The process of fertilization involves a sperm fusing with an ovulated oocyte (Pic.1). The process begins with ejaculation during copulation, when hundreds of milions of sperm are released into the vagina. Millions of these sperm are overcome by the acidity here and milions more may be blocked from entering the uterus by thick mucus in cervix. Of those that do enter, thousands of sperm are destroyed by female immune system. Sperms have to reach the distal part of fallopian tube, because oocyte cannot survive the 72 hour journey to the uterus. They are reduced to a few thousand survivors on this journey. Their effort is facilitated by uterine contractions - usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the fallopian tube for another 3–5 days. Hence, fertilization can still happen a few days before ovulation. In comparison, an oocyte can survive independently for only approximately 24 hours following ovulation. Intercourse more than a day after ovulation will therefore usually not result in fertilization.

Capacitation

During the journey, fluids in the female reproductive tract prepare the sperm for fertilization in a process called capacitation. The fluids improve the motility of the sperm. They also modify the sperm membrane, thinning the membrane in such a way that will help facilitate the release of the digestive enzymes, which are essential penetration the oocyte. Sperm must undergo the process of capacitation in order to be able to fertilize an oocyte. If they reach the oocyte before completion of capacitation, they will be unable to penetrate the oocyte’s thick outer layer of cells, because freshly ejaculated sperm is unable or poorly able to fertilize. 

Reaching the oocyte

To stream the right direction to the oocyte, sperms response to increasing temperature of distal parts of female reproduction tract and to chemical attractants released from the corona radiata cells. To reach the oocyte itself, the sperm must overcome two protective layers surrounding the oocyte. The sperm first pass through the cells of the corona radiata. First few sperm undergo a spontaneous acrosomal reaction, which does not require contact with the zona pellucida. The reaction digest the corona radiata by the digestive enzymes. That’s why the first sperm, which reaches the oocyte, does not fertilize. Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida. This initiates a process called the acrosomal reaction in which the enzyme-filled „cap“ of the sperm head, called the acrosome, releases its stored digestive enzymes (Pic. 2). These enzymes clear a path through the zona pellucida, which surrounds the oocyte, that allows sperm to reach the oocyte. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte’s plasma membrane (Pic. 3). After that, both plasma membranes fuse and whole sperm enter the oocyte cytoplasm. 

The block of polyspermy

When the first sperm fuses with the oocyte, it is necessary to prevent polyspermy, which means penetration by more than a single sperm. This is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes. This is incompatible with further development. First block of polyspermy is very fast. It involves a near instantaneous change in sodium ion permeability upon binding of the first sperm, depolarizing the oocyte plasma membrane and preventing the fusion of additional sperm cells. The fast block sets in almost immediately and lasts for about a minute, during which time an influx of calcium ions following sperm penetration triggers the second mechanism, the slow block. This one is called as the cortical reaction, which involves the release of cortical granules from the oocyte. The cortical reaction leads to a modification of the zona pellucida and establishes a permanent barrier to sperm entry, because of the hardening of zona pellucida. 

The zygote

Although sperm reaches the oocyte, the oocyte has not yet completed meiosis. All secondary oocytes remain arrested in metaphase of meiosis II until fertilization. Only upon fertilization as an activation stimulus by penetrated sperm does the oocyte complete meiosis. The unneeded complement of genetic material that results is stored in a second polar body that is eventually ejected. At this moment, the oocyte has become an ovum, the female haploid gamete. The two haploid nuclei derived from the sperm and oocyte and contained within the egg are decondensed and transformed to form pronuclei. They expand and replicate their DNA in preparation for first cell cycle. The pronuclei then migrate toward each other and then together to the centre of the oocyteTheir pronuclear membranes disintegrate, and both genetic materials fuse a shuffle to form an unique individual. This is the final step of the fertilization and results in a formation of diploid zygote contains all the genetic instructions necessary to develop into a foetus.

Twins

In some cases, there could be two offsprings procuded by a single pregnancy, also called as twins. Twins can be developed from just one zygote (monozygotic, "identical“ twins), when a zygote is splitted in two embryos, or can be developed from two different oocytes and each of them is fertilized by separate sperm (dizygotic, "fraternal“ twins).

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Sources

Fertilisation ―sourced from Wikipedia licensed under CC BY-SA 3.0
Twin ―sourced from Wikipedia licensed under CC BY-SA 3.0
Human fertilization ―sourced from Wikipedia licensed under CC BY-SA 3.0
Cortical reaction ―sourced from Wikipedia licensed under CC BY-SA 3.0
Fertilization ―sourced from Boundless licensed under CC BY-SA 4.0
Fertilization in humans ―by Ttrue12 licensed under CC BY-SA 3.0
Acrosome reaction diagram ―by LadyofHats licensed under CC BY-SA 3.0
Sperm-egg ―by unknown licensed under CC0 1.0
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