For successful conception, sperm need to make the most remarkable journey through the bodies of men and women. It is a journey that scientists have only recently been able to understand in any detail, with new discoveries being made all the time. This article outlines our current knowledge and explains what makes for a winning sperm, but also why the majority of sperm never make it at all.
The journey of sperm begins inside the testicles. Males begin to produce sperm at the start of puberty at around 12 or 13 years old. It is a process that requires a slightly cooler temperature, which is why testicles hang outside men's bodies. Nor is it a quick process: the production of sperm takes about 70 days. Exactly how many sperm come off the production line is also critical to how fertile each man is.
In simple terms, the larger a man's testicles the greater the number of sperm produced per unit time. This is because inside bigger testicles there are more of the sperm pre-cursor cells (called spermatogonia) that divide and multiply and go on to make new sperm. Also, there are more of the intricate labyrinths of tubes and nurse cells that are critical for the production of sperm. Quite simply, the bigger the factory the bigger the quantity of sperm produced.
about 1,000 sperm roll off the production line with every heartbeat
In a typical fertile male, about 1,000 sperm roll off the production line with every heartbeat. However, before they are finally ready for release, sperm first spend another week or so passing through a long winding tube called the epididymis. Here finishing touches are added to them, including modifications to the molecules on their surface that ultimately will help when they reach the egg. In addition, there are also some 'tweaks' to the machinery of the tail that will help them swim better when the time comes.
Once sperm leave the epididymis, they can remain alive, healthy and motionless for several weeks in the man's body without being released. Men have the capacity to store many billions of sperm, but if sperm are stored for too long, then they can start to degenerate and die. This is a natural process designed to dispose of old sperm. However, if ejaculation does not happen often, then older degenerating sperm can damage newer sperm leaving the epididymis. Therefore, scientists recommend that men ejaculate every two or three days to keep the reservoir of sperm in optimal condition.
When ejaculation finally happens, about 250 million of the sperm being stored are propelled by muscular contractions through a tube called the vas deferens and then into the tube passing down the penis (the urethra) that eventually leads to the outside world. It is the muscular contractions that contribute to the feeling of orgasm that men experience during intercourse. Typically, there are several spurts of fluid from the end of the penis at orgasm. The majority of sperm are in the first spurt, with the second and third containing fluid from the prostate gland and other structures called the seminal vesicles. It is now that the sperm begin swimming for the first time.
Once inside the vagina, sperm must escape quickly if they are to survive. Perhaps surprisingly, the environment of the vagina is quite acidic. This is an important defense mechanism to protect women's bodies from attack by bacteria and viruses in everyday life. However, following sexual intercourse, these acids would also quickly kill sperm if the fluids from the prostate and seminal vesicles did not give them some protection.
Therefore, in those few minutes immediately after ejaculation it is the fastest, healthiest or just plain luckiest sperm that have enough time to swim out of the vagina and into the relative safety of the cervix. Slow, deformed or unlucky sperm are left behind in the vagina and quickly die. A woman's cervix acts as an amazing filter and it's easy to see how men who produce too few 'mucus penetrating sperm' may find it difficult to get their partner pregnant and therefore may suffer infertility as a consequence.
only about 1 in 100 of the sperm ejaculated into the vagina will ever make it into the relative safety of the cervix
As a general rule of thumb, scientists think that only about 1 in 100 of the sperm ejaculated into the vagina will ever make it into the relative safety of the cervix. However, to make matters more complicated, this will also depend on the hormone profile of the woman and how close she is to releasing an egg from her ovaries. It is only in those few days immediately before ovulation that a woman's cervical mucus allows sperm inside. Outside this fertile window, even the strongest and fastest sperm will find it impossible to get inside.
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Entering the cervix is, however, only the very start of a sperm's journey through a woman's body. The next step is how to get through the womb and into one of the Fallopian tubes. Although it is a journey of only about 10 or 12 centimeters, there are a number of hazards along the way.
The first hurdle for sperm that successfully enter the womb is whether to go left or right. This is because women have two Fallopian tubes (one at either side) but since only one of her ovaries will release an egg in any given month, sperm must swim into the right one. To make their way down a Fallopian tube toward an ovary that isn't going to release an egg would be a wasted journey! Thankfully, to increase the odds of sperm getting it right, the woman's body comes to the rescue.
Briefly, as the egg gets closer to being released from the ovary, some of the hormones that are produced to help the egg grow also enter the woman's bloodstream. These trigger muscles in the wall of the womb closest to it start rhythmically contracting upward, slowly moving sperm gently toward the entrance of the Fallopian tube on the same side. Because the muscles on the other side (which isn't growing an egg) receive fewer hormones, they don't contract as much meaning sperm generally don't go in that direction.
However, getting to the opening of one of the woman's Fallopian tubes, on the side of her body where the egg is about to appear from, is only part of the journey. Here the woman's body has another trick up her sleeve. Although the entrance to the Fallopian tube is a tiny opening only a few sperm heads wide, it acts as a valve only letting certain sperm inside. This again means that the woman's body is selecting sperm to make sure that only the best ones get close to the egg.
Scientists think that sperm are selected to enter the Fallopian tubes in two ways. First, some experiments have shown that if sperm swim too erratically then they are unable to get inside. Sperm that swim like this are nearing the end of their life and so perhaps they are being excluded because they won't live long enough to find an egg. Second, there is some evidence that sperm with certain molecules on their surface can't get into the Fallopian tube either. Quite how this works is unclear, but again it shows that not just any sperm can complete the journey. Of the many millions of sperm that were ejaculated into the vagina, only perhaps a hundred get this far!
Once inside the Fallopian tubes, sperm are in a relatively safe place in comparison to the rest of their journey so far. Here the female body helps them survive and remain in a healthy condition until the egg is ready to be released. For example, several research studies now suggest that inside the Fallopian tubes, sperm tend to stick to the inner walls by the tip of their head. Studies have shown that sperm that stick tend to live longer and have better quality DNA in their heads than sperm that don't.
Some scientists believe that when sperm attach and stick to the sidewalls of the Fallopian tube, this gives the woman's body a signal to say sperm have finally arrived. Others have suggested that once sperm are stuck, new molecules are produced by the cells which line the inside of the Fallopian tube which may ultimately help in fertilization or embryo development. Whilst it is hard to know for sure, one thing is clear: if sperm are to fertilzse an egg they must free themselves from the sidewalls as soon as it appears.
Quite how long sperm can remain stuck to the sidewalls of the Fallopian tube is unclear. But to pull themselves away, they increase the beat of their tail to give them extra thrust. This type of swimming is called hyperactivation and it's very important in these final stages of a sperm's journey.
Once free from the sidewalls, then sperm need to make their way down the Fallopian tube in the direction of the ovary in order to find an egg that will be shortly coming in the other direction. Quite how they do this is not completely clear, but probably involves one or both of the following mechanisms:
First, inside the Fallopian tube there is probably a subtle change of temperature that occurs once the egg is released from the ovary. Experiments in the laboratory suggest that sperm are 'positively thermotaxic' which means they preferentially swim to a warmer place. It has been suggested that after ovulation has occurred it is one or two degrees Celsius warmer in the middle of the Fallopian tube (where the egg will eventually arrive) so sperm will naturally make their way there.
Second, it has also been suggested that sperm are able to detect subtle chemical signals, either given off by the egg once it is released from the ovary or that are released from the ovary at the same time as the egg (perhaps in the fluid that bathes the growing egg). The response of sperm to these signals is called chemotaxis and if true, it will be a powerful attractant to guide the sperm to the egg.
probably only half a dozen sperm ever get this far!
Either way, one or both of these mechanisms will serve to make sure that there are a few sperm present in the middle of the Fallopian tube in time for the egg to appear. This is important because, in comparison to sperm, the egg only has a limited life and once released from the ovary will need to be fertilized within a few hours if conception is to occur. Therefore it is critical that sperm get to the right place in the Fallopian tube before the egg arrives. However, probably only half a dozen sperm ever get this far!
In comparison to sperm, the egg is quite a large cell - the largest in the human body. In addition, it is released from the ovary surrounded by a cloud of cells called the cumulus through which the sperm must first penetrate before they can reach and make contact with the egg surface. To assist sperm to get through the cumulus, the sperm must again use the erratic and frenetic swimming style (hyperactivation) that they used to pull away from the sidewalls of the Fallopian tube.
At this point in the sperm's journey there are perhaps only one or two sperm close to the egg and with any chance of fertilizing it. However, for the successful sperm, there is one more hurdle left: how to get inside the egg once it reaches the egg surface. To some extent, the hyperactivated tail beat that helped the sperm get free of the Fallopian tube side walls and through the cloud of cumulus cells will also assist here. But for additional help there is a bag of enzymes on the very top of the sperm head that bursts once the winning sperm makes contact with the outer surface of the egg.
The enzymes help dissolve the outer membrane of the egg and in combination with the powerful tail thrusts, assist the sperm in penetrating the egg and getting inside. Once this is achieved, then there is an immediate chemical reaction inside the egg that suddenly hardens the egg membrane and stops other sperm from following it. If two sperm were to fertilize the egg, the egg would then contain too many chromosomes and in the resulting confusion embryo development would fail to occur.
Over the following few hours the sperm tail is detached (this stays outside) and the chromosomes inside the sperm head separate and line up with those from the egg to start the process of forming a new individual. At this point the sperm's journey has ended, but the journey of the embryo has only just begun.
See how the race of the winning sperm unfolds in our infographic
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