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Fertilization

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Reproduction is the process by which an organism produces young ones similar to itself. When this process involves only one parent and produces clones that are morphologically and genetically similar to the parent, it is termed an asexual mode of reproduction. When two gametes one from a male and the other from a female parent fuse together, it produces progenies possessing variations. This is called the sexual mode of reproduction. The various steps involved in sexual reproduction are as follows:

  • Gametogenesis: It is the process of the formation of gametes. 
  • Insemination: It is the process of the transfer of sperm into the female genital tract. 
  • Fertilization: It is the fusion of the male and female gametes in the female reproductive tract to form a zygote. 
  • Implantation: It is the attachment of blastocyst (developed zygote) into the uterine wall.
  • Gestation: It is the period of development of embryo in the uterus.
  • Parturition: It is the process of delivery of a fully developed baby from the mother’s womb. 

Let us have a glance over the topic of Fertilization.

Fertilization

Human Reproduction Process

 

Fertilization in humans refers to the fusion of male and female gametes that facilitates the development of a new organism. During this process, semen comprising thousands of sperms is inseminated into the female’s vagina during coitus. The sperms move towards the uterus and reach the opening of the fallopian tube. Only a few sperms succeed in reaching the fallopian tube and the rest are digested and absorbed into the female reproductive tract. The process of release of a secondary oocyte from a mature Graafian follicle is called ovulation. Many sperms adhere to the surface of the secondary oocyte, however only one is able to enter it. During meiosis II, the sperm enters the secondary oocyte. After completion of meiosis, the secondary oocyte is known as the egg. Both sperm and egg are viable for a limited period of time. Sperm remains alive for 48-72 hours in the female reproductive tract, whereas the egg can be fertilized for 24 hours after it is released. Fertilization in most animals is similar to that in humans. 

Modes of Fertilization

Animals also produce gametes for fusion. But the fusion of gametes may take place inside or outside the body. Based on this, fertilization is of two types-internal and external fertilization.

  • Internal Fertilization: If the fusion of gametes takes place within the female parent it is called internal fertilization. In humans and most animals like cats, pigs, dogs, etc the fusion of gametes takes place internally. In this type, the zygote is formed within the mother and gets its nourishment from her. 
  • External Fertilization: When the fusion of sperm and egg takes place outside the body of the female parent, it is called external fertilization. Only very few organisms exhibit this type of gamete fusion. For eg: fish, frogs, etc. Since the fusion of gametes takes place in the external environment, the zygote faces many challenges for its survival. It has to obtain nourishment from outside and must also find ways to prevent environmental challenges.

Process of Fertilization

Let’s discuss the process of fertilization in further detail. 

For fertilization to occur the two gametes must be brought together. This is achieved by the process of copulation. Semen from the male’s penis is released into the female’s vagina. This process is called Insemination. Sperms have a longer lifetime in the male’s body. However, they can survive for only 48-72 hours in a female’s body. Capacitation of sperm begins in the female genital tract. It is the process of removal of all protective layers of the sperm and making only the spermatozoa available for fertilization with the female gamete. Capacitation includes the following processes:

  • Removal and weakening of the cholesterol membrane present over sperm’s acrosome begin.
  • Dilution of factors that help in the decapacitation process occurs. Decapacitation takes place in the male reproductive tract and is the process of the buildup of sperm.
  • Calcium ions enter the sperm. This activates it to move up vigorously to the ampulla-isthmus junction of the fallopian tube. The ovum in the secondary oocyte stage reaches there simultaneously. Both gametes must be transferred to the oviduct simultaneously for successful fertilization to occur. Hence all copulations don’t result in pregnancy.
  • Many sperms adhere to the surface of the ovum. The acrosome of the sperm releases hydrolytic enzymes called sperm lysins which digest the coverings of the ovum (Acrosomal reactions). These enzymes are as follows:
    • Hyaluronidase: It digests hyaluronic acid which cements the follicle or granulosa cells.
    • Corona Penetrating Enzyme (CPE): It dissolves the corona radiata.
    • Zona lysin/Acrosin: It digests the zona pellucida.
  • As the sperm’s acrosome makes contact with the oocyte, an outgrowth called the fertilization cone/ cone of reception starts to develop. 
  • When the sperm head touches the fertilization cone, it causes sodium ion channels on the ovum membrane to open. This causes depolarization of the ovum membrane. This constitutes the fast block that prevents many sperms to fuse with a single ovum. Calcium ions enter the ovum.
  • The envelopes of sperm and ovum get digested and now the whole sperm enters into the cytoplasm of the ovum. 
  • The entry of calcium ions into the ovum causes the extrusion of the cortical granules. This is called the cortical reaction. 
  • Calcium ions also make the ovum impermeable to the entry of a second sperm. This is called the Zona reaction.
  • Both the cortical and zona reactions constitute a slow block that checks polyspermy.
  • As the sperm enters the ovum, it initiates the completion of the meiotic division of the secondary oocyte which leads to the formation of a second polar body and an ovum. Sperm’s entry breaks down MPF (Metaphase Promotion Factor) and activates the APC (Anaphase Promotion Complex).
  • Now the sperm fuses with the ovum and fertilization take place which results in the formation of a diploid zygote

Significance of Fertilization

  • To maintain the diploid number of chromosomes fertilization is necessary. Meiosis takes place and produces haploid gametes. These gametes fuse together to restore the original diploid nature by the process of fertilization.
  • When two different gametes fuse new combinations are created. Variations are essential for the process of evolution.
  • During fertilization, the sex of the baby is determined.
  • Fertilization works as a stimulus provider for the ovum to complete its maturation.
  • Fertilization introduces centrioles that were missing in the ovum.

FAQs on Fertilization

Question 1: Where does the capacitation of sperm occur?

Answer:

In the Female genital tract, capacitation of sperm occurs.

Question 2: Fertilizin is a chemical substance produced from?

Answer:

Fertilizin is a chemical substance produced from Mature eggs.

Question 3: When does the extrusion of the second polar body from the egg nucleus occur?

Answer:

After entry of sperm before completion of fertilization.

Question 4: A reaction of granules content that harden the zona pellucida and ensures sure block to polyspermy is?

Answer:

Cortical Reaction is a reaction of granules content which harden the zona pellucida and ensures sure block to polyspermy.

Question 5: In oogenesis, the haploid egg is fertilized by sperm at which stage?

Answer:

At the Secondary oocyte, the haploid egg is fertilized by sperm.

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Last Updated : 17 Oct, 2022
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