Fertilizations And Implantation

Fertilization and implantation are the 2 important events in human reproduction, which is the biological process of producing new individuals from a union of male and female gametes. This complex process involves the fusion of gametes, the development of a zygote, and the growth and differentiation of the embryo, which ultimately leads to the formation of a fetus within the uterus. 

Fertilization in Humans

Human fertilization is a complex natural life process that involves the fusion of male and female gametes to form a zygote, which develops into an embryo. It is a crucial step in human reproduction, and its success determines the genetic makeup of the offspring.

The male gametes are produced inside the testes and are released into the female reproductive tract during sexual intercourse. The female gamete, or the secondary oocyte, is produced in the ovaries and released by the mature Graafian follicle into the fallopian tubes. The sperm cell must swim through the cervix and uterus to reach the fallopian tubes (More specifically, the fertilization talked place at the ampullary isthmic junction), where it meets the female gamete, which ultimately results in the formation of a zygote. 

The process of fertilization takes place in a defined time frame since the viability of both the gametes is very less. Inside the female reproductive tract, sperm can remain alive only for 24-72 hours, whereas after releasing it into the fallopian tube, a secondary oocyte can remain alive for 24 hours. 

Steps in Fertilization in Humans

The process of Fertilization can be broadly categorized into the following major processes.

Acrosomal Reaction

After being released into the female reproductive tract, the sperm undergoes various modifications, one of that is capacitation (several modifications occur at the sperm plasma membrane, which enables it to fuse with the female gamete.). Followed by the process of Capacitation, the Acrosomal reaction takes place when the sperm binds to the plasma membrane of the secondary oocyte. The head of the sperm contains enzymes that help it penetrate the outer layer of the egg cell, called the zona pellucida. Once the sperm has entered the egg cell, the zona pellucida undergoes a chemical change that prevents other sperm from entering and hence prevents the Polyspermy. Furthermore, Ca²⁺ ions play a crucial role in the formation of the acrosome; other than this, pH, temperature, and Mg²⁺ also play a key role in the accomplishment of the process.  

Cortical Reaction

It is one of the preventive measures that oocyte takes to prevent polyspermy. Once the fusion of the plasma membrane of the egg and sperm, the cortical reaction occurs on an immediate basis.  Cortical granules are modified forms of egg Golgi apparatus which contain several proteases and are stored between the egg plasma membrane and zona pellucida. Once after the fusion of the plasma membrane of the egg and sperm, the cortical granules get released inside the space, which results in the hardening of zona pellucida, which ultimately prevents the entry of other sperms. 

Sperm Entry

The sperm enters the secondary oocyte through a projection known as the cone of reception, which is formed while the sperm contacts the secondary oocyte. 

Karyogamy

The entry of sperm results in the release of a second meiotic arrest of the secondary oocyte. This results in the formation of a haploid ovum and a secondary polar body (going to degenerate).  Karyogamy is the fusion of both male and female pronuclei. The head of the sperm contains the male pronuclei, which is the only part of the sperm that enters into the cytoplasm, and the rest of the parts get degenerated. This fusion takes place inside the ovum cytoplasm. 

Egg Activation

When once sperm invade the ovum, then sperm activates the metabolism i.e., Protein synthesis and other cellular respiration.

Implantation

Implantation is a process of attachment of fertilized egg to the uterine lining, also known as the endometrium, in response to the hormonal changes that help in the growth of the endometrium. After the successful event of fertilization, the zygote starts dividing mitotically and forms a 2-cell stage and proceeds to divide mitotically to finally form an 8-16-cell stage known as Morula. Furthermore, the morula grows to form a blastocyst. 

The blastomere in the blastocyst gets arranged to form an outer layer known as trophoblast and an inner layer called as inner cell mass. The trophoblast layer then attaches to the endometrium. The endometrium, in response to the several hormonal changes that take place during the menstrual cycle, grows and provides a nourishing platform for the blastocyte to get attached to the endometrium.  

Factors affecting the process of Implantation

Implantation is a very delicate process which gets affected by a variety of factors; i.e., hormonal imbalance is one of the main factors that make implantation more difficult. The increased level of progesterone may lead to the abandonment of implantation. Furthermore, the structural abnormalities in the uterus can make it more difficult for the blastocyst to implant. 

FAQs on Fertilization and Implantation

Question 1: What are the four major steps in the process of fertilization?

Answer:

The process of fertilization can be broadly categorised into following four major steps:

• Acrosomal reaction (Includes sperm activation also known as Capacitation, and fusion of male and female gametes plasma membrane.) 
• Cortical reaction 
• Sperm Entry 
• Karyogamy

Question 2: For how much sperm and secondary oocyte are viable inside the female reproductive tract?

Answer:

After getting released into the female reproductive tract the viability of sperm varies from 24 to 72 hours, whereas in the case of secondary oocyte they are viable for about 24 hours after being released by the mature Graafian follicle into the fallopian tube.

Question 3: What results in the release of a second meiotic arrest of secondary oocyte? 

Answer:

The sperm entry results in the release of second meiotic arrest and ultimately leads to the formation of an ovum and a secondary polar body. 

Question 4: What is Implantation?

Answer:

It is a step of fertilization in which the embryo implant in the endometrium of the vagina. This stage is known as implantation.