Process of fertilization – a journey of sperm to egg

Posted On June 29, 2021March 13, 2022

Introduction

Most people always have a curiosity to know the process of fertilization. They have a desire to understand the journey of sperm in the female reproductive system. 

I think you are also one who wants to know – what exactly happens after sexual intercourse and how does sperm interact with the ovum!

In my last post, I explained what is a female period.  In that post, I clarified all doubts regarding the menstrual period. It usually happens when a female does not fertilize with sperm. 

But here, we will see what happens if the egg is fertilized with sperm and how does this entire process of fertilization happens. 

Let’s take a closer look at this fascinating topic. 

What is fertilization in the human body?

When a female egg is successfully meet with male sperm during sexual intercourse, it may become fertilized. This process of fusion of male and female gamete is called fertilization. 

Male gamete (sperm) + Female gamete (ovum/egg)

What is the structure of sperm and ovum?

Generally, sperm are highly motile cells (moves fast), whereas ovum are less motile cells (moves slow) due to their structure. 

Structure of sperm

The standard length of human sperm is 0.055 mm (or 60 mu). Sperm comes with whitish fluid is called semen. 

Semen is usually rich in fructose. This fructose provides energy to sperm. Fructose is one of the components of seminal fluid. For example, if someone ejaculates 1 ml semen, it contains around 10% sperm and 90% different fluid. 

Semen (1ml) = 10% sperm + 90% different seminal fluid

The sperm is made up of three main parts: head, middle piece, and tail. 

• Head (flat and oval shape) – The outer portion of the sperm head contains an acrosome. It releases hyaluronidase (hydrolytic enzyme), which helps in penetration of the ovum membrane for fertilization. 

Inside, the head portion of sperm contains a nucleus. It has a single set of unpaired chromosomes (23 chromosomes) are called haploid (n). 

• Middle piece (between head and tail) – It is a powerhouse of your sperm. It contains a lot of mitochondria which provides energy for the movement of sperm. 

• Tail – It is longer in length, also called flagellum. This part helps in the movement of the entire sperm. 

Structure of ovum

Ovum is much larger (0.1 mm in diameter) in comparison to sperm. The mature ovum is covered with a plasma membrane made up of lipoprotein. 

The surrounding plasma membrane contains a layer of glycoprotein which is called zona pellucida. It acts as a gatekeeper for sperm. 

The outer portion of a zona pellucida is covered with corona radiata. It provides physical protection to the ovum. 

Ovum also contains a single set of unpaired chromosomes (23 chromosomes) called haploid (n). It is present in the nucleus. 

Where does fertilization occur in human females?

The most common site of fertilization in a human being is an ampullary region of the uterine tube or fallopian tube. The fallopian tube is a part that connects the ovary and uterus. 

When you do sex with your partner, the sperm goes to the female genital tract and store in the vagina. In one ejaculation (sticky liquid shoot out from the penis), 200 to 300 million sperms come out. 

These troop of sperms store in the vagina. Sperm motility is generally slow in the vagina due to the acidic environment of the vagina. 

But it becomes super-active as soon as it goes to the uterine cavity because the uterus has an alkaline or warm environment. 

Out of 300 million sperms, only 200-300 hundred sperm reach the site of fertilization.

The fallopian tube has three main parts – infundibulum (closest to the ovary), ampulla (middle segment), and isthmus (closest to the uterus).

The ampulla is the leading region where sperm meets the ovum and fertilization occurs. 

What is the process of fertilization in humans? How human fertilization occurs?

Once the sperm reaches the site of the isthmus. Sperm become less motile and stop their own migration. 

As ovulation occurs, the ovum release with the corona radiata layer. This corona radiata present surrounding the ovum, and it releases a special type of chemical – chemoattractant. 

This chemoattractant attracts the sperm, and it becomes again motile. Then sperm moves towards the ovum in the ampulla area.  

At this stage, the process of fertilization takes place. There are various steps involves in this process –

Step -1 Capacitation (preparation of the sperm)

The freshly ejaculated sperm are not able to fertilize an egg. Because normally, the head of sperm is fully embedded in glycoprotein and seminal plasma proteins.

Due to this, sperm can’t enter the ovum. So, there is a need to remove all this stuff during fertilization. 

When the sperm interacts with the fallopian tube’s mucus surface, it removes glycoprotein coat and seminal plasma proteins from the head of the sperm. 

The process of clean up the head of sperm is called capacitation. Only capacitated sperm can pass through the corona radiata of the ovum. 

Even in-vitro fertilization, some chemicals are used to remove the glycoprotein coat. 

Step -2 Acrosome reaction (penetration of zona pellucida)

Once a capacitated sperm has penetrated the layer of corona radiata, it starts to penetrate the zona pellucida membrane by binding on zona proteins like ZP3. 

As soon as sperm binds on ZP3, then sperm releases enzymes, including acrosin and trypsin-like substances. Thus, the acrosome reaction in sperm is triggered by acrosin and trypsin-like substances. 

All these enzymes work together to help in the digestion of the zona pellucida membrane. 

During this reaction, the acrosomal cap of sperm disappears. 

Step -3 Fast block to polyspermy (electrical barrier)

Polyspermy means if an egg is fertilized by more than one sperm. It very rarely happens in human beings. So, to prevent this polyspermy, the ovum makes an electrical barrier. 

As soon as sperm reaches the plasma membrane of the ovum, then sodium channels open, and an influx of sodium ions occurs. It makes a layer of sodium charge (Na⁺²) around the ovum wall, which acts as an electrical barrier. This electrical barrier prevents polyspermy. 

Step -4 Slow block to polyspermy (mechanical barrier)

As soon as sperm enters the cytoplasm of the ovum, then cortical granules move towards the surface of an ovum. These cortical granules contain lysosomal enzymes (also called hydrolytic enzymes).

Then cortical granules release lysosomes in the entire wall of the ovum and bring changes in the zona pellucida. 

Eventually, zona pellucida and the plasma membrane of the ovum become impenetrable to other sperms. It also inactivates the ZP3 protein and degenerates the tail of sperm.

This entire process is called cortical reaction or zona reaction. 

Step -5 Fusion of male and female gamete

This is the final step where the genetic material of sperm fuses with the genetic material of the ovum. 

After entry of nuclear material of sperm, ovum finishes its secondary meiotic division. As a result, the ovum produces some polar body and definitive ovum. 

This definitive ovum contains a single copy of a chromosome (22+X) and it fuses with the sperm chromosome (22+Y).

 one copy of female gamete (haploid) + one copy of male gamete (haploid)

These haploids fuse together and form a zygote.

Male haploid (n) + Female haploid (n)

↓

Diploid (2n)

↓

Zygote

Takeaway 

In this article, I have described the process of fertilization in human beings. Therefore, if you want to become pregnant, it is essential to understand the process of fertilization. 

Knowing about this entire process can help you get pregnant. 

In my next article, I will explain the development of the embryo. If you found this post informative, please share it on social media. 

Sources –

1. T. W. Sadler. Langman’s medical embryology. 9^th edition. First week of development: ovulation to implantation. Chapter-2. Page-31.

2. Christopher J. De Jonge and Christopher L. R. Barratt. The Sperm Cell. Cambridge university press, New York. First edition 2006. Regulation of capacitation. Chapter – 6. Page- 147.