How to get pregnant? The answer might seem simple. But what is really going on? A while ago we explained how important the female cycle is for the chance of conception. This time we will give you the answer from the perspective of a sperm cell. Our new expert, Dorte Egeberg, explains:
It begins with millions of sperm cells
Millions of sperm cells are ejaculated into the female reproductive tract, where they gain their full ability to swim. To reach the site of fertilization, the sperm cells have to pass through several functional obstacles, many of which require the ability to swim. Hence, the total number of motile sperm cells is a very important parameter to study in the laboratory evaluation of semen quality . The sperm cells are deposited in the vagina enclosed in seminal plasma, a viscous fluid containing factors protecting and nourishing the sperm cells. Inside the female the seminal plasma coagulates and in this way helps the sperm cells staying inside the female in close vicinity to the cervix. Mucus extends from the cervix, enabling passage of the sperm cells from the semen coagulum up into the cervix. Only sperm cells capable of swimming can penetrate the cervical mucus. Furthermore, the mucus excludes the seminal plasma from entering the upper female reproductive tract . This process is mimicked in the processing of IUI-ready samples in the sperm laboratory, where the motile sperm cells are separated from seminal plasma, dead or otherwise abnormal cells. Once through the cervix, the sperm cells swim across the uterus and at the junction between the uterus and the fallopian tubes they have to pass another mucus plug.
About 1 out of 1.000.000 sperm cells enters the fallopian tubes
In humans, of the millions of sperm cells deposited after coitus, only a very few sperm cells have the potential to reach the fallopian tubes. Once inside the fallopian tubes the sperm cells follow clues excreted from the supportive cells of the egg, the cumulus cells . On the way the sperm cells undergo a series of biochemical and functional modifications, collectively referred to as capacitation, which renders the sperm cells ready for fertilization. Eventually, the sperm cells met the barrier of cumulus cells surrounding the egg cell. To pass through the cumulus cells, the sperm cells have to use their very special stroke, known as hyperactivation. But this is not the last obstacle for the sperm cells to pass, i.e. surrounding the egg is an impenetrable shell, the zona pellucida . In order to pass the zona pellucida, the sperm cells have to undergo a very special process called the acrosome reaction, where hydrolytic enzymes are released from a deposit at the top of the sperm cell. These enzymes are capable of breaking down the zona pellucida and in this way allowing the last step of fertilization .
Only one sperm cell is needed for fertilization
In the end, only one sperm cell will fertilize the egg. The success of a sperm cell to finalize its primary goal depends on several parameters where each parameter is believed to explain a minor proportion of the total fertilizing ability, and that only sperm cells that possess all these properties will essentially be able to fertilize the egg.
- 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.
- Suarez, S.S. and A.A. Pacey, Sperm transport in the female reproductive tract. Human Reproduction Update, 2006. 12(1): p. 23-37.
- 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.
- 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.
- 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.