Sci will admit that the sperm paper she picked for this week really is an excuse to use this image:
But really, sperm swimming is an important questions in terms of fertility. An ovum is only viable for about 24 hours, and a female only produces one per month. Sperm are viable for a little longer than that, but still, timing is rough. And once you get past the timing, you get into sperm getting lost, sperm swimming in circles, sperm getting stuck in the vaginal mucosa, and sperm getting to the egg only to be thwarted by the zona pellucida and corona radiata (the corona radiata is a group of cells surrounding the egg, while the zona pellucida is a tough, thick protein lattice on the outside of the egg, inside the corona radiata. Tough barriers).
(Who's betting on the double-headed one?!)
So when it comes down to it, it takes more than a straight swimmer to make it to the egg. But what, exactly, does it take?
Force, et al. "Membrane fluidity and lipid content of human spermatozoa selected by swim-up method". International Journal of Andrology, 2001.
So some of you may be thinking "what? Why is there more than swimming? It's just two cells bumping into each other and getting it on, right?" Well, no. In fact, there's a LOT more to fertilization than just the smooshing of cells. Observe:
Sperm enter the vagina (I assume most people know how this can be accomplished). On average, a man will drop between 150 and 600 MILLION sperm in a given ejaculation. Of those, there can be ONLY ONE.
(There is only One)
The lucky sperm that gets up the vagina, past the cervix, through the uterus, and into the right fallopian tube where that month's egg is hangin' out, then has a further obstacle course to get through.
1) The corona radiata: made of granulosa cells from the ovarian follicular formation, these little cells protect the egg after ovulation has occurred.
2) Now the sperm hits the zona pellucida, a tight lattice of proteins immediately surrounding the egg. Fortunately, the sperm has brought its toolkit. Hitting the zona pellucida makes the calcium levels inside the plasma membrane of the sperm head rise, and makes the sperm release a little acrosomal packet in the sperm head. This contains a boatload of enzymes which chew through the zona pellucida.
(OM NOM NOM)
3) Once the zona pellucida of the egg is dissolved, the sperm and egg membranes touch. The egg puts out little microvilli to surround the sperm head, and the membranes fuse, triggering the sperm to release its genetic contents into the egg.
The whole thing looks like this:
So obviously there's more to sperm than just swimming ability. There's swimming in a straight line (preferably pretty fast), having the right intracellular responses (like the rise in calcium triggering the release of the acrosomal enzymes, as well as the response to release the genetic material into the egg), and having a cell membrane that is capable of making all this happen.
In this particular study, the authors wanted to look at what made sperm special OTHER THAN how well they swam. Most infertility in men is thought to arise from sperm that are bad swimmers, but obviously there is much more at stake than that. So in this case, the authors took a bunch of sperm from men of normal fertility and men of reduced fertility, and compared them. But they didn't want their data to be messed up by whether or not the sperm could swim. They had to make sure they selected only the best swimmers. So they used the "swim-up method".
In the swim-up method, you take some media, and cover a sample of sperm with it (in this case, sperm was acquired via masturbation when the subjects had refrained from ejaculation for 3-6 days prior). You then incubate your sperm and media all nice and warm. This warm liquid environment makes the sperm swim, and the sperm swim up into the media. Because you keep it incubated for a limited time, this ensures you get the best sperm, the kind that moves well. So for this study, the authors took swim-up sperm and analyzed them for the other things that sperm are required to have.
But they did run into some problems. First of all, the men with fertility issues had far fewer swim-up sperm than those that didn't have fertility problems, suggesting that the main problem is still whether or not your sperm can swim. In fact, the men with fertility issues had SO few sperm by the swim-up method that the authors had trouble getting a sample! But even when they did, the sperm in the infertile men that DID swim STILL had problems.
The healthy sperm from the controls had cell membranes with relatively reduced membrane area with a very fluid membrane, implying that the healthy sperm that swam well were also pretty small and compact. In contrast, the unhealthy sperm not only didn't swim well, but they had reduced membrane fluidity. This might be the reason that the sperm, even though they can swim, can't fertilize the egg. Without the right amount of membrane fluidity, it's possible signals might not go through, and the sperm and egg might never get a change to merge.
This also could have some implications for men who have reduced fertility, and decide to try the turkey baster method or something. If the sperm are suffering from membrane problems as well as swimming issues, fertility will be reduced unless you're actually directly fusing the sperm and egg in the lab. And reduced membrane fluidity and swimming in sperm could mean other possibilities of ill-health, such as bad genetic copy, which might still make fertility difficult even if you directly fuse sperm and egg.
So remember, when working with sperm, swim isn't everything (though it's sure a lot). Your sperm can't just swim, they've got to be sane in the membrane (can you tell I've been waiting all week to use that line?!)
Force, A., Grizard, G., Giraud, M., Motta, C., Sion, B., & Boucher, D. (2001). Membrane fluidity and lipid content of human spermatozoa selected by swim-up method International Journal of Andrology, 24 (6), 327-334 DOI: 10.1046/j.1365-2605.2001.00309.x