Overview of Fertilisation - Fertilisation and Implantation in Humans
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:
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.
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
Fish (e.g., Salmon)
Frogs
Sea Urchins
Starfish
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
Humans
Dogs
Cats
Birds
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
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.
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.
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.
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.
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).
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.
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)
Where does fertilisation in humans usually occur?
A. Uterus
B. Cervix
C. Fallopian Tube
D. Ovary
Answer: C. Fallopian Tube
Which ion plays a key role in the acrosomal reaction?
A. Sodium
B. Calcium
C. Potassium
D. Magnesium
Answer: B. Calcium
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
Which parent determines the sex of the child in humans?
A. Mother
B. Father
C. Both equally
D. Grandparents
Answer: B. Father
What is the outcome of fertilisation?
A. Zygote formation
B. Follicle formation
C. Gamete division
D. Menstruation
Answer: A. Zygote formation
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FAQs on Fertilisation and Implantation in Humans: Process, Types, and Examples
1. What is fertilisation in humans?
In humans, fertilisation is the natural process where a male gamete (sperm) fuses with a female gamete (ovum or egg) to form a single diploid cell called a zygote. This marks the very beginning of a new human life.
2. Where does fertilisation usually take place in the female reproductive system?
Fertilisation most commonly occurs in the ampulla, which is a specific, wider section of the fallopian tube. The egg waits here after ovulation, and sperm travels to this site to meet it.
3. What are the main steps in the process of human fertilisation?
The process of fertilisation involves several key steps:
- Sperm Penetration: The sperm must first pass through the outer layers of the egg, the corona radiata and the zona pellucida.
- Acrosome Reaction: The sperm releases enzymes to dissolve a path through these layers.
- Membrane Fusion: The plasma membranes of the sperm and egg fuse, allowing the sperm's nucleus to enter the egg.
- Cortical Reaction: Once one sperm enters, the egg's membrane changes to prevent any other sperm from fertilising it.
- Formation of Zygote: The genetic material from the sperm and egg combine to form a complete set of DNA in the new zygote.
4. What is implantation and when does it happen?
Implantation is the process where the developing embryo, now called a blastocyst, attaches itself to the inner lining of the uterus, known as the endometrium. This typically happens about 6 to 10 days after fertilisation.
5. What is the main difference between fertilisation and implantation?
The key difference lies in the event and location. Fertilisation is the fusion of the sperm and egg, which happens in the fallopian tube. Implantation is the attachment of the resulting embryo to the uterine wall, which happens in the uterus about a week later.
6. Why is it so important that only one sperm fertilises the egg?
It's crucial for only one sperm to fertilise the egg to ensure the resulting zygote has the correct number of chromosomes (46 in humans). If more than one sperm entered (a condition called polyspermy), the embryo would have extra genetic material and would not be able to develop properly.
7. How does the fertilised egg get from the fallopian tube to the uterus?
After fertilisation, the new zygote begins to divide as it travels down the fallopian tube. This journey is helped by two things: the gentle sweeping motion of tiny hair-like structures called cilia on the tube's lining and mild muscular contractions of the tube's wall, which together push the embryo towards the uterus.
8. How does the uterus prepare itself for implantation?
After ovulation, the hormone progesterone signals the uterine lining (endometrium) to become thick, spongy, and rich with blood vessels. This creates a welcoming and nourishing environment, perfectly prepared for the blastocyst to attach and begin growing.
9. What happens if fertilisation doesn't occur after ovulation?
If the egg is not fertilised by a sperm within about 24 hours of ovulation, it starts to disintegrate. Without the hormonal signals that a pregnancy would produce, the thickened uterine lining is no longer needed and is shed during the next menstrual period.
10. What if the fertilised egg implants somewhere other than the uterus?
If the embryo implants outside the main cavity of the uterus, it is called an ectopic pregnancy. Most commonly, this happens in a fallopian tube. This is a serious medical condition because the tube cannot stretch like the uterus and may rupture, and the pregnancy cannot continue to term.
