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Understanding The Sperms' Journey to the Egg

What is the goal of a sperm cell?

The goal of the sperms’ journey to the egg is to fertilize it. To do so, the sperm cell must pass through a long and challenging path. This is one of the reasons why the total number of motile sperm cells is very important, and a key parameter for a man’s potential to reach pregnancy with his partner.

Fig 1 Sperms journey to the egg

The sperms’ journey to the egg begins with millions of sperm cells that are released into the female reproductive tract during intercourse. The sperm cells gain their full ability to swim when they are ejaculated into the reproductive tract [1],[2].

Upon ejaculation, the sperm cells are enclosed in a fluid called seminal plasma or semen, which is a mix of fluids from the testes, seminal vesicles, prostate, and the bulbourethral glands. The fluid contains elements which protect the sperm cells during their journey towards the egg. The semen thickens and helps the sperm cells stay inside the woman – as close as possible to the cervix, which is the “gate” to the egg. Liquid extends from the cervix, allowing the sperm cells from the semen to swim into the cervix. Only the strongest sperm cells will make it this far. Once through the cervix, the sperm cells swim across the uterus and into the fallopian tubes as illustrated in figure 1.

Fig 2

Sperm to the egg

Only the strongest make it this far

Of the millions of sperm cells deposited during intercourse, only very few have the potential to reach the fallopian tubes. Once inside the tubes, the sperm cells follow signals (a process called chemotaxis) from the supportive cells (called the cumulus cells) of the egg [3]. On the way, the sperm cells undergo a series of biochemical and functional changes. These prepare the sperm cells for fertilization.

Eventually, the sperm cells meet a barrier of cumulus cells surrounding the egg. To pass through this barrier, the sperm cells must use their very special ‘stroke’, known as hyperactivation. The sperm cell then has to pass another barrier called the ‘zona pellucida’, an additional layer of the egg. To pass this barrier the sperm cells now must undergo a process called the acrosome reaction. That’s where a deposit at the top of the sperm cell releases enzymes. These enzymes will break down the zona pellucida barrier, allowing the sperm to penetrate the egg [4], [5]. See figure 2.

When the sperm cell’s head is inside the egg, the tail of the sperm detaches. At that moment, the zona pellucida becomes impermeable to other sperm cells.

When inside the egg, both the egg and the sperm prepare for a genetic fusion. After this, the 23 chromosomes from the egg and the sperm cell respectively, fuse together. They generate the 1st cell that gives rise to new life, called the zygote.

After fertilization, the zygote moves through the fallopian tube to attach itself to the inner wall of the uterus.

What about twins? Is the sperms’ journey to the egg different?

Twin pregnancies happen either when two fertilized eggs attaches to the uterus simultaneously (dizygotic twins). Or it happens when one fertilized egg splits into two embryos (monozygotic twins).

With the ExSeed Home Sperm Test, we assess how many sperm cells there are in a semen sample and how many of them are swimming. As you may see from this post, these are important parameters in determining if the sperm cells will reach the egg.

As always, don’t hesitate to contact us if you have any comments or questions!

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