperm and membrane of secondary oocyte. (Syngamy) 
7. Phagocytosis of sperm by the secondary oocyte. 
8. Completion of meiosis-II of the secondary oocyte during phagocytosis to form ovum and simultaneously it releases 2nd polar body. 
9. Structural changes in zona pellucida through cortical reaction and discharge of cortical granules in perivitelline space forms fertilization membrane, which prevent polyspermy. 
10. In the event of fertilization complete sperm enters inside the ovum. (By phagocytosis) 
11. It is followed by plasmogamy, karyogamy, and amphimixis i.e., completion of fertilization.

Effects of fertilization:  

If fertilisation occurs and the foetus is deposited in the endometrium, the growing placenta's trophoblast cells secrete the hormone human chorionic gonadotrophin (hCG). Like LH, this hormone supports the corpus luteum and its release of progesterone and estradiol. These two hormones monitor the disintegration of the uterine endometrium; the lack of monthly flow (the "period") is the first indicator of pregnancy. The placenta produces enough progesterone and estradiol for a normal pregnancy by the 16th week of pregnancy, and the now-ineffective corpus luteum shrinks. 

Significance of Fertilization: 

1. The secondary oocyte completes its second maturation division on coming in contact with the sperm. 
2. The Amphimixis process leads to the formation of a diploid zygote to restore the normal diploid number of the chromosomes. 
3. The centriole of sperm after entering into egg induces the egg to undergo cleavage. 
4. The paternal and maternal characters are transmitted to the offspring through the process of fertilization. 
5. The peripheral changes occurring in the egg prevent the further entry of sperm into the ovum, thus checking polyspermy. 

Amphimixis: 

Nucleus develops and becomes pronuclei. Male and female pronuclei fuse with each other to form a synkaryon or zygote with a genetic complement (2n,2C). It takes approx. 24 hrs. Proximal centriole of sperm initiates cleavage. 

Miscarriage:  

The most prevalent cause of miscarriage at around 10 – 12 weeks of pregnancy is premature corpus luteum degradation (miscarriage means abortion). 

Sex determination: 

Human female- XX; Human male-XY 

If a male gamete containing an X-chromosome fertilizes a female gamete the female baby will have (XX). If a male gamete containing Y-chromosome fertilizes a female gamete the male baby will have (XY). Thus, the sex of the baby is determined by the father and not by the mother.