Anatomy and physiology

Describe the process of fertilization in human beings: A Step-by-Step Guide

Process of fertilization Posted On
Posted By SUMIT SHARMA

Introduction

All life on this planet is because of sexual reproduction. It starts with the fertilization of a female egg by a male sperm. Today, we will describe the process of fertilization in human beings. 

Many people have a curiosity to know the process of fertilization. They have a desire to learn what exactly happens after sexual intercourse and how sperm interact with the ovum. 

Fertilization is basically an interaction between an ovum (egg) and a sperm. The egg comes from a female individual and the sperm comes from a male. When they meet or join together, this joining makes a new cell called a Zygote. It is a very first step in making a baby.

Understanding human fertilization is important for personal reproductive health, family planning, pregnancy prevention, infertility problems, etc. 

This post will describe the process of fertilization in human beings step by step. 

Let’s take a closer look at this process.

Structure of sperm 

In the human body, the sperm is the smallest cell. It is a highly motile cell, which moves fast due to its structure. 

The standard length of human sperm is 50 to 60 micrometers (µm) which is equivalent to 0.05 to 0.06 millimeters (mm). 

The sperm is a male reproductive cell or gamete, which is also called spermatozoon (pleural – spermatozoa). 

The sperm cells (or spermatozoa) always come 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 of semen that 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. 

describe the process of fertilization in human beings

  • Head part of sperm (flat and oval shape) 

The outer portion of the sperm head contains an acrosome. It releases hyaluronidase (hydrolytic enzyme), which helps in the penetration of the ovum membrane for fertilization. 

The head portion of sperm contains a nucleus. It has a single set of unpaired chromosomes (23 chromosomes) and is called haploid (n). 

 

  • Middle part of sperm (between head and tail) 

It is a powerhouse of your sperm. It contains a lot of mitochondria which provide energy for the movement of sperm. 

 

  • Tail part of the sperm

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

 

Structure of ovum

The ovum, also known as an egg cell, is the female gamete (sex cell) responsible for fertilization in sexual reproduction.

Unlike sperm, the ovum is a less motile cell. It moves slowly due to its structure. 

The ovum is the largest cell of the human body. The size of a mature ovum is approximately 0.1 mm (millimetres) in diameter.

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. 

describe the process of fertilization in human beings

Explain the process of fertilization in human beings

The journey of sperm through the female reproductive tract is a complex process, and many factors influence their ability to reach and fertilize the egg. 

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. 

 

Journey of sperm in the female reproductive tract

When you have sex with your partner, the sperm goes to the female genital tract, which stores in the vagina. 

It will be shocking to know that one ejaculation releases 200 to 300 million sperm. Here, ejaculation is a sticky liquid that shoots out from the penis. 

These troops of sperm are stored in the vagina. Sperm motility is generally slow in the vagina due to the acidic environment of the vagina. 

As soon as sperm goes to the fallopian tube, it becomes super-active because the uterus has an alkaline or warm environment. 

There are 300 million sperm during one ejaculation, but only 200-300 hundred 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).

describe the process of fertilization in human beings

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

Once the sperm reaches the site of the isthmus, sperm becomes less motile and stops their own migration. 

When ovulation occurs, the ovum is released with the corona radiata layer.

The corona radiata release a special type of chemical called a chemoattractant, which surrounds the ovum.

The chemoattractant attracts sperm, which becomes motile again.

Following that, sperm travels to the ovum in the ampulla area. This is the place where the process of fertilization begins. 

 

Steps of fertilization

There are various steps involved to describe the process of fertilization in human beings –

Step -1 Capacitation – Preparation of the sperm

Freshly ejaculated sperm are initially unable to fertilize eggs because their heads are typically embedded in glycoproteins and seminal plasma proteins. 

This prevents sperm from entering the ovum. Therefore, all of these materials must be removed during fertilization.

As sperm interacts with the mucus surface of the fallopian tube, it sheds the glycoprotein coat and seminal plasma proteins from its head. This process, known as capacitation, cleans up the sperm head, allowing only capacitated sperm to penetrate the corona radiata of the ovum. 

Even in in-vitro fertilization, certain chemicals are utilized to eliminate the glycoprotein coat.

 

Step -2 Acrosome reactions – Penetration of zona pellucida

After a capacitated sperm successfully penetrates the corona radiata layer, it begins to penetrate the zona pellucida membrane by binding to proteins such as ZP3. 

Upon binding to ZP3, the sperm releases enzymes, including acrosin and trypsin-like substances. 

These enzymes collectively trigger the acrosome reaction in the sperm, which aids in the digestion of the zona pellucida membrane. 

Consequently, the acrosomal cap of the sperm disappears during this reaction. 

 

Step -3 Fast blocks to polyspermy – Electrical barrier

Polyspermy refers to the fertilization of an egg by more than one sperm, a rare occurrence in human beings. 

To prevent polyspermy, the egg creates an electrical barrier. 

When a sperm reaches the egg’s plasma membrane, sodium channels open, leading to an influx of sodium ions. 

This creates a layer of positive sodium charge (Na+2) around the egg’s wall, serving as an electrical barrier that effectively prevents polyspermy.

describe the process of fertilization in human beings

Step -4 Slow block to polyspermy (mechanical barrier)

As soon as sperm enters the cytoplasm of the egg, some cortical granules move towards the surface of an ovum. 

These cortical granules contain lysosomal enzymes, also known as hydrolytic enzymes. 

Subsequently, the cortical granules release lysosomes throughout the egg’s wall, inducing alterations in the zona pellucida.

This process makes the ovum impenetrable to other sperms. It also inactivates the ZP3 protein and degenerates the tail of sperm.

This series of events is termed the 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. 

Following the entry of sperm’s nuclear material, the ovum completes its secondary meiotic division.

As a result, the ovum produces some polar body and a definitive ovum. 

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

describe the process of fertilization in human beings

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

These haploids fuse together and form a zygote.

Male haploid (n) + Female haploid (n)

Diploid (2n)

Zygote

A zygote is the initial cell formed by the fusion of two gametes. This is a first step in making a baby. 

 

Conclusion

Fertilization is an important step in making new human life possible. The journey of sperm to the female reproductive system is not easy. 

In this post, I mentioned the various steps to describe the process of fertilization in human beings.

We learned how sperm meets with the ovum and what happens after the fusion of gametes.  

It is the only informative article to make you understand about process of fertilization. 

If you want to do pregnancy planning or to avoid pregnancy, it would be better to consult your doctor. 

Please share this post, if you found it informative. 

 

FAQ

Q1. 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 sperm travels to the ovum in the ampulla area. This is the place where the process of fertilization begins. 

Q2. What happens when sperm enters into the female reproductive system?

The entry of sperm into the female reproductive system marks the beginning of the fertilization process. Here, the sperm gets fused with the egg or ovum and makes a zygote.

Q3. What is the fallopian tube also called?

The fallopian tube is also known as the Oviduct or Uterine tube. It plays a crucial role in the process of fertilization. 

Q4. What is the fertilized egg called?

The fertilized egg is called a zygote. It is formed when a sperm cell successfully penetrates and merges its genetic material with an egg cell during fertilization.

Q5. What happens after fertilization in humans?

When the fertilization process is completed, it makes a fertilized egg, called a Zygote. Once the zygote forms, it starts to divide and multiply the cells. 

 

Sources –

1. T. W. Sadler. Langman’s medical embryology. 9th 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.

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