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Fertilisation and Implantation: Events and Mechanism of Fertilisation, Cleavage of Zygote, Implantation, Practice Problems and FAQs

Fertilisation and Implantation: Events and Mechanism of Fertilisation, Cleavage of Zygote, Implantation, Practice Problems and FAQs

We all like babies, their smiles will make everyone happy. We all know that through our parents we came to this world. But do you know how a human female gets pregnant? Yes, you are correct. We have studied in lower classes that it is possible through a sexual intercourse between the male and femle partners. 



                                        Fig: Pregnancy

The human male transfers the sperm in the vagina of the female by a process called insemination. This happens in the sexual intercourse. So this is the first step towards pregnancy. The sperms then travel through the reproductive tract towards the egg. Once the sperm meets the egg it fuses with it. The fusion of male (n) and female (n) gametes is considered as fertilisation. If fertilisation occurs properly, then we can say that there is a chance that the female can become pregnant or the female may conceive the child. Why is it so? Because there are many other supporting factors also involved to achieve pregnancy. So in this article, let’s understand the processes related to pregnancy like fertilisation and implantation in detail.

Table of contents:

  • Fertilisation
  • Events of fertilisation
  • Mechanism of fertilisation
  • Significance of fertilisation
  • Specificity of fertilisation
  • Cleavage of zygote
  • Implantation
  • Practice Problems
  • FAQs

Fertilisation

Fertilisation is described as the fusion of male (n) and female (n) gametes to form a diploid cell (2n) called zygote. Gametogenesis is the process through which haploid male and female gametes are produced. During fertilisation, offspring receive 50% of genetic material from each parent. Therefore, offspring resemble their parents.



                                                                 Fig: Fertilisation

Events of fertilisation

There are three events involved in fertilisation as follows:

  • Journey of ovum
  • Journey of sperm
  • Fusion of sperm and ovum

Journey of ovum

The secondary oocyte released from the ovary is carried towards the fallopian tube. The finger-like structures of the fallopian tube called fimbriae collect the ovum and push it to the fallopian tube. The fimbriae moves closer to the ovary at the time of ovulation. 

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Inside the fallopian tube, ciliated cells are present that transports the secondary oocyte towards the uterus. 

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The secondary oocyte reaches the ampulla region where it awaits for the sperm for fusion.This area is known as the site of fertilisation.

Journey of sperm

Just like ovum, sperm also travel to meet the ovum. This happens during sexual intercourse or copulation. It is an act in which male and female come together to perform a sexual activity. The act of copulation occurs in four steps:

Erection

Erection is the first step of copulation. During this, penis gets erect and stiffened. This occurs due to the rush of blood into sinuses. After penis gets erect, it is inserted into the female vagina.



                                                                 Fig: Erection

Stimulation

Stimulation of vagina occurs when penis is inserted in it. Orgasm is a sensation that happens when sexual excitation reaches its climax. This is necessary for ejaculation to take place. The release of lubricating fluid from the penis and vagina is aided by stimulation. 



                                                             Fig: Stimulation

Ejaculation

After stimulation, ejaculation of sperm happens. Ejaculation is the discharge of sperm by the penis in a wave-like contraction. A single ejaculate normally contains 3 - 4 ml of semen which possess 200 to 300 million sperms. 



                                   Fig: Ejaculation

Insemination

Millions of sperm are deposited in the vaginal canal during insemination. These sperm swim quickly through the vaginal canal. They enter the cervical canal after passing through the cervix. In the acidic environment of the vaginal canal, some of these sperm will die. But many will survive, because of the protective components found in the fluids that surround the sperm. The sperm then must swim through the cervical canal, towards the uterus, and finally to the fallopian tubes. The sperm die as a result of the mucus in the uterus, reducing the sperm count even more. The contractions of the uterus help the sperm travel further in the reproductive tract. However, sperm undergoes some changes before reaching the fallopian tube. Leucocytes present in the vaginal epithelium engulf millions of sperms, therefore only a few thousand sperms find their way to reach the fallopian tubes. 



                                                             Fig: Insemination

Capacitation

It is a type of sperm modification during which the sperm ability increases to penetrate and fertilise the ovum. It is commonly referred to as the process of sperm activation. The coating substances deposited on the surface of the sperm particularly present on the acrosome is worn away during capacitation by the fluid present inside the uterus. The receptor sites on the acrosome are exposed and sperm become active to penetrate the egg.



                                                           Fig: Capacitation

The sperms that reach the uterus have higher probability of fertilisation. Sperms continue their journey from here. Few sperms migrate towards the left fallopian tube after entering the uterus, whereas others move towards the right fallopian tube. 



                          Fig: Movement of sperms from uterus to fallopian tube

Only a few hundred sperm are able to reach the fallopian tube. This is due to the acidic environment of the female reproductive tract and mucus. Sperm swim further to reach the ampulla where the ovum awaits for fertilisation.



                                           Fig: Movement of sperm to ampulla

Fusion of sperm and ovum

The fusion of gametes occurs in the ampulla region of the fallopian tube. Fertilisation occurs within 12 to 24 hours post ovulation. This is because an oocyte is viable only about 24 - 48 hours after ovulation. Similarly, sperm is viable for about 72 hours after insemination in the vagina.

Fertile period

Fertile period is the time period during which there is a higher chance to get pregnant if intercourse takes place. Fertile period is a three days window i.e., from two days before ovulation to one day after ovulation.

Mechanism of fertilisation

The mechanism of fertilisation occur in three steps as follows:

  • Penetration of oocyte
  • Activation of oocyte
  • Amphimixis

Penetration of oocyte

A single and first spermatozoon has the ability to fertilise and activate the oocyte. That spermatozoon must have an intact acrosome. The spermatozoon binds with the sperm receptors which are ZP3 molecules (Zona pellucida sperm-binding protein 3) present on the zona pellucida layer. Zona pellucida is a thick layer that surrounds the oocyte. The binding of spermatozoon with receptors ruptures the acrosome because this forms the pores in the acrosome. This process is called acrosome reaction. Optimum pH, temperature, calcium (Ca2+) and magnesium (Mg2+) ion concentration are essential for fertilisation. 

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Chemical events during fertilisation

A sperm cell must penetrate the different layers of oocyte for the process of fertilisation. These layers include granulosa cells, corona radiata, and zona pellucida. Zona pellucida is a clear glycoprotein layer present between the corona radiata and oocyte’s plasma membrane. Sperm can pierce the corona radiata cells and come into touch with the zona pellucida because of the presence of acrosomal enzymes and vigorous tail motions.

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Enzymes of acrosome and its action

The following are the major enzymes called sperm lysins present in the acrosome that assist in fertilisation:

Hyaluronidases

They act on the ground substances of follicle cells.

Corona penetrating enzymes

They dissolve corona radiata.

Zona lysines or acrosins

They help to digest the zona pellucida. 

These enzymes digest a pathway through the outer layers of the oocyte's surface. The sperm and oocyte membranes begin to merge when they come into contact with that surface. Oocyte activation is triggered by this phase.

Oocyte activation

Oocyte activation involves the series of changes that occur in the metabolic activity of the oocyte. The oocyte activation can occur in three steps. These are listed below:

  • Formation of reception cone
  • Metabolic activation
  • Prevention of polyspermy

Formation of reception cone

The oocyte's plasma membrane, or oolemma, raises slightly to form a reception cone or fertilisation cone for arriving sperm. It receives the sperm. The distal centriole present in the sperm divides and forms two centrioles for generating the mitotic spindle for cell division. In the mammalian secondary oocyte centriole is absent. 



                                               Fig: Formation of reception cone

Metabolic activation

It involves the activation of enzymes, which results in a rapid increase in the metabolic rate of the cell. A huge proportion of mRNA strands have been inactivated by specific proteins in the cytoplasm. 

Prevention of polyspermy

Polyspermy is the entry of more than one sperm into an oocyte. After the entry of one spermatozoon, the entry of another spermatozoon is blocked. This can be achieved by two mechanisms. These are fast blocks to polyspermy and slow blocks to polyspermy. Polyspermy can be blocked by the depolarisation of the oocyte membrane due to an increased permeability to sodium ions. Exocytosis of secretory vesicles or cortical granules  from the oocyte is stimulated by depolarisation. This depolarisation ensures monospermy (entry of only one sperm into the egg). Depolarisation also causes release of intracellular calcium ions.

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Cortical granules fuse with the plasma membrane of the oocyte. This causes release of their enzymatic content between the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida and inactivate the sperm receptors which help in the prevention of polyspermy. This exocytosis of vesicles and the chemical reactions is called cortical reaction. 

Amphimixis

When the sperm cell penetrates the secondary oocyte, it first completes its meiosis II. That means the oocyte which was arrested in metaphase II completes its meiosis. The secondary oocyte divides into two structures: one is a larger ovum and the other is a small and second polar body. The second polar body fragments and disintegrates. After completion of meiosis, the nuclear material remaining in the ovum reorganises and this is called female pronucleus. Simultaneously, the nucleus of the spermatozoan swells and forms a male pronucleus. The rest of the sperm cell breaks down. 

Spindle fibres form as the male pronucleus migrates toward the cell's centre. Amphimixis is a process during which the two pronuclei fuse and produce a single diploid nucleus (2n). This diploid nucleus contains 46 chromosomes and is known as the fertilised ovum or zygote. This is the ‘moment of conception’.

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Significance of fertilisation

The significance of fertilisation are as given below:

  • It restores the normal diploid condition of human beings.
  • Sperm entrance causes the secondary oocyte to complete its meiosis, which had been stopped at metaphase II.
  • It results in formation of diploid zygote.
  • It initiates cleavage. 
  • It helps in combining the characters of two parents and leads to variations.
  • It introduces the centrioles to mature ovum. 

Specificity of fertilisation

The adhesion of sperm to the egg of the same species occurs through agglutination. Ovum secretes a chemical called fertilizin which is a glycoprotein. The sperm possesses some proteins on its surface called antifertilizin which is composed of amino acids. The fertilizin of an egg is able to interact with the antifertilizin of sperm belonging to the same species. 

Cleavage of zygote

Soon after fertilisation, zygote undergoes rapid mitotic cell divisions. This is called cleavage. Each daughter cell formed after cleavage is known as blastomere. These cell divisions which are called cleavage are quick. The growth phase of daughter cells is very short and therefore , their size does not increase. The nuclear to cytoplasmic ratio increases. 

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The zygote's initial division begins about 24 hours after fertilisation and ends about 6 hours later. After first division, each and every successive cell division takes slightly less time. The second cleavage is complete by the second day following fertilisation, and there are four cells. The cleavage forms 16 cells by the end of the third day. 

Morula

The morula (mulberry) is a solid sphere of cells with no central cavity formed by successive cleavages. The zona pellucida still surrounds the morula, which is about the same size as the original zygote.



              Fig: Morula

Blastocyst formation

The dividing zygote travels to the uterus in four days. Morula floats in the uterine lumen for 3 days before entering the phase of compaction. The glands of endometrium of the uterus secrete a glycogen-rich fluid. This fluid enters the morula through zona pellucida when it enters the uterine cavity. This glycogen-rich fluid is known as uterine milk. It gets stored in the cytoplasm of the blastomeres of the morula at the 32-celled stage along with the other nutrients. This provides nourishment to the developing morula. Blastomeres reconfigure themselves around the blastocyst cavity which is a huge fluid-filled chamber and also known as blastocoel. The blastocyst has hundreds of cells but the size of the blastocyst is the same as that of the original zygote. 

Cells of the blastocyst

The cells of blastocyst are divided into two types such as the embryoblast and the trophoblast. 

Embryoblast

The embryoblast is also known as the inner cell mass. It is situated internally and eventually develops into the embryo. It is normally seen attached to the embryonic or animal pole. The opposite side of the blastocyst is called the abembryonic pole. 

Trophoblast or trophectoderm

The layer of trophoblast cells is the outer superficial layer. It forms a sphere-like wall surrounding the blastocyst. The cells present here help to provide nutrition to the embryo. They later form the extra embryonic membranes like chorion and amnion. The cells of the trophoblast that are in contact with the inner cell mass are referred to as cells of Rauber.



                                                         Fig: Blastocyst

Implantation

The endometrium layer of the uterus becomes thick to support the implantation. The uterine cells divide quickly once the blastocyst is attached and cover the blastocyst. As a result, the blastocyst embeds itself in the uterine endometrium. This completes the procedure of implantation. The blastocyst gets nutrition and grows into a foetus.



                                Fig: Implantation of blastocyst

Practice Problems

Q 1. Identify the correct statement regarding the cleavage of the fertilised egg in humans.

a. Cleavage starts in the uterus
b. Cleavage starts when fertilised egg is in the fallopian tube
c. Cleavage results in the formation of embryo with 8 - 16 blastomeres
d. Cleavage starts in the cervix

Answer: After 24 hours of fertilisation, rapid mitotic cell divisions start which is known as cleavage. It starts when the fertilised egg or zygote is in the fallopian tube. These cell divisions are quick and do not increase the size of the daughter cells. By the end of the third day, post fertilisation, it results in a cluster of 16 cells. Hence, the correct option is b.

Q 2. From the given options, identify the reaction that ensures prevention of polyspermy.

a. Acrosomal reaction
b. Implantation
c. Cortical reaction
d. All of the above

Answer: Polyspermy is the entry of more than one sperm into an oocyte. After the entry of one spermatozoon, the entry of another spermatozoon is blocked. Cortical reaction is described as the exocytosis of vesicles with cortical granules outside the oocyte membrane. Cortical granules fuse with the plasma membrane of the oocyte. This causes release of their enzymatic content between the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida and inactivate the sperm receptors which help in the prevention of polyspermy. Hence, the correct option is c.

Q 3. Which term is used for the preparation of sperm prior to the penetration of the ovum?

a. Spermiation
b. Spermiogenesis
c. Capacitation
d. Amphimixis

Answer: It is a type of sperm modification during which the sperm ability increases to penetrate and fertilise the ovum. It is commonly referred to as the process of sperm activation. The coating substances deposited on the surface of the sperm particularly present on the acrosome is worn away during capacitation by the fluid inside the uterus. The receptor sites present on the acrosome are exposed, so that the sperm become active to penetrate the egg.. Hence, the correct option is c.

Q 4. Identify the correct sequence of events that occur inside the human female reproductive system.

a. Ovulation → Fertilisation → Zygote formation → Blastocyst
b. Fertilisation → Ovulation → Zygote formation → Blastocyst
c. Ovulation → Fertilisation → Blastocyst → Zygote formation
d. Ovulation → Zygote formation → Fertilisation → Blastocyst

Answer: The rupture of the mature Graafian follicle and release of secondary oocyte is caused by rapid production of LH from the anterior pituitary. This is referred to as ovulation. Fertilisation is described as the fusion of male and female gamete and that will produce a diploid cell called zygote. Gametogenesis is the process through which haploid male and female gametes are produced.. Zygote undergoes rapid mitotic cell divisions. This is called cleavage. Each daughter cell formed after cleavage is known as blastomere. The embryo having 8 - 16 blastomeres is known as morula which transforms into blastocyst. The blastocyst gets implanted in the uterus. Hence the correct option is a. 

FAQs

Q 1. How does conception occur?
Answer: Conception occurs when the two pronuclei fuse and produce a single diploid nucleus. The fusion of pronuclei is known as amphimixis. This diploid nucleus contains 46 chromosomes and is known as the fertilised ovum or zygote.

Q 2. Why does sperm need a capacitation?
Answer: Capacitation is a type of sperm modification during which the sperm ability increases to penetrate and fertilise ovum. The coating substances deposited on the surface of the sperm particularly present on the acrosome is worn away during capacitation by the fluid inside the uterus. The receptor sites present normally on the acrosome are exposed, so that the sperm become active to penetrate the egg 

Q 3. Which side of the blastocyst embed in the endometrium?
Answer: The blastocyst embed in the endometrium from the side of the inner cell mass. The uterine cells divide quickly once the blastocyst is attached and cover the blastocyst. This process is known as implantation.

Q 4. What happens when more than one sperm penetrates the ovum?
Answer: If more than one sperm enters the egg cell, it will lead to a condition called polyspermy. Due to this, multipolar spindles are formed. As a result, there is a faulty separation of chromosomes during the process of cell division and form a non-diploid cell. This will stop the process of development. 

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Related Topics

Gametogenesis: Oogenesis, Practice Problems and FAQs 

Pregnancy, Embryonic development, Placenta, Parturition, Lactation, Practice Problems and FAQs 

The female reproductive system: Ovaries, Fallopian tubes, Uterus, Vagina, Practice Problems and FAQs 

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