What is Fertilization? Steps, Process & Facts

Fertilization is the process in which the female and male gametes fuse to form a zygote, which later develops into an embryo. 

During intercourse, the male sperms are ejaculated into the female that passes through the fallopian tube and combines with the ovum’s (egg) zona pellucida layer, forming a zygote (fertilized egg). 

The male body produces thousands of sperms. Once the zygote is formed, it is deposited in the uterus, where it continues to grow into a fetus.

What are the steps involved in fertilization?

The entire fertilization process involves multiple steps from sperm maturation to zona response and post-fertilization events. Let’s take a closer look:

1. Sperm Capacitation – This step refers to the process of preparing sperm for fertilization. The motility of the sperm increases as it becomes hyperactive. This phase ensures that the sperm penetrates the egg.
2. Sperm-Zona Pellucida Binding – In this step, the sperm encounters an ovum and binds into the zona pellucida layer of the ovum to form a receptor-ligand response. No more sperm can get through this layer after this coupling reaction.
3. Zona Pellucida Penetration– The structure of the sperm’s head is in such a way that it assists in breaking through the walls of the ovum. It also stimulates ovum maturation, suspended during one stage of meiosis. Next stages happen just after the egg is fertilized.
4. Acrosome Reaction– The head of sperm, called the acrosome, carries several Zona Pellucida digesting enzymes that support it in penetrating farther into the ovum layers. With deeper entry, the head of the sperm shrinks. 
5. Cortical Reaction– At this stage, the egg is activated after being frozen during the metaphase stage of meiotic division II. Cortical granules unite with the Zona Pellucida, resulting in exocytosis.
6. Zona Reaction – This step marks the end of fertilization as the Zona Pellucida layer hardens.

What is the process involved in fertilization?

There are three main phases of fertilization.

1. Chemotaxis
2. Sperm activation/acrosomal reaction
3. Sperm/egg adhesion 

These three phases ensure that the egg and sperm find each other and that only one sperm penetrates the egg.

Ovulation happens once a month during the menstrual cycle in humans and is essential for fertilization. The first stage of fertilization begins when this cycle releases an egg cell from the ovaries.

When sperm finds an oocyte, it adheres to the zona pellucida, a thick layer of a jelly-like extracellular matrix made up of glycoproteins that envelopes the egg. The acrosome reaction is triggered when a specific molecule on the surface of the sperm attaches to a ZP3 glycoprotein in the zona pellucida. The acrosome reaction produces hyaluronidase, which ferments the hyaluronic acid around the oocyte and lets sperm to pass through.

The cortical granules within the egg fuse with the cell’s plasma membrane and are released into the zona pellucida following successful sperm embedding, prompting the surface to become rigid and impermeable. This step is known as the cortical reaction, and it ensures that just one sperm cell penetrates and fertilizes the egg.

The outer layer and tail of sperm dissolve once they have successfully reached the egg. To create a haploid ovum, the oocyte goes through meiosis. The genetic material of sperm and egg, each of which has 23 chromosomes, is fused, resulting in a diploid cell with 46 chromosomes, termed as a zygote. The zygote then goes through mitosis, which is the recurrent cellular division required for an organism’s growth, to produce a blastocyst, which is implanted into the uterine wall, kicking off the pregnancy.

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Here are a few facts about fertilization!

Let us take a look at some interesting facts about fertilization:

1. A woman’s whole egg supply (many millions) is produced while she is still in the womb. By the time they reach puberty, the population is shrunk to between a quarter and half a million. Only about 400 of them will be released during her lifespan by ovulation.
2. Each testicle can produce up to 1,500 sperm per second.
3. Since most healthy sperm can survive for several days in the female reproductive canal, intercourse can take place up to 3 days before ovulation and still result in fertilization.
4. Each time an adult male ejaculates, he produces millions of sperm, and the egg is 500 times larger than the sperm’s head.