Fertilisation is the process of reproduction that involves the combination of the male gamete and the female gamete, which results in the formation of a zygote. This process of fertilisation involves both mitosis and meiosis. Fertilisation is a process of sexual reproduction, which is very significant for the continuation of the generation of a particular species whether it be plants, animals, or humans.

This article will learn more about fertilisation types, fertilisation process in plants, animals and humans, and the process of implantation with diagram.

What is Fertilisation?

Definition: Fertilisation is defined as the fusion of two haploid gametes, i.e., male gametes with female gametes, to give rise to a new diploid individual organism or offspring and initiate its development. It is also known as generative fertilisation, syngamy or impregnation.

Types of Fertilisation

The process of fertilisation occurs differently in plants and animals.

In plants, mainly in angiosperms, fertilisation is a process of sexual reproduction which occurs after pollination. For plants, fertilisation can be defined as fusing the haploid male gamete in the pollen grains with the haploid female gamete (egg/ovum) to form a diploid zygote. Fertilisation is always internal in angiosperms.

In animals, fertilisation is of external fertilisation and internal fertilisation. Both of these processes are based on the site of syngamy. Fertilisation in humans occurs after insemination, where the sperm is released that gets fused with the ovum to form the diploid individual.

Fertilisation in Plants

In the fertilisation process, flowers play a significant role as they contain the male and female reproductive structures called stamen and pistil that produce the haploid male and female gamete. 

In angiosperms, fertilisation occurs after pollination, i.e., the transfer of pollen grains to the stigma of the same or the different flowers but the same species. This transfer of pollen can take place naturally or with the help of pollinating agents like honey bees, butterflies, birds, wind, water, etc.

Events Leading to Fertilisation in Angiosperms

The following events lead to the fertilisation in Angiosperms:

Development of Pollen Grains and Male Gametes

Each anther contains four pollen sacs in which the pollen grains are produced. The pollen grain contains the male gametes. In the process, the pollen grains have to get transferred to the stigma of the same plant species.

When the pollen matures, its nucleus divides into two nuclei. The smaller generative nucleus is the male nucleus, and the larger tube nucleus is the vegetative nucleus. In over 60% of angiosperms, the pollen grains are transferred to the stigma at this two-celled stage.

Development of an Embryo Sac

An ovary may contain one or many ovules. Each ovule contains an embryo sac mother cell, a large diploid cell. It undergoes meiotic division and gives rise to four haploid nuclei, out of which only one remains functional and the other three degenerate. The functional haploid nucleus grows and undergoes mitotic divisions to produce eight haploid nuclei. The structure formed is known as the embryo sac or female gametophyte.

An embryo sac contains seven-celled or eight-haploid nuclei, out of which three nuclei are present at the micropylar end called egg apparatus (\(1\) egg cell present at the middle and the other \(2\) cells present on either side of the egg are called synergids). The synergids have special thickenings at the micropylar tip called filiform apparatus. A group of three nuclei is present at the chalazal end of the ovule (present opposite to the micropylar end), known as antipodal cells. The remaining two haploid nuclei are present at the centre of the embryo sac known as polar nuclei, which ultimately fuse at fertilisation to form a single diploid nucleus.

Study About Sexual Reproduction

Pollen Germination

After the pollen grains land on the stigma, the events that lead to pollen germination are as follows:

1. If stigma is receptive and the pollen lands on its compatible species, then it begins to germinate.
2. The intine (inner integument of a pollen grain), along with its contents, emerges from the germ pore as a short cytoplasmic outgrowth called germ tube and grew in the form of a pollen tube.
3. The pollen tube is directed towards the ovary by chemicals, calcium-boron-inositol sugar complex, and, hence is called positively chemotropic and negatively aerotropic (grows away from the air).
4. The sucrose solution secreted by the epidermal cells of the stigma acts as the stimulus for the formation of the pollen tube and provides nutrition.
5. The pollen tube produces certain enzymes that digest the tissues of the stigma and style and enables the passage of the pollen tube to the ovule.
6. The pollen tube has two nuclei – the vegetative or tube nucleus and the generative nucleus.
7. The tube nucleus, which is present at the tip of the pollen tube, helps direct the pollen tube’s growth and ultimately disintegrates.
8. The generative nucleus divides mitotically to produce two male nuclei called male gametes.
9. The pollen tube grows and enters the ovule through a small opening called the micropyle.
10. The pollen tube enters into one of the synergids through the filiform apparatus that guides the entry of the pollen tube.

Double Fertilisation in Angiosperms

1. After entering the embryo sac, one of the synergids disintegrates.
2. The pollen tube releases the two male gametes into the cytoplasm of the synergids.
3. First Fertilisation: One of the male gametes \(\left({\text{n}}\right)\) fuses with the egg nucleus \(\left({\text{n}}\right)\) to form the diploid zygote \(\left({{\text{2n}}} \right)\). This process is the first fertilisation process or also known as syngamy.
4. Second Fertilisation: The other male gamete moves to the central cell where it fuses with the two polar nuclei or the secondary diploid nucleus (formed by the fusion of \(2\) polar nuclei) and forms a triploid nucleus called the primary endosperm nucleus (PEN). This process is known as the second fertilisation process or triple fusion.
5. Since two sets of fertilisation occur, that is syngamy and triple fusion; it is called double fertilisation. It is unique to flowering plants.
6. It was discovered by the Russian scientist S.G. Nawaschin in \(1898\).

Events after Double Fertilisation

1. The diploid zygote \(\left({{\text{2n}}} \right)\) divides mitotically to form the embryo of the seed.
2. The triploid nucleus \(\left({{\text{3n}}} \right)\) or primary endosperm nucleus (PEN) gives rise to a mass of tissue that develops into the endosperm of the seed. The Endosperm provides nourishment to the growing embryo.

Floral Parts After Fertilisation

After the fertilisation process is complete, the fate of sepals, petals, stamens and carpels are as follows:

1. The petals, stamens, style, and stigma wither and generally fall off.
2. The calyx may either fall off or may remain intact in a dried and shrivelled form, as in the case of tomato, guava, etc.
3. The ovary enlarges to form fruit, and the ovary wall forms the fruit wall. The ovary wall may either form a dry and hard fruit wall or a fleshy fruit wall.
4. Ovules form the seeds.

Fertilisation in Animals

Fertilisation in most animals is similar to that in humans. Animals produce haploid gametes, which fuse to restore the normal diploid number of chromosomes. This process is also known as syngamy, which occurs after the insemination process in animals.

Syngamy or Fertilisation in Animals

Based on the site of syngamy or fertilisation, whether the fusion of gametes takes place inside or outside the body, fertilisation is of two types: external fertilisation and internal fertilisation.

External Fertilisation: It refers to the fusion of gametes outside the body of organisms. In most aquatic organisms like fish and amphibians like frogs, fertilisation occurs in water. There needs to be good synchronism to release male and female gametes in the surrounding water, enhancing fertilisation chances. Normally, both the gametes are released in large numbers, as the chances of survival of both the zygote and the offspring are dependent on the predators and other environmental conditions.

Internal Fertilisation: It refers to the fusion of male and female gametes inside the body of an organism. It occurs in all higher animals like reptiles, birds and mammals. The female gamete (egg) is formed inside the body of the female organism, where it fuses with the male gamete (sperm). The male gamete is motile so that it can reach the female gamete. In animals, male gametes are produced in large numbers while the eggs are produced in very few numbers.

Parthenogenesis

Parthenogenesis is a special type of sexual reproduction process in which the growth and development of embryos occur without fertilisation by sperm. Parthenogenesis occurs naturally in some plants, some invertebrate animal species like bees, wasps, ants, aphids, etc., and a few vertebrates such as fishes, amphibians, and reptiles very rarely in some birds.

In honey bees, female bees (workers and queens) develop by fertilisation and thus are diploid, while male bees (drones) develop directly from eggs without fertilisation.

Parthenogenesis has been induced artificially in a few species, including fishes and amphibians. In plants, parthenogenesis apomixis is where the seeds develop without fertilisation from unfertilised egg cells. Parthenogenetic offspring can be haploid or diploid depending on the cell of the female gamete from which it develops.

Fertilisation Process in Humans

In human beings, fertilisation is internal that occurs high up in the fallopian tube at the ampullary-isthmic junction. During insemination, millions of sperms (about \(200-400\) million) are deposited in the vagina but only one out of it fertilises the ovum.

The motile sperms swim rapidly, through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube. The ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilisation takes place.

Fertilisation can only occur if the ovum and sperm are transported simultaneously to the ampullary-isthmic junction.

Capacitation of Sperms

The sperms remain fertile for only about \(12-24\) hours in the female reproductive tract. Thus, within this period, sperm need to reach the Fallopian tube to facilitate fertilisation. The vagina triggers the motility of sperms and is aided further by muscular contractions of the walls of the uterus and oviducts. Viscous liquid secretions from the secretory cells of the oviduct mucosa further increase sperm motility.

A sperm undergoes a number of changes called capacitation, in order to fuse with the ovum to complete the fertilisation process. This involves the changes in the membrane covering the acrosome.

Physicochemical Events during Fertilisation

The fertilisation process involves three major steps:

1. Penetration of the sperm into the ovum.
2. Activation of the ovum.
3. Fusion of the sperm and egg nucleus.

Penetration of the sperm into the ovum: As the sperm reaches the ovum, it brings about the sequence of events described below:

1. Breaking of the membrane covering the acrosomal region of the sperm.
2. Release of chemical substances or enzymes called sperm lysins from the acrosome.
3. Sperm lysins contain many chemicals:
   1. Hyaluronidase – an enzyme that acts and dissolves the ground substance that binds follicle cells of the ovum together and form corona radiata.
   2. Corona penetrating enzymes – these enzymes dissolve the corona radiata region.
   3. Zona lysin or acrosin is an enzyme that helps digest the zona pellucida region.
4. As sperms contact zona pellucida, the acrosome turns inside out. It releases chemicals that dissolve the vitelline and the egg’s plasma membrane at the point of contact.
5. The sperm enters the egg cytoplasm.
6. Entry of more sperms is prevented by chemical changes on the egg’s surface.
7. Zona pellucida, along with the vitelline membrane, thickens and separates from the plasma membrane. It ensures fertilisation of the egg with only one sperm called monospermy.
8. Some physical and chemical changes take place in response to egg-sperm fusion.
9. First, the egg becomes activated, and depolarisation of its membrane occurs.
10. Second, the egg shows a cortical reaction. The enzyme-rich cortical granules below the egg membrane, fuse with the membrane and release their contents into the space between the egg and vitelline membrane (perivitelline space).
11. The hardened vitelline membrane, now called fertilisation membrane serves as a barrier for the entry of the other sperms.
12. This reaction makes the egg impervious to any second sperm and prevents polyspermy (i.e., entry of more than one sperm into the ovum).

1. Activation of the ovum: As the sperm enters the ovum (secondary oocyte), it gets activated and undergoes the second meiotic division. As a result, the secondary oocyte produces a haploid ovum (ootid) and a polar body. The polar body ultimately degenerates after fertilisation is completed.
2. Fusion of the sperm and the egg nucleus: The fusion of the sperm pronucleus and the egg pronucleus restores the diploid number of chromosomes through fertilisation or syngamy. The single diploid cell formed is called a zygote. It contains both the paternal and maternal chromosomes. It is the starting cell of the new multicellular individual.

Implantation

Immediately after fertilisation, the zygote undergoes the first mitotic division or the cleavage division which converts a large cytoplasmic mass into a number of small cells clustered together forming a morula.

In about \(4-5\) days, the morula takes the shape of the blastocyst. During the next \(2\) days, the zona pellucida disappears and at about \({{\text{7}}^{{\text{th}}}}{\text{-}}{{\text{8}}^{{\text{th}}}}\) day after ovulation, the blastocyst gets embedded in the endometrial lining of the uterus. This process is called implantation. As the embryo is implanted, the pregnancy is established.

Significance of Fertilisation in Organisms

The significance of fertilisation are as follows:

1. Fertilisation maintains the diploid number of chromosomes in the newly formed individual.
2. It is important for the continuation of the species.
3. It gives rise to genetic variations by mixing the characters from the two parents that leads to evolution of a new species.
4. It helps to maintain the biodiversity of organisms.
5. Fertilisation also helps determine the sex of the offspring.

Summary

After all, fertilisation is not a necessary process to maintain the life of an individual, unlike the essential life processes such as nutrition, respiration or excretion. On the other hand, a lot of energy will be spent by an organism in the fertilisation process. Then why should individuals waste energy? It is because if there were to be only one, non-reproducing member of a particular kind, it is doubtful that we would have noticed its existence. So, by the process of fertilisation, new individuals are created with few or more variations that look very much like themselves.

PRACTICE QUESTIONS RELATED TO FERTILIZATION

Frequently Asked Questions on Fertilisation

Q.1. What is the sole purpose of fertilisation?
Ans: The sole purpose of fertilisation is to restore the normal diploid number of chromosomes in the new individual by the combination of two haploid chromosome sets of two individuals.

Q.2. What are the four steps of fertilisation in humans?
Ans: The four steps of fertilisation in humans are as follows: i) Capacitation of sperms, ii) Penetration of sperm into the ovum, iii) Activation of ovum, and iv) Fusion of sperm and egg nucleus.

Q.3. What is triple fusion in plants?
Ans: Triple fusion is defined as the fusion of one haploid sperm nucleus \(\left({\text{n}}\right)\) with the two polar nuclei \(\left({{\text{2n}}} \right)\) present centrally in the embryo sac to produce the primary endosperm nucleus \(\left({{\text{3n}}} \right)\). This process involves the fusion of three haploid nuclei, so it is known as triple fusion.

Q.4. What is double fertilisation?
Ans: In flowering plants, fertilisation takes place twice. Firstly, one sperm nucleus \(\left({\text{n}}\right)\) fuses with the egg nucleus \(\left({\text{n}}\right)\) to form the diploid zygote \(\left({{\text{2n}}} \right)\). Secondly, the other sperm nucleus \(\left({\text{n}}\right)\) fuses with the two polar nuclei \(\left({{\text{2n}}} \right)\) present centrally in the embryo sac to produce the primary endosperm nucleus (PEN). So, this process is known as double fertilisation.

Q.5. What are the different types of fertilisation?
Ans: There are two different types of fertilisation – external fertilisation and internal fertilisation.

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