The idea of sperm racing to the egg is not entirely true. (Image credit: CHRISTOPH BURGSTEDT/SCIENCE PHOTO LIBRARY via Getty Images)
Fertilization of an egg is often imagined as a spectacular swimming race: millions of sperm compete to reach the egg as quickly as possible, and only one of them – the fastest, strongest and healthiest sperm – wins the race and penetrates the egg, getting the opportunity to pass on its genes to future offspring.
But is this really true? Do sperm really rush to the egg?
Yes and no, David J. Miller, a professor of animal science at the University of Illinois at Urbana-Champaign, told Live Science. “The female reproductive tract plays a key role in this process.”
Sperm does move during this process, but “the bulk of the movement is actually provided by contractions of the female tract,” Miller explained. “For example, there are uterine contractions that are very similar to contractions in the gastrointestinal tract and can move fluid through the uterus.”
A 1996 study showed how effective these contractions are, Miller added. Scientists inserted sperm-sized pellets into the uteruses of 64 women, and some of them reached the fallopian tubes — the site where fertilization usually occurs — in just a few minutes.
It makes sense that sperm need extra help because while the sperm is moving in one direction, the egg needs to move in the opposite direction to meet them, Sabine Koelle, a professor of anatomy and developmental biology at the School of Medicine and Health Sciences at University College Dublin, told Live Science. The egg can't swim, so it gets help from tiny hairs known as cilia.
“The cilia vibrate to move the oocyte,” or egg, Kelle explained. “Because the sperm are coming from the opposite direction, they have to overcome” the current created by the cilia.
In fact, the movement of an individual sperm is not so much a forward effort as it is a desire to move inward, toward the middle of the tract, Kelle said. If sperm get too close to the edges, they get stuck and lose momentum.
However, just because a sperm arrives first doesn’t guarantee that it will be able to fertilize an egg. “Sperm need to go through a final maturation process that occurs in the female tract, and that is time-dependent,” Miller explained. “So the sperm that ‘win the race,’ so to speak, need some extra time before they can fertilize an egg.”
“They may not be there when they complete this maturation,” Miller added. “They may be replaced by other, slower-moving sperm that have had time to complete this process.”
However, even these less mature sperm are more successful than the vast majority of sperm that reach the egg. Because the female reproductive tract helps propel the sperm, it also eliminates the unlucky ones from the race.
“Less than 1% — maybe up to 2 or 3% — of the sperm that are actually injected make it to where the egg is,” Miller said. “Many of them are washed back out of the tract. Some of them are destroyed by immune cells in the uterus because the sperm are foreign.”
Up to 70% of sperm don't even make it past the cervix, Kelle said. “The sperm get stuck there and can't get free,” she added.
For the few sperm that make it into the fallopian tube, the goal is to travel as far as possible and then attach to the wall, waiting for the egg to arrive.
Sourse: www.livescience.com
