Conceiving: the man's role
Sperm, a man's contribution to the conception of his child, are made in his testes (testicles). A man begins making sperm at puberty under the influence of testosterone from the testes, and luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from his pituitary gland. LH and FSH act on the testes as they do on the ovaries. A man goes on making sperm throughout his life.
Inside each testis is a network of minute tubes that contain the cells from which spermatids are created. These tubes connect with about eight larger tubes; these are the efferent ducts that carry the developing sperm into the epididymis where they mature and grow their tails. The sperm travel from the epididymis through a tube called the vas deferens before ejaculation. On this journey sperm are mixed with secretions from other glands to form semen. The semen acts as a vehicle for the sperm to carry them to the female reproductive tract.
The testes also produce hormones, the most important being the male sex hormone testosterone, which is the most powerful of the androgens. These hormones are responsible for male secondary sexual characteristics such as facial hair and deepening of the voice, and for male and female sex drives.
The mature sperm
Each individual sperm is only about one-twentieth of a millimetre long, so cannot be seen by the naked eye. It's shaped like a tadpole and has a strong tail, five or six times longer than its head, which it uses to move itself along. The tail is attached to the head by a short middle section or body. This contains special cell components called mitochondria, which are its energy-producing apparatus. A sperm's head is dark in colour because it contains so much genetic material.
The newly formed sperm pass into the epididymis at the rear of each testis, where they mature. From the epididymis, matured sperm travel up a tube called the vas deferens, which leads to the seminal vesicle, a small, sac-like structure near the bladder. When a man ejaculates, seminal fluid (semen) is discharged from the penis via the urethra. Semen is made up of sperm, mixed with fluid produced by the seminal vesicle and fluids secreted by the prostate and other glands.
Most men ejaculate about 3.5ml (that's about two-thirds of a teaspoonful) of semen when they make love, but the range is 2-5ml ( ½ -1tsp). Each millilitre contains 20-150 million sperm, of which a high proportion are abnormal in shape. Only about three-quarters of the sperm are motile (able to wriggle). A man's sperm production does speed up at times of sexual activity, but if ejaculation is very frequent, sperm numbers do decrease, which lowers his fertility.
Reaching the egg
Although sperm can move 2-3mm per minute, their actual speed varies with the acidity of their environment - the higher the acidity, the slower their movement. Vaginal secretions are slightly acidic; so sperm ejaculated into the vagina probably move quite slowly until they reach the more friendly alkaline environment of the uterine cavity. Having got through the hostile acidic conditions of the vagina, they then face a longer and more dangerous journey before they reach the egg way down a fallopian tube. Of about 300 million sperm in an ejaculation, only a few hundred will actually reach the egg. Most of the rest trickle out of the vagina, or are destroyed by vaginal acidity. Others may be destroyed by cleansing cells within the uterus, enter the wrong Fallopian tube, or go into the correct tube but miss the egg altogether.
Making sperm for conception
The process of making sperm takes place inside a man's testes and is known as spermatogenesis.
After puberty the testicles make sperm continuously at the rate of about 125 million sperm a day. The whole process, from the initial generation of a sperm to its maturing and ejaculation, takes about seven weeks.
Although the numbers and quality of sperm produced decline from the age of 40, men in their 90s have fathered children.
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