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.

Semen released during ejaculation has sperms and some secretions. The coagulated semen now undergoes liquification and sperms become active. The mechanism of fertilization is as follows :

1. Movement of sperm towards egg :

• It involves capacitation of sperms reaching the vagina. Here as many as 50% are demotilised / broken / destroyed.
• Remaining sperms undergo capacitation. This process requires 5-6 hours. Acrosome membrane becomes thin, Ca++ enters the sperm and their tails begin to show rapid whiplash movements.

• As a result of capacitation, sperms become extra active and begin moving upwards from the vagina to the uterus and the oviducts.

• Prostaglandins activate the sperms, and the vestibular secretions of the female also enhance sperm motility.

• The sperms swim at an average speed of 1.5 to 3.0 mm/min. Sperms reach the ampulla as a result of their own swimming and partly by the contraction of the uterus and fallopian tubes stimulated by oxytocin in the female.

• After capacitation, the sperms may reach the ampulla within 5 minutes. Sperms can remain viable for 24-48 hours (Ovum for about 24 hours).

2. Entry of sperm into the egg:

• Out of 200 to 400 million sperms, only a few hundred manage to reach the ampulla. Though many sperms reach the ampulla, only a single sperm fertilizes the ovum.
• A sperm, after reaching the egg/ovum, comes to lie against it. Its acrosome releases lysins: hyaluronidase and corona penetrating enzymes.
• They separate and dissolve the cells of the corona radiata, allowing the sperm head to pass through the zona pellucida of the egg.
• The zona pellucida has fertilizin receptor proteins (ZP3, ZP2). The fertilizin binds to specific acid protein-antifertilizin of the sperm.
• It brings about the attraction of sperms to the egg to enhance fertilization. The fertilizin-antifertilizin interaction is species-specific.
• Thus, the fertilizin-antifertilizin reaction is also called a compatibility reaction.

Acrosome reaction:

• As the sperm head touches the zona pellucida in the animal pole region, its acrosome covering ruptures to release lytic enzymes, acrosin or zona lysin.
• These enzymes act on the zona pellucida at the point of contact, causing an egg reaction.
• A small fertilization cone/cone of reception is formed on the egg membrane, and the sperm head comes in contact with this cone.
• This results in the production of a weak wave of depolarization. The plasma membranes of both cells dissolve at the point of contact.
• The sperm nucleus and centrioles enter the egg, while other parts remain outside.
• As soon as the sperm head touches the vitelline membrane, a cortical reaction is activated, changing the vitelline membrane into a fertilization membrane by deactivating the sperm receptors of the zona pellucida.
• A distinct perivitelline space is created around the fertilization membrane. This prevents any further entry of other sperms into the egg, i.e., polyspermy is avoided.

3. Activation of ovum:

• The ovum before fertilization was at the metaphase II stage. With contact of the sperm head to the vitelline membrane of the egg, it gets activated to resume and complete its meiosis II, releasing the second polar body.
• The germinal vesicle organizes into the female pronucleus. At this stage, it is the true ovum or egg.

Fusion of egg and sperm:

• The coverings of the male and female pronuclei degenerate, allowing chromosomal pairing.
• This results in the formation of a synkaryon by the process called syngamy or karyogamy, thus forming the zygote.
• The proximal centriole received from the sperm helps in the formation of the synkaryon spindle and the cleavage of the cell into two blastomeres.

• 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. 

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. Fertilization involves the fusion of male and female gametes from the two parents. This fusion results in variations that are significant to evolution.
4. The centriole of sperm after entering into egg induces the egg to undergo cleavage. The sex of the offspring is determined.
5. The paternal and maternal characters are transmitted to the offspring through the process of fertilization. 
6. The peripheral changes occurring in the egg prevent the further entry of sperm into the ovum, thus checking polyspermy. 

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). 

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.