When we last left the egg, it had been nurtured in its follicle in the ovary, and escorted into the fallopian tube to await the sperm. The sperm's life story, as we'll see, is dramatically different.
Sperm: An Army on a Mission
After spending 74 days being formed in spermatogenesis, sperm spend another two weeks in "boot camp" -- becoming proficient swimmers as they wind their way through twisted, coiled tubes in the testis, and at last reach a holding area around the prostate gland.
The journey ahead of them is a long and dangerous one; a sperm must not only find the egg and survive to reach it, but beat out millions of other sperm to get there first!
|Largest cell in the body
|Smallest cell in the body|
~5 microns, 55 microns with tail
|A woman is born with
a lifetime supply of eggs
(She's an egg warehouse)
|A man makes sperm every|
day of his life from puberty
(He's a sperm factory)
|One egg is released
per cycle (usually)
|20 to 150+ million sperm are|
in each ejaculate
|The egg can live about
24 hours after ovulation
|Sperm can live up to 5 days|
under ideal conditions
(Cannot move by itself)
|Moves by thrashing it's whiplike tail|
The only human cell with a flagellum
|Has 23 chromosomes, the baby's genetic heritage from its mother
||Has corresponding 23 chromosomes, the baby's genetic heritage from its father|
|The egg's 23rd chromosome
is always an X
|A sperm's 23rd chromosome |
is either an X or a Y
Ejaculation - Welcome to the Vagina
The journey begins with ejaculation, and while it does get the sperm started on their way, it's no joyride -- the sperm are catapulted into the vagina at amazing speed and under intense shearing forces. (Many sources say the sperm shoot out at 28mph/45kph, but I think that's an urban legend.)
If ovulation is not occurring or approaching, the sperm are doomed on their mission, and find themselves in very hostile surroundings. The vagina's acidity will cause the sperm to perish within hours. Those who continue towards the cervix will be trapped in thick, sticky cervical mucus, and any who manage to struggle all the way to the cervix will find the entrance to the womb firmly closed.
We can hardly blame the female reproductive tract for being so unfriendly -- after all, bacteria and other invading organisms would like to make a home there.
But as ovulation approaches, the vagina becomes more hospitable, becoming more alkaline and less acidic. Cervical mucus changes from a dense, impenetrable barrier into a watery, slippery fluid; at a microscopic level, the mucus actually forms tiny channels, like swimming lanes, guiding the sperm forward. This type of cervical mucus is called egg white cervical mucus (EWCM), because it is clear and slippery. It can also be amazingly copious, and you may have noticed it literally dripping from your vagina on the day before and day of ovulation, an indication that you are in the most fertile days of your cycle.
Even under the most favorable conditions, the sperm have a daunting distance to travel. Although only about 4 inches (10 cm) lie between the sperm and egg, the extreme itti-bittiness of the sperm make this distance a marathon. If the sperm were the size of a salmon, the trip would be 43 miles (69 km)! The sperm's whiplike tail, 10 times the length of its head, can propel it at about 3mm per hour.
Millions of sperm have been ejaculated to make the attempt; 20 to 150 million sperm in a single ejaculate is considered the normal range, but it could be as much as several times more -- even up to a billion sperm.
Why are millions of sperm needed to fertilize one egg?
Because they don't ask for directions. -- GoAskAlice.com
Sperm are manufactured constantly in the testes, regardless of how often a man has sex. Temperature is a critical factor in sperm production; normal body temperature is just a little too warm for optimal sperm production, and so the testes are housed in the scrotum, outside of the body at 2 to 3 degrees lower than normal body temperature. A too-hot scrotum can drastically impair sperm production, which is why tight pants are sometimes blamed for impaired fertility. Other factors, such as drug use, smoking, excessive drinking, obesity, and chemical exposure, have been linked to poor sperm production.
Of the millions of sperm launched into the vagina, not all are Top Gun quality, so to speak. Some will already be dead, and even in normal, healthy semen up to 40% may be defective, malformed or unable to swim forward properly. These are quickly left behind, eliminated from the gene pool.
Healthy sperm propel themselves upward in the vagina, nurtured by fluid in the semen, and bathed in favorable cervical mucous. Fluid in the semen also helps suppress the female autoimmune system, but her white blood cells can still attack the helpless travelers at any moment. Sperm cells must also avoid getting trapped in the numberless folds and crevices throughout the reproductive tract.
Heat Seeking Guided Missiles
How do the sperm know which way to go? Recent research reveals that sperm have olfactory sensors -- a sense of smell -- and may be guided to the egg by chemical attractants. This "scent" emitted by the egg has yet to be detected, but research continues to attempt to do so, because it could yield new treatments for infertility, and new methods of birth control. Evidence also suggests that sperm are guided by heat, and that the upper part of the fallopian tube is warmer than the bottom, leading the sperm to the egg.
When sperm reach the top of the vagina, they encounter the cervix, the entrance to the uterus. Now open in honor of ovulation, sperm continue through the cervix and into the uterus. Only 1% of the millions of sperm ejaculated into the vagina will actually make it into the uterus. Once here, they get a helping hand. Contractions of the uterus convey the sperm to the fallopian tubes much faster than they could swim themselves.
The fallopian tubes, also called oviducts, lead from the uterus to the ovary -- and is where the sperm and egg will meet. A few inches in length, the inside of the fallopian tube is no larger than a pin head, but quite roomy for the tiny sperm.
After a journey lasting about 10 hours, only a few hundred sperm will make it this far. At last, the goal is in sight.