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.
Anatomy of a testicle
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
Sperm in the testis
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.
Sperm in the epididymis
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.
Sperm waiting in the vas deferens
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.
Journey of Sperm
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.
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