Fertilization: Definition, Types, Steps, & Post Fertilization Events

It is the nature of all living organisms to continue their species through reproduction. A species needs to reproduce to keep the population growing and prevent it from extinction. In human, reproduction takes place when the fertilized zygote is successfully implanted in the endometrial layer of the womb.

Fertilization is an essential process as without it, zygote formation would not be possible. This process ensures the transmission of genetic information from one generation to the next and genetic variation, making it essential for the survival of life.

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Sexual Reproduction in Human

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A type of reproduction in which the gametes of two parents fuse together to form a zygote is known as sexual reproduction. In this type of reproduction two parents are involved in the generation of a new organism. Sexual reproduction causes genetic variation which is beneficial for survival in extreme conditions. Both mitosis and meiosis type of cell divisions occurs throughout the process of sexual reproduction. Thus, majority of higher level organisms perform sexual reproduction.

Formation of Germ Cells

There are different processes for generation of germ cells in each sex. In human females the maturation of germ cells occurs two times, that is once after birth and other after puberty, while in human males maturation occurs only once, that is after puberty. The human female germ cell is called ovum or egg while the human male germ cell is called sperm.

Process of Spermatogenesis

Process of Oogenesis

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What is Fertilization?

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The biological process that involves the fusion of a male and a female gamete which gives rise to a new organism is called fertilization. Successful fertilization and implantation are the indications of pregnancy in human. The fusion of gametes or syngamy is followed by zygote formation in humans.

• The genetic materials from both the parents are combined during the time of fertilization.
• The site of fertilization in humans is female oviduct or fallopian tube.

Types of Fertilization

Based on the location and surrounding environment, the process of fertilization is categorized into two types: Internal fertilization and external fertilization.

1. Internal Fertilization

A type of fertilization that takes place inside the female body is called internal fertilization. It is also known as in-vivo fertilization. This type of fertilization is a common method of fertilization in human.

2. External Fertilization

When the fusion of male and female gametes takes place in an external or laboratory environment, it is called external fertilization. This process of fertilization is also called in-vitro fertilization.  With this approach, an infertile parent can also become pregnant and have a baby. 

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Process of Fertilization in Human 

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Fertilization begins with insemination process, which is the transfer of semen (fluid containing sperms and some gland’s secretions) into the vagina. The sperms travel up to the fallopian tube only after surviving through the difficult environment in the vagina, cervix and uterus.

• The survived or most healthy sperms have to reach up to the ampulla region of the fallopian tube where the fertilisation occurs. 
• The sperm’s head contains an enzyme which allows the sperm to enter the ovum by passing all the layers which are meant to protect the egg.
• Only one sperm can fertilise the egg and when it does, the outermost layer, that is zona pellucida of egg will harden up to prevent the entry of any other sperms.

Steps of Fertilization in Human

The process of fertilization in humans involves several chemical and physical reactions for it to be successful. The steps involved in the fertilization process are:

1. Acrosomal Reaction

The plasma membrane of the secondary oocyte and the sperm are fused together so that the contents of the sperms can enter. This happens due to the acrosomal reactions taking place in the sperm. The plasma membrane of the sperm then binds with that of the secondary oocyte, causing the plasma membrane of the oocyte to depolarize. This step is very crucial in order to prevent polyspermy.

2. Cortical Reaction 

Once the plasma membranes fuse, the oocyte quickly starts to show cortical reactions. Cortical granules present under the plasma membrane of the oocyte, then fuse with the plasma membrane and release cortical enzymes between the zona pellucida and plasma membrane. The cortical enzymes harden the zona pellucida to prevent polyspermy.

3. Sperm Entry 

The secondary oocyte forms a projection known as the cone of reception. It then receives the sperm at the point of sperm contact.

4. Karyogamy

The head of the sperm which contains the nucleus detaches from the main body(called male pronucleus). The tail and the second polar body degenerates. The nucleus of the ovum is known as female pronuclei.

Both male and female pronuclei fuse and their nuclear membranes degenerate. The fusion of the chromosomes of male and female gametes is called karyogamy. The ovum is now fertilized and is known as a zygote.

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Post Fertilization Events in Human

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Zygote is the end result of a successful fertilization in humans. The process of fertilization is followed by implantation in humans. This zygote will move down to the uterus where it will implant into the thickened uterine walls for further development.

• After this, cell division will start which will lead the zygote to develop into an embryo. This has three types of cell layers and those are ectoderm, endoderm and mesoderm.
• These layers form different parts of the body of an organism. The meat loaf type structure formed after the successful fertilization is called foetus.
• This foetus is connected to the mother through a disc like structure on the uterus wall called placenta which helps in exchanging substances between mother and foetus.
• Placenta acts as a connecting link between mother and a foetus through which required nutrients are provided to the developing foetus. 
• The foetus develops inside the womb for approximately 9 months and after that the fully developed baby is delivered out of the vaginal opening.