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The term "sperm capacitation" refers to the process male sperm go through after being ejaculated into the female reproductive tract. Chemical processes take place that enable the sperm to penetrate and fertilize an egg. These processes are not fully understood, but it is believed that they involve the full maturation of the sperm cells so that they can successfully join with a mature female egg.
Sperm capacitation is a term first used in 1952 by Colin Russell Austin when it was noted that more sperm were able to fertilize the eggs of certain animals if they were inserted into the female several days prior to ovulation rather than just after ovulation. This led him to suspect that there must be some sort of chemical maturation process sperm must undergo before successful fertilization can occur. Further research has supported this conclusion.
It is believed that this process happens in intervals. Not all sperm that are ejaculated in a single act mature at the same rate, which allows for a more steady supply for mature sperm when ovulation does occur. Some sperm may undergo the process first, and if no egg is released soon enough, these first mature sperm will die off. Another group go through capacitation afterward, and this cycle may continue for several days. Sperm may live in the female genital tract for up to five or more days under ideal conditions.
When sperm capacitation takes place, changes in the sperm behavior also occur. Their tails begin to move more vigorously and their heads may move quickly back and forth. This helps to propel them faster into the fallopian tubes, making fertilization more likely.
Other factors may also play a role in how well sperm function inside of the female body. The pH balance of the vagina and fallopian tubes may have a big impact on motility. Sperm have to be adequately protected by ample cervical mucus and seminal fluid. The fluid should also be the right consistency to surround the sperm and should contain adequate levels of fructose in order to feed and energize them.
Sperm motility and health can be observed in a fertility clinic. Sometimes, these cells may be abnormally shaped or have other abnormalities that can lead to lowered mobility. A lack of capacitation may also occur if sperm cells are not able to fully mature. These issues could hinder fertility if severe enough.