The lifecycle of a sperm
This image from a colour-enhanced scanning electron micrograph shows the lucky sperm that have survived the journey up the fallopian tubes to swarm all over the egg. Only one sperm from the 100-million-plus ejaculated will penetrate the membrane of the egg and fertilise it.
A single sperm has a short and fraught life and it's highly unlikely it will ever achieve its goal of fertilising an egg. No wonder men produce so many of them.
Every day men's bodies are busy making millions of sperm that are eager to share a bit of their host's DNA with a warm and receptive egg. But the chances of any of those sperm ever meeting an egg let alone fertilising one are miniscule. Not only do these sperm have to survive ejaculation and be competent swimmers with a sense of direction, they have to navigate through a virtual minefield of obstacles planted by the woman's body. We follow a sperm through its short and crowded life.
Each of the testes contains thousands of tiny tubes called seminiferous tubules. Inside these tubules, sperm are being constantly formed from puberty onwards. Tiny cells known as spermatogonia line the seminiferous tubules; these cells begin to enlarge to form spermatocytes, which then divide to become young spermatozoa, later developing into sperm. Once a sperm is "born" in the seminiferous tubules, it lives there for about 70 days.
After leaving the seminiferous tubules, the sperm migrate through the testicular duct system, which takes them to the epididymis, where they continue to mature. Once mature, the sperm are ready to move to the ejaculatory duct, remaining there until the moment of ejaculation. At this time the sperm mixes with seminal plasma, made of secretions from the seminal vesicles and the prostate gland. This plasma nourishes the sperm and its lubricating effect helps them swim.
The survival rate of sperm on the journey to the uterus is extremely low. For conception to occur, the sperm must travel through the opening of the cervix, into the uterus and then into the fallopian tubes to meet the egg.
The sperm make this journey by swimming they move at a rate of about 3mm a minute, which is super fast given their microscopic size. Even so, the journey from the cervix to the ovum takes several hours, although contractions of the woman's uterus can help.
Once a sperm is in the uterus or fallopian tubes, it can live for up to five days. Conception can occur any time during this period. Of the hundreds of millions of sperm ejaculated, only a few thousand make it into the uterus and only about 50 of these actually survive to reach the egg.
Throughout most of a woman's monthly cycle, the cervix is plugged by a thick, viscid mucous that sperm find difficult to get through. Only at ovulation does the cervix become more receptive to the millions of sperm knocking on the door. But just because they've entered the uterus doesn't mean the sperm will have an easy ride from then on: they have to swim deep into the uterus for the openings to the fallopian tubes which many of them will never find, dying or getting lost instead.
And of course these tubes aren't some simple straight downhill ride to the egg. The journey is convoluted, with many twists and turns and hurdles for the sperm to navigate. It's a difficult, highly risky journey that only the strongest and healthiest sperm will be able to survive; nature's way of ensuring the survival of the fittest.
Once at the "pot of gold" the handful of survivors swarm the egg, much like ants covering a basketball. Finally, if all goes well and conception happens, one lucky sperm penetrates the egg and fertilisation occurs, shutting out any other sperm.
Sperm facts in brief
A man's ejaculate contains more than 100 million sperm, of which, in a healthy sample, less than 40 percent could be normal. Many millions will die, drop off, swim in the wrong direction or get lost, with only 50 making it to where the egg should be. The egg is only open to fertilisation for 12 to 24 hours per cycle, but the sperm can lie in wait for a healthy egg for up to five days.
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