Fertilisation and Implantation in Humans: Process, Types, and Examples

Fertilisation is the process where male and female gametes unite to form a new individual. In humans, this union occurs inside the female reproductive system, primarily in the fallopian tube. Once the egg (ovum) and sperm fuse, their genetic material combines to form a single diploid cell called a zygote. This zygote then travels to the uterus, where it implants and begins its development.

Fertilisation in humans is an example of internal fertilisation. However, fertilisation in animals can occur in two main ways: external fertilisation and internal fertilisation. The types of fertilisation in animals vary depending on the species and their environment.

Fertilisation in Animals: External Vs Internal

Fertilisation in animals broadly falls under two categories:

1. External Fertilisation:

• Occurs outside the female’s body, often in water.

• Examples include many fish and amphibians releasing eggs and sperm into the environment for fertilisation.

2. Internal Fertilisation:

• Occurs inside the female’s body, providing a more protected environment for the developing zygote.

• Common in mammals, birds, and reptiles. Humans are a prime example.

These two types of fertilisation in animals help different species adapt to their surroundings. In aquatic animals, external fertilisation can be advantageous for dispersal, while land-based creatures often use internal fertilisation to protect offspring.

5 Examples of External Fertilisation Animals

1. Fish (e.g., Salmon)

2. Frogs

3. Sea Urchins

4. Starfish

5. Corals

All these rely on water for the fusion of gametes outside the body. Releasing eggs and sperm into the water environment can allow for greater numbers of offspring but also increases vulnerability to predators and environmental factors.

5 Examples of Internal Fertilisation Animals

1. Humans

2. Dogs

3. Cats

4. Birds

5. Reptiles (e.g., Snakes)

In these internal fertilisation animals, fertilisation happens inside the body of the female. This method typically results in fewer offspring but offers enhanced protection and nourishment to the developing embryo.

Process of Fertilisation in Humans

1. Release of Gametes

• During coitus, the male releases semen, which contains millions of sperm, into the female vagina.

• Meanwhile, an egg is released from one of the ovaries once every menstrual cycle.

2. Sperm Journey

• Sperms travel through the cervix and uterus to reach the fallopian tube.

• Only a few sperm reach the egg, as many are lost on the way.

3. Fusion of Egg and Sperm

• The egg released from the ovary is viable for about 12–24 hours.

• Sperm can remain alive in the female reproductive tract for about 48–72 hours.

• If a sperm meets the egg during this time, internal fertilisation takes place in the fallopian tube.

4. Acrosomal Reaction

• The head of the sperm has an acrosome containing enzymes.

• These enzymes help the sperm penetrate the outer layers of the egg.

• Calcium ions play an important role in triggering these reactions.

5. Cortical Reaction

• Once the first sperm successfully fuses with the egg’s membrane, the egg’s outer layer changes.

• This prevents other sperms from entering (prevents polyspermy).

6. Completion of Meiosis in the Egg

• The secondary oocyte completes its second meiotic division upon sperm entry, forming the mature ovum and a second polar body.

• The sperm’s head (male pronucleus) fuses with the egg’s nucleus (female pronucleus). This fusion is known as karyogamy.

7. Zygote Formation

• When the male and female pronuclei fuse, a diploid zygote is formed.

• This zygote starts undergoing cell divisions (mitotic divisions), marking the beginning of embryonic development.

Implantation in Humans

• After fertilisation, the zygote undergoes multiple divisions as it travels down the fallopian tube.

• By the time it reaches the uterus (around 3–4 days later), it has formed a cluster of cells called an embryo (specifically a blastocyst).

• This embryo then attaches to the endometrial lining of the uterus, a process known as implantation.

• Successful implantation is crucial for the continued growth and nourishment of the embryo throughout pregnancy.

Importance of Fertilisation

• Genetic Variation: Fertilisation mixes genetic material from two parents, increasing diversity.

• Restoration of Chromosome Number: The haploid chromosomes from both parents combine to form the diploid number required for the offspring.

• Initiation of Embryonic Development: The fusion of gametes triggers the first stages of a new life.

What If Fertilisation Does Not Occur?

• If fertilisation in animals (specifically in humans) does not happen within the viable window, the egg degenerates.

• In humans, the uterine lining is shed during menstruation when no implantation occurs.

• The cycle then repeats in the next menstrual cycle.

Additional Insight: Sex Determination in Humans

• In humans, the father’s sperm determines the sex of the child.

• If the sperm carries an X chromosome, the resulting child is genetically female (XX).

• If it carries a Y chromosome, the resulting child is genetically male (XY).

Quick Quiz (With Answers)

1. Where does fertilisation in humans usually occur?
   A. Uterus
   B. Cervix
   C. Fallopian Tube
   D. Ovary
   Answer: C. Fallopian Tube

2. Which ion plays a key role in the acrosomal reaction?
   A. Sodium
   B. Calcium
   C. Potassium
   D. Magnesium
   Answer: B. Calcium

3. What happens to the uterine lining if fertilisation does not occur?
   A. It remains thick
   B. It is shed through menstruation
   C. It forms multiple layers
   D. It calcifies
   Answer: B. It is shed through menstruation

4. Which parent determines the sex of the child in humans?
   A. Mother
   B. Father
   C. Both equally
   D. Grandparents
   Answer: B. Father

5. What is the outcome of fertilisation?
   A. Zygote formation
   B. Follicle formation
   C. Gamete division
   D. Menstruation
   Answer: A. Zygote formation

Related Topics You May Find Helpful

• Cell Division

• Sex Determination

• Human Reproductive System