The Testicular Factory
Sperm are produced in the male testicle (testis). The testis is filled with tiny coiled tubes (seminiferous tubules) through which sperm move as they are produced. Each seminiferous tubule is lined with sperm nurse cells (Sertoli cells) and sperm stem cells (Spermatagonia). Outside the tube and between the tubes lie testosterone-producing cells (Leydig cells), whose product is essential to regulating sperm production. Hormones produced by the Leydig cell stimulate sperm production indirectly, by acting on the nurse cells, which in turn regulate the sperm cells. From start to finish, it takes approximately 70 days to make sperm, and sperm in various stages of development can be found throughout the testis.
Waves of Sperm Production
Spermatagonia stem cells start to divide at staggered times and continue dividing throughout a man's lifetime. Spermatagonia have to accomplish two things: make more germ cells to keep the process going and kick some of the germ cells down the path to make mature sperm. Germ cells also jumble up or recombine the genes on the chromosomes in new ways. Germ cells accomplish this by undergoing both mitosis (duplication of DNA) and meiosis (recombination of DNA) in the early stages of making sperm. This is why men don't run out of sperm and why not all of Dad's kids look exactly alike.
Specialized for One Job
The sperm starts out looking like a typical round cell and ends up with highly specialized features for one job: to fertilize an egg. Excess cytoplasm is removed to reduce drag. The cell nucleus containing the sperm's chromosomes is tightly packaged and housed in the elongated sperm head. Most excess organelles are removed with the cytoplasm, but the powerhouses of the cell (mitochondria) are positioned right behind the head like engines behind a rocket. Cellular structures that package enzymes (Golgi apparatus) are recycled to form the acrosomal cap, an enzyme-filled packet on the front of the head that help the sperm penetrate the cells surrounding the egg. Other cellular structures (the centrioles) are also recycled to create the sperm tail which is used for propulsion and steering.
The newly produced sperm is not much of a swimmer and doesn't have much fertilizing ability yet. These features are acquired when the sperm pass through the epdidymis, a tightly coiled spermatic duct that sits atop the testis. Special proteins are added to the sperm membrane and others are removed, resulting in a highly specialized and outfitted cell that can swim to the egg, bind to the egg and penetrate the egg. With these finishing touches, sperm are ready to go with the next ejaculation.
Production of semen and ejaculation
Sperm cells are only the smallest part of the fluid that is ejaculated. Other fluids are added to the sperm as the sperm are ejaculated. The ejaculated sperm are pushed through the ejaculatory duct and fluid from various glands (seminal vesicles, prostate, and Cowper's gland) are added along the way. The majority of fluid in semen comes from the seminal vesicles and prostate gland. A typical ejaculate is between a teaspoon and tablespoon of fluid containing at least 30 million sperm. Withdrawal birth control methods tend not to work very well because the sperm-rich fraction of the ejaculate is the very first part of the ejaculate.