Spermatogenesis: Equal X's and Y's

The process by which sperm are formed guarantees that an equal number of X and Y-sperm are produced; this is because X-sperm and Y-sperm aren't manufactured separately, but result from the division of an XY parent cell.

Normal cells in your body are reproducing all the time, by a process called mitosis: the cell's DNA replicates (makes an exact copy of itself), then divides down the middle, resulting in two cells that are identical to the original cell.

Obviously, mitosis won't work for creating sperm cells, because a sperm cell somehow has to wind up with only half the usual number of chromosomes. Further, even if you could somehow start out with such a cell, if it just made copies of itself, they would all be genetically identical; all of a couple's offspring would be clones of each other.

The answer is a specialized form of cell division called meiosis, used only in the formation of sperm cells (which is called spermatogenesis) and oocytes (oogenesis).

In the testes, sperm is produced by cells called spermatagonium. These cells reproduce themselves in the usual way, by mitosis, so that a man doesn't run out of them; after all, he'll be producing sperm his entire life, starting from puberty.The Sperm Cell

 

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!

 

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.
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 makes this distance a marathon. If the sperm were the size of a salmon, the trip would be 43 miles (69 km)! The sperm's whip like 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.
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 mucus. Fluid in the semen also helps suppress the female autoimmune system, but white blood cells can still attack the helpless invaders at any moment. Sperm cells must also avoid getting trapped in the numberless folds and crevices throughout the reproductive tract.

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