The sperm journey to the egg

The goal of a sperm cell is to fertilize an egg. 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

The journey 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].

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

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, which prepare the sperm cells for fertilization.

Eventually, the sperm cells meet a barrier of cumulus cells surrounding the egg. To pass through the barrier created by the cumulus cells, 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, where enzymes are released from a deposit at the top of the sperm cell. These enzymes will break down the zona pellucida barrier, allowing the sperm to penetrate the egg [5].

When the sperm cell’s head is inside the egg, the tail of the sperm is detached and 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 23 chromosomes from the sperm fuse to 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.

See figure 2.

But what about Twins….

Twin pregnancies happen either when two eggs are fertilised and attached to the uterus simultaneously (dizygotic twins) or when one fertilised 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 the description above, 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!



[1] Sakkas, D., et al., Sperm selection in natural conception: what can we learn from Mother Nature to improve assisted reproduction outcomes? Hum Reprod Update, 2015. 21(6): p. 711-26.

[2] Suarez, S.S. and A.A. Pacey, Sperm transport in the women reproductive tract. Human Reproduction Update, 2006. 12(1): p. 23-37.

[3] Eisenbach, M. and L.C. Giojalas, Sperm guidance in mammals – an unpaved road to the egg. Nat Rev Mol Cell Biol, 2006. 7(4): p. 276-85.

[4] Liu, D.Y., C. Garrett, and H.W. Baker, Acrosome-reacted human sperm in insemination medium do not bind to the zona pellucida of human oocytes. Int J Androl, 2006. 29(4): p. 475-81.

[5] Overstreet, J.W. and W.C. Hembree, Penetration of the zona pellucida of nonliving human oocytes by human spermatozoa in vitro. Fertil Steril, 1976. 27(7): p. 815-31.

[6] Dorte Louise Egeberg Palme, et al.; Viable acrosome-intact human spermatozoa in the ejaculate as a marker of semen quality and fertility status, Human Reproduction, Volume 33, Issue 3, 1 March 2018, Pages 361–371







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