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It's only about a teaspoon of liquid, but it contains about 3 million sperm. They are immediately in peril. The vagina is acidic, so they must escape or die. They start to swim, at least some of them. Even in a healthy man, 60% of the sperm can be less than perfect, for these guys the journey is over. But what of the rest? What are the chances that one tiny sperm will reach and fertilize an egg? Sperm are often portrayed as brave little warriors forging their way through hostile territory to conquer the egg. Nothing could be further from the truth.
For each challenge the sperm face, success is to a great extent controlled by the woman's body, and even the egg itself. Take the sperm's first obstacle, the cervix, passageway to the uterus. Most of the time it's locked shut, packed with mucus that keeps bacteria and sperm out. But for just a few days a month, after ovulation, the mucus becomes watery and guides the sperm through. Arriving inside the uterus, the sperm are still about 6 inches away from their goal, about a two day swim. But undulations of the uterine muscles propel the sperm into the fallopian tube within 30 minutes. Even a sperm that reaches the tube in record time has no guarantee of fertilizing an egg. Ovulation could still be days away. It's the slowpokes, caught up in the cillia lining the tube who may have a better chance. It's probably here that chemicals in the woman's body alter the sperm's outer coating. Only the sperm that are altered can get a date with the egg. The sperm are released gradually over the course of a few days, so that any given time only a couple hundred sperm will move on.
If all goes well then farther up the tube they will find the egg. But it's heavily chapeoned by support cells, and the chaperones are picky. Only some of the sperm are let through. Those that make it will face another challenge. Underneath the cloud of cells, the egg is encased in a thick protein shell called the zona. To fertilize the egg, the sperm must break through the zona, but even the strongest cant do it by brute force alone. The egg demands a proper introduction. Proteins protruding from the sperm's cap must hook up precisely with a set of proteins on the egg's surface. If they match, the sperm is held fast, and undergoes a dramatic transformation. It sheds its outer coating releasing powerful enzymes that dissolve a hole in the zona, allowing the sperm to push it's way though. The final hurdle passed, the sperm still does not thrust it's way into the egg itself. Rather the membranes of the two cells fuse, and the egg draws the entire contents of the sperm inside.
tl;dr: it's OVER for spermcels.
from Life's Greatest Miracle by PBS https://www.pbs.org/wgbh/nova/video/lifes-greatest-miracle
For each challenge the sperm face, success is to a great extent controlled by the woman's body, and even the egg itself. Take the sperm's first obstacle, the cervix, passageway to the uterus. Most of the time it's locked shut, packed with mucus that keeps bacteria and sperm out. But for just a few days a month, after ovulation, the mucus becomes watery and guides the sperm through. Arriving inside the uterus, the sperm are still about 6 inches away from their goal, about a two day swim. But undulations of the uterine muscles propel the sperm into the fallopian tube within 30 minutes. Even a sperm that reaches the tube in record time has no guarantee of fertilizing an egg. Ovulation could still be days away. It's the slowpokes, caught up in the cillia lining the tube who may have a better chance. It's probably here that chemicals in the woman's body alter the sperm's outer coating. Only the sperm that are altered can get a date with the egg. The sperm are released gradually over the course of a few days, so that any given time only a couple hundred sperm will move on.
If all goes well then farther up the tube they will find the egg. But it's heavily chapeoned by support cells, and the chaperones are picky. Only some of the sperm are let through. Those that make it will face another challenge. Underneath the cloud of cells, the egg is encased in a thick protein shell called the zona. To fertilize the egg, the sperm must break through the zona, but even the strongest cant do it by brute force alone. The egg demands a proper introduction. Proteins protruding from the sperm's cap must hook up precisely with a set of proteins on the egg's surface. If they match, the sperm is held fast, and undergoes a dramatic transformation. It sheds its outer coating releasing powerful enzymes that dissolve a hole in the zona, allowing the sperm to push it's way though. The final hurdle passed, the sperm still does not thrust it's way into the egg itself. Rather the membranes of the two cells fuse, and the egg draws the entire contents of the sperm inside.
tl;dr: it's OVER for spermcels.
from Life's Greatest Miracle by PBS https://www.pbs.org/wgbh/nova/video/lifes-greatest-miracle
