For a sperm to fertilise an egg, they have to travel a distance of about 10 or 12 centimetres from the vagina to half way along one of the woman’s Fallopian tubes. We have all seen films and cartoons of sperm madly swimming to get there in some kind of race, but is it really like that?
For those sperm that are able to get through the thick mucus of the cervix (see Part one of The Great Sperm Race), their next challenge is to navigate through the woman’s womb and find the opening to one of her Fallopian tubes. The womb is a bit like an upside down pear, with a small channel running through the middle. It’s here where the baby will grow if she does get pregnant, but for a woman trying for a baby it represents a major hurdle for sperm.
The first problem for sperm that enter the womb is whether to go left or right. At the top of the womb are the entrances to the Fallopian tubes, one on each side, down which an egg travels once it is released from the ovary. But since only one egg is normally ovulated each month, the sperm needs to know from which side it will come. Sperm that get it wrong will have a wasted journey. Thankfully, the woman’s body gives some signposts.
As the egg gets closer to being released from the ovary, some of the hormones produced there enter the woman’s bloodstream. These stimulate some of the muscles in the wall of the womb on that side to start contracting. The direction of contraction is from the cervix to the Fallopian tubes and this gently moves any material in the womb upwards. However, because the muscles on the other side (which isn’t growing an egg) receive fewer hormones, they don’t contract as much. This means that sperm are moved preferentially to toward the entrance to the Fallopian tube on the side in which the egg is going to appear after ovulation. Ingenious!
But it’s not plain sailing for any sperm that makes it to the entrance of the Fallopian tubes: they still have to get inside. To do this, sperm have to pass through an opening only a few sperm heads wide. This is easier said than done because we now think the entrance to the Fallopian tube has a strict door policy – a bit like an exclusive nightclub!
First if the sperm swims too erratically – perhaps like someone who is a bit drunk – they will simply have trouble in finding the entrance in the first place. If their tail thrashes about too much, it will get caught on the sides or bounce off from the edges. Think of trying to throw a basketball into the net – to maximize your chances of getting it in, the ball has to be in an almost perfect trajectory.
Second, if sperm have the wrong molecules on their surface – a bit like wearing the wrong clothes – by some unknown mechanism they won’t get through either. Perhaps the Fallopian tube has its equivalent of nightclub bouncers on the door!
We think that sperm pass through the woman’s womb and into the Fallopian tube quite quickly, and probably in less than an hour. Speed is certainly of the essence, as lurking in the womb are white blood cells – assassins waiting to trap and kill sperm that are too slow. Sperm that don’t find their way quickly are disposed of and are never seen again.
In Part 3 of this story, we will look at what happens in those final stages of the sperm’s journey that take place in the relative safety of the Fallopian tubes. Stay tuned!
Author: Allan Pacey BSc, PhD, FRCOG, Professor of Andrology at the Department of Human Metabolism, Academic Unit of Reproductive and Developmental Medicine, The University Of Sheffield