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Fertilization process in human

 What is fertilization?

Fertilization is the process by which the fusion of  haploid male gamete ( sperm) and haploid female gamete (ovum) takes place and formation of diploid zygote formed. It may be external ( in frog )or internal .
Fertilization takesplace in ampula of fallopian tube of human being.

Process of fertilization:

During copulation male discharges semen in to the vagina of the female. Nearly 300 million sperm are released with semen. From vagina the sperms are moved towards the ampula of fallopian tube partly by the spiral movement of the tail of sperm and partly by the help of muscles contraction of uterus. Here in the fallopian tube the sperm lives only 3 to 4 days. It took 12 to 24 hours for sperm to fertilize an egg. 

From this the process of fertilization takesplace in various stages. 

1.What is capacitation??

In order to fertilize an egg the sperm acquire some capacity in fallopian tube which increases the motility of the sperm capable to undergo acrosomal reaction. 

Biochemical and functional changes during capacitation-

1.Removal of cholesterol occurs which increases the fluidity of plasma membrane. 
2. Flowing out of potassium occurs due to which the variation in membrane potential of sperm results. 
3. This allows calcium and bicarbonate channel to enter the sperm. 
4. Bicarbonate channel influx results increase in the pH of sperm , calcium channel and bicarbonate channel activates adenylate cyclase .
5. Adenylate cyclase makes cAMP from ATP
6. The rise of cAMP activates protein kinase A (PKA) which mediates in phosphorylation of tyrosine kinase.

2.Acrosome Reaction 

The process by which the content of  acrosome are released by exocytosis , which release various Proteases that lyse zona pellucida, is known as acrosome reaction .
 
After the process of capacitation,  the sperm encounters the egg and penetrate egg's zona pellucida.
The zona pellucida of human egg contains four types of zona proteins ZP1,ZP2,ZP3,Zp4 . These are produced by growing oocyte.
Sperms without acrosomal reaction binds with ZP3 ,which induces sperm to undergo acrosome reaction. And sperms with acrosome reaction binds with ZP2.

The enzyme Hyaluronidase (sperm lysin) released from acrosome dissolves zona pellucida  and the sperm reaches the plasma membrane of the egg.
                         

3.Cortical Reaction 

After the fusion of sperm with plasma membrane of egg, all the contents of sperm are drawn in to the egg.
Fusion with a sperm activates the egg , cortical granules release their contents by exocytosis and this process is known as cortical reaction. 
This mechanism has similarity with acrosomal reaction. 
During the fertilization process Intracellular calcium ion of the egg increases and the higher calcium environment results the fusion of cortical granules with egg plasma membrane,  releasing their contents in to it.

4.What is amphimixis( karyogamy)??

The egg containing N- acetylglucosaminidase enzyme capable of cleaving  N -acetylglucosamine from ZP carbohydrate chains .
Sperm binds with N-acetylglucosamine and metaphase II of meosis II resumes  producing  diploid zygote. 

Prevention of polyspermy 

About 200-300 million of sperm are released on the vagina but nearly about 200 reach their destination to ovum in fallopian tube. Although many sperm can bind to an egg, normally,  one inject its cytosol, nucleus and other organelles in to egg cytoplasm. 
Polyspermy is the condition in which more than one sperm fuses with egg. This causes multipolar mitotic spindle formation,  resulting in faulty segregation of chromosomes during the first mitotic cell divisions.
Internal fertilization act as a block for polyspermy. 
Sperms are migrated from vagina to oviduct and stay there in oviducal sperm reservoirs,  mediated by isthmus. 
Oviductal sperm reservoirs help to prevent polyspermy and increases viability of sperm for an extended period of time (even up to months in some species).
In mammals , several changes occur in zona pellucida to block polyspermy. The ZP is inactivated so that no longer bind sperm or induce acrosome reaction after the fusion of one.

The process of mixing up of the chromosomes  of a sperm and an ovum resulting a diploid zygote nucleus is known as amphimixis or karyogamy.
The sperm centriole enters the egg along with nucleus, and a centrosome forms around them.
Then the centrosome duplicates to form two centrosomes takes part in spindle formation in mitotic division. 

What are the significance of fertilization. 

1. The diploid number of chromosome is maintained  i.e 46 in human being.
2.The ovum is stimulated to mature. 
3. Fertilization cause variation in organism.
4. The sex of embryo is determined by fertilization. 
5. It interduces centrioles which is absent in ovum.And also donates mitochondria. 
6. Fertilization membrane prevents polyspermy.

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