Fertilization

Fertilization is the process of union of male and female gametes that occur in the ampulla of the fallopian tube. The result of fertilization is the production of a unicellular embryo called a zygote. After the formation of the zygote, the prenatal development starts. Scientists discovered the dynamics of human fertilization for the first time in the 19th century.

The process of fertilization begins with the ejaculation of sperm during copulation, followed by ovulation, and terminates with the production of a zygote. But some exceptions are also present such as in Vitro fertilization, gametes intrafallopian transfer, zygote intrafallopian transfer, ejaculation without copulation, or copulation shortly after ovulation.

Approximation of gametes

During sexual intercourse, a man ejaculates about 200 to 300 million sperm into the female vagina. Only 200 to 300 sperm survive and hold the secondary oocyte. The trip of the sperm from the vagina into the ampulla of the fallopian tube is promoted by prostaglandin present in the semen. After ovulation, the oocyte gets to the ampulla by the motion of cilia and contraction of muscles of the fallopian tube.

The oocyte cumulus cells produce chemoattractants and make again the sperm motile. Most sperm due within 24 hours. Before fertilization, the sperm must gain the ability to disintegrate the oocyte obstacle and this can be done by the process of capacitation which is called the capacitation of the sperm.

Stages

The process of fertilization generally completes in five steps. These steps are,
1. Binding to corona radiata
2.  zona reaction
3.  Capacitation of the sperm
4.   Cortical reaction
5.    Fusion

Capacitation of the sperm and acrosome reaction

As we know that due to different factors freshly ejaculated sperm are unable to fertilize an egg. To Fertilize the egg the sperm undergoes a series of changes. These series of changes in the sperm that occurs in the female reproductive system are called the capacitation of the sperm. The capacitation of the sperm lasts for several hours. In the process of capacitation, the sperm motility increases and its membrane is destabilized for the acrosome reaction, sperm penetrates through the zona pellucida. Zona pellucida is a tough membrane that surrounds the oocyte.

binding to corona radiata.
Corona radiata is a layer of follicle cells that surrounds the outside of the secondary oocytes. The sperm reaches the egg and binds to the corona radiata.

Zona pellucida Reaction Or zona reaction.
After the binding to the corona radiata, the sperm attached to the pellucida that triggers the acrosome of the sperm to burst and release enzyme. These enzymes will help help the sperm to get through the zona pellucida.

Cortical reaction
Once a sperm goes inside the zona pellucida the cortical granules present inside the egg fuses with the plasma membrane of the cell causing the release of enzymes. These enzymes act on the zona pellucida and make it hard and impermeable to the other sperm.  Due to the cortical reaction, the fertilization of an egg by more than one sperm is prevented.

Fusion
When the sperm reaches the cytoplasm of the oocyte its tail and outer coating disintegrate. The egg or oocyte then undergoes its second meiotic division and produces a haploid ovum. The sperm nucleus then fuses with the egg and fusion occurs. the process of fusion completes in following steps

1. Cell membrane. Firstly the cell membrane of both gametes fuses.

2. Transformation. For the fusion of genetic materials, the sperm and egg undergo a transformation reaction. The egg completes its second division and changed into the mature ovum and its nucleus is called a pronucleus.

When the male pronucleus has formed the sperm tail and mitochondria degenerates.

3.  Replication. The male pronuclei and the female pronuclei migrate toward the center of the oocyte and replicate its DNA to prepare the zygote for its first meiotic division.

4. Mitosis. The membrane of the gametes dissolves and the male and female chromosomes come in contact with each other. At that time mitotic spindle forms between them that capture the chromosomes before they dispersed in the cytoplasm. Due to the process of mitosis, the cell gathers its genetic material.

What is the fertilization age?

Fertilization is the process that is most commonly marked by the anatomists as zero-point in the prenatal development of the embryo and the resultant age is called fertilization age. The fertilization age is also known as embryonic age, fetal age, or developmental age.

Opposite to that is the gestational age that marks the last menstrual cycle as zero-point.

If you want to calculate the gestational age then add 14 days to fertilization age and vice versa.

What are the results of fertilization?

The primary results of the fertilization are

1. Completion of the second meiotic division. When the sperm reaches the secondary oocyte the oocyte completes its second meiotic division and produce a mature ovum and a polar body. The polar body goes into the perivitelline space.

2. Restoration of diploid chromosomes. The male and female pronuclei fuse in the process normal diploid chromosomes are restored.

3.  Perseverance of the sex of the new individual. The oocyte contains x chromosome while the sperm contains x and y chromosomes.

When an oocyte is fertilized by a Y bearing sperm then the result will be a man baby and if oocyte is fertilized by X bearing sperm then the result is a female baby.

From this, we concluded that the father is responsible for the perseverance of the sex of the infant rather than the mother.

Diseases that might occur in fertilization

Polyspermy
In science, polyspermy is a condition in which an egg is fertilized by more than one sperm. Organisms normally contain diploid chromosomes but on the other hand, the zygote formed from polyspermy contains three or more copies of each chromosome.

Blocking polyspermy
In human reproduction, polyspermy occurs very rarely. The human reproductive system prevents polyspermy by two mechanisms. 1. Declining the number of sperm that swims to the viaduct. 2. Blocking of the sperm in the fertilized egg.