Sperm: swimming and surviving

How do sperm move, and what determines who wins the race to the egg?
23 November 2020

Interview with 

Allan Pacey, The University of Sheffield


First up in our exploration of cell movement, we're talking about the cells that got us all here: sperm! Allan Pacey is a sperm expert from The University of Sheffield and he filled Chris Smith in on all the details...

Allan - Yeah, a tadpole’s quite a good way to look at most sperm types. There are many variations on the theme. But it's the vehicle if you like that delivers the male set of chromosomes to the female set of chromosomes. The DNA's in the, what might be the tadpoles head, there's then a middle power pack of mitochondria. That's in a thing called the midpiece. And then there's a long tail. Incredibly small human sperm, you'd get 20 of them in a millimeter, if you could hold them down and line them up.

Chris - And that tail is there to provide propulsion. Isn't it? Because one of the key things the sperm has to do is to move.

Allan - Yeah, absolutely, so how does that happen is a good question. So imagine the stick of rock that you might buy at the seaside, chop it in half, it says Blackpool or Brighton through the middle. If you were to chop a sperm tail in half, you would see a series of tubules - they're called microtubules. Those tubules slide together, and that's what gives the tail the kind of whip shape motion that we generally recognise.

Chris - And do sperm do that all the time? Are they constantly swimming even when they're being made in the testicles, for example, or can they turn that on and off?

Allan - So before sperm are released they're not swimming. They're being stored. But when they're released, it's the removal of the chemistry of the male body and it's the stimulatory chemistry of the female body that switches them on. Now, sperm will swim until they die pretty much. In human terms, in higher mammals, sperm often swim close to the egg, and then they tend to power down and they wait for the egg. It's a really elegant kind of holding mechanism if you like.

Chris - Like go an enormous distance, don't they? Because from the point of entry into the female reproductive tract, in a human, they've got to get all the way up through the uterus. And then to the top part of the oviduct, the channel that carries eggs from the ovary, where it's come from in order to meet the egg. When they actually meet the egg, how do they know they've met the egg and how do they actually then do the business of fertilising it?

Allan - Well, actually sperm get quite a helping hand from the female's body to get to the site of fertilisation. So there's muscular contractions of her body that help the sperm get there. And then once they get into that oviduct, there's temperature differences within the oviduct that cause the sperm to move towards the egg. It's slightly warmer towards the egg end. There's a chemical signal that helps them get there. We're not quite sure what that is, but there are chemicals that are released from the egg or the cells around the egg that draw the sperm in. And then there's also fluid movements that push the sperm along towards the egg. A man, a human male will ejaculate 50 or 60 million sperm into a woman. By the time you get to the oviduct, there's probably only around half a dozen.

Chris - Well, if just a handful make it, does that mean then that basically a massive selection process has happened? All those sperm that started the race - it is literally survival of the fittest. It's the best sperm that makes it?

Allan - To some extent that's right. It is a big race, but it's very highly coordinated. So the point that ejaculation happens, then it's only nicely shaped sperm that enter the cervix. If the sperm are malformed - got two tails, two heads, or they just that bit slightly differently made, then they're not going to get through that cervical mucus. It's very thick. You need to be kind of aerodynamically and fluid mechanically very sleek to get through that. In the uterus sperm are under attack from white blood cells. So, you know, you've got to get to the fallopian tubes pretty quickly if you're going to avoid death by leukocytes. And then when they sense these signals from the egg, those sperm then change the way that they swim. They start to hyper activate -we call them the tailies - beating around and providing great force and energy to help that final winning sperm get through the outer coat of the egg. It's probably only at the last few millimeters of sperm getting towards the egg that there's actually a sperm by sperm kind of race if you like, but there's probably only about half a dozen sperm at that point in the race.

Chris - Yes. It's a miracle that any of us are here really, when you think about it, isn't it? But you brought up the question of sperm shape. Now a few years back, I was in Norway and I went to visit a gentleman there who's a biologist who works on bird sperm. And I was very surprised when he told me that if you look at birds that are very promiscuous and they basically they'll go with anything that's willing, they have very different shaped sperm, he said, to birds that are highly monogamous, like swans, for example, they mate for life. What's he getting at?

Allan - So promiscuity within a species does impose some sperm-specific selection pressures. So if you're a male in a species where there's lots of competition for females, you better make sure your sperm is the best. And that evolutionary pressure means that across the animal kingdom, there's been some quite elegant solutions to the sperm problem and quite subtle refinements that might give males a competitive edge. Within primates, there's a classic example. So chimps, highly promiscuous, they have big testicles and the sperm they produce swim really fast and they're very sleek and elegant. Gorillas, not particularly promiscuous. And they by comparison have pretty terrible sperm. Humans? Kind of in the middle. We've got a reasonable number of pretty well sperm, nicely made sperm. Um, but we've also got a lot of badly made sperm too. So depending on where you are on this kind of promiscuity scale, it does have a huge impact on what the sperm look like and how well they're made.


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