Male Reproductive Organs

Male Reproductive Organs

In purely practical terms, the male of the human species seems to contribute little to reproduction. His only donation is the male sex cell or gamete, the sperm. This is a tiny cell compared to the female’s egg. The female carries the foetus for nine months in her womb, and in most societies she continues to care for and breast-feed the baby after birth. However, the 23 chromosomes carried by the sperm cell provide half the genetic material for the offspring, so in evolutionary terms at least, men and women are equal partners in the future of the species.

The role of the male’s reproductive system, in the biological sense, is to manufature billions of sperm (spermatozoa) and transport them safely and effectively to the female reproductive system during sexual intercourse. Compared to the female, reproductive organs in the male are relatively simple. The ‘factories’ that make the sperm are two avoid organs called the testes. A system of tubes and glands transports the sperm to the penis, the exit and point of transfer of sperm into the female’s body.

The male and female systems are different in many ways, despite their common origin in the embryo. The male organs are almost entirely external to the body, whereas the female organs are internal. In men, sperm production continues unremittingly from puberty through adult life into old age; female egg production is cyclical and of limited duration. Another important difference between men and women is the close association, in the male, between the reproductive and the urinary systems. A central canal that runs through the penis, the urethra, performs the dual role of transporting urine from the bladder, and sperm from the testes, to the outside of the body.

The testes

The male reproductive process begins in the testes, where sometimes more then 250 million sperm are manufactured every day.

The two testes (testicles) are egg-shaped organs; each is about 2.5cm across and 5cm long and weighs about 25grams. They are suspended between the legs in a sac, the scrotum. The testes develop in the embryo from a ridge of tissue lying at the back of the abdomen – tissue that develops into ovaries in female embryos. When the two male testes have developed they begin to move slowly down the abdomen and through a canal, called the inguinal canal, to the groin. Normally they descend completely and lie in the scrotum just before birth. In about 10 per cent of boys, however, one or both of the testes fail to descend in a condition known as cryptorchism* and for which the cause is unknown. The condition does not, however, cause the baby any pain or problems with urinating. Treatment may be necessary to move them into the scrotum, if they have not descended of their own accord during the first few years of life, and regular medical check-ups are carried out by the doctor until this happens. If undescended testes are not corrected infertility may be the eventual result. If one or both of the testes has not descended into the scrotum by the time the boy is about five years old an operation known as an orchidopexy may be carried out. This usually involves a stay in hospital of about three days. Once the testes are descended, there should be no further problems.

In about 75 per cent of men the right testis hangs lower than the left although the reason for this is not clear. The testes are suspended in the scrotum by a cord that runs down the inguinal canal. The suspending cord, the spermatic cord, comprises two blood vessels (the testicular artery and vein), a lymph vessel and a tube called the vas deferens which is responsible for transporting sperm from the testes to the penis. The entire spermatic cord is enclosed by a sheath of muscle, the cremasteric muscle. In cold conditions this muscle tightens to draw the testes up closer to the body. This also occurs during sexual intercourse.

It is important that the testes stay at a temperature a few degrees lower than the internal body temperature, in order to allow effective sperm production. If the testes should become too hot – through a fever, or a hot bath, or even by wearing tight body-hugging clothing – sperm production is markedly reduced. Prolonged overheating of the testes is one possible cause of temporary infertility because insufficient sperm are being produced.

Sperm production

Sperm are produced by the division of cells that line tiny tubes packed into the testes. These tubes, the seminiferous tubules, are tightly coiled and arranged into lobes. There are about 500 tubules in each testis, with a combined length of about 600m. Spherical spermatogonia cells divide in several intermediate stages to form spermatids and finally tadpole-like sperm. The whole process of sperm production, from the spermatogonia cells in the testes to the fully mature motile sperm, takes about six to eight weeks. Each sperm is about 0.05mm long and very thin, so it is invisible to the naked eye (the egg cell is a relative giant at 0.1mm in diameter). It consists of a head, neck-piece, middle-piece and long tail, and contains one single set of 23 human chromosomes.

The immature sperm are not yet able to propel themselves by their tails and in the next stage of their development are carried along to the epididymis. This comma-shaped structure lies curved over the back of the testis and is composed of a tightly-coiled tube that serves as the primary storage site for sperm. It is here that the sperm become fully mature and mobile. This mobility is of vital importance in order to make fertilization possible. The sperm have to endure the equivalent of a marathon swim before reaching the egg. Being only 0.05mm long they have to travel more than 2,000 times their own length from the point of ejaculation through to the uterus and the Fallopian tube. Sperm can survive in the epididymis for about a month, after which they are destroyed.

Testosterone production

The testes have a second and equally vital role in addition to sperm production. It is the manufacture and secretion of the male sex hormone, testosterone. An entirely separate set of cells, called Leydig or interstitial cells, scattered between the seminiferous tubules, are responsible for making this hormone.

The cells are stimulated to start making testosterone at puberty by messages sent from part of the brain which direct the pituitary gland to secrete another hormone, gonadotropin. In boys, puberty usually starts between the ages of 10 and 14.

Testosterone is the key to male sexual maturity. Although erection and ejaculation can occur during childhood, it is not until testosterone production has begun that a boy becomes sexually mature and capable of producing sperm.

Effects of testosterone

The hormone testosterone has several functions. It affects the development of the sexual organs, bodily development and the distribution of muscles and fat, and it influences psychological development. It also triggers sexual desire and arousal.

The penis and scrotum both grow under the influence of testosterone over a period of about two years some time between the ages of 11 and 17. Each testicle may grow from about Ito 15gm in weight during this time. Less obviously, testosterone also causes the prostate

gland to enlarge and the seminiferous tubules to mature and produce sperm.

Testosterone is responsible for the changes we call puberty. It encourages hair growth on the face, chest, armpits and around the genitalia. It promotes a sudden enlargement of the voice-box, or larynx: the cartilage layers that make up the larynx grow and the elastic vocal cords that run down the centre of the larynx thicken, so that the boy’s voice eventually cracks or ‘breaks’.

If the testicles have to be removed before puberty the boy will develop none of the secondary sexual characteristics. His voice will not break and he will not produce sperm. If removal occurs after puberty, the voice remains deep and certain sexual characteristics persist. However, fat will be deposited on the buttocks and breasts, hair will start to disappear, and the man ceases sperm production.

Testosterone continues to be produced throughout adult life and is important in maintaining many of the sexual characteristics developed at puberty. As the man gets older the Leydig cells begin to decline in number and so less testosterone is produced. By the age of 50 only half of the maximal amount of testosterone is produced. This is one of the reasons why sexual arousal usually takes longer in older men compared to young men.