Egg fertilization is typically referred to as a legendary swim satisfy: Millions of sperm swim as quickly as they can towards the egg till one– the fastest, greatest, healthiest sperm of them all– wins the race and wriggles into the egg, with the reward of passing its genes to future offspring.

Is this truly how it occurs? Do sperm actually race to the egg?

Yes and no, David J. Millera teacher in the animal sciences department at the University of Illinois Urbana-Champaign, informed Live Science. “The important player in all of this is actually the female reproductive tract.”

Sperm do swim throughout this procedure, however “the major movement is actually provided by contractions of the female tract,” Miller described. “There are contractions of the uterus, for example, that are much like contractions of the GI tract that can move fluid through the uterus.”

A 1996 research study detailed simply how effective these contractions are, Miller kept in mind. Researchers transferred sperm-size beads into the uteruses of 64 females, and a few of the beads took a trip all the method to the fallopian tubes– where fertilization normally happens– within minutes.

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It makes good sense that sperm would require some additional aid, due to the fact that while the sperm swim in one instructions, the egg requires to take a trip in the opposite instructions to satisfy them, Sabine Koellea complete teacher of anatomy and developmental biology at the University College Dublin School of Medicine and Medical Sciences, informed Live Science. The egg can’t swim, so rather small hairs called cilia assist it along.

“Cilia beat to transport the oocyte,” or egg, Koelle stated. “Because the sperm are coming from the opposite direction, they have to struggle” versus the present produced by the cilia.

A private sperm’s motion is less of an effort to move forward and more of an effort to move inwardtowards the middle of the system, Koelle stated. If the sperm get too near the sides, they stick and lose their forward momentum.

Simply since a sperm is the very first to show up does not imply it gets to do the fertilizing. “Sperm require some final maturation that occurs in the female tract, and it’s time dependent,” Miller stated. “So the sperm that ‘win the race,’ so to speak, need more time before they can actually fertilize the egg.”

“They may not be there when they’ve completed that maturation,” Miller included. “They might be replaced by some of the other more slowly transported sperm that have had time to complete that maturation.”

Even those less-mature sperm are more effective than the large bulk of sperm that get transferred. As the female reproductive system presses the sperm along, it likewise cuts unfortunate people from the swim group.

“Less than 1% — maybe up to 2 or 3% of the sperm that are actually deposited — make it all the way to where the egg is,” Miller stated. “A lot of them are flushed back out from the tract. Some are eaten up by immune cells in the uterus, because sperm are foreign.”

Approximately 70% of sperm do not even make it past the cervix, Koelle kept in mind. “The sperm are stuck there and can’t free themselves,” she stated.

For those couple of sperm that make it into the fallopian tube, the objective is to get as far as possible and after that stay with the wall as they await the egg to show up. This is another location where the female reproductive organs are picking winners: Scientists have actually discovered that normal-looking sperm are most likely to bind to the wall, Miller stated, and binding to the wall supplies some metabolic advantages that increase their life expectancy.

Once the egg shows up, the fallopian tube– likewise understood as the oviduct– permits just healthy-looking sperm to unstick from the wall. “As soon as a sperm is not OK, the oviduct doesn’t release it,” Koelle stated. “It’s the main selector of good sperm.”

This isn’t an ideal system, naturally. “ Obviously, we have genetic diseases that come through sperm. So it’s not always true that the fittest are the best ones genetically,” Miller stated.

Every action of the method, the female reproductive system is doing its finest to weed out the less-fit sperm so that just healthy sperm reach the egg. Because method, fertilization is less like a race and more like a task interview.

“There’s certain qualifications that you need to be able to apply for the job,” Miller stated. “But also, the sperm that have those qualifications would have to have them at the time the job is open — the time when the egg is ovulated.” In the end, it’s the female reproductive system that picks the finest prospect.

Ashley Hamer is a contributing author for Live Science who has actually blogged about whatever from area and quantum physics to health and psychology. She’s the host of the podcast Taboo Science and the previous host of Curiosity Daily from Discovery. She has actually likewise composed for the YouTube channels SciShow and It’s Okay to Be Smart. With a master’s degree in jazz saxophone from the University of North Texas, Ashley has a non-traditional background that offers her science composing a special viewpoint and an outsider’s perspective.