Читать книгу Zita West’s Guide to Getting Pregnant - Zita WEST, Zita West - Страница 57

Sperm are produced in the seminiferous tubules of each testis. The two testes are contained in the scrotal sac, next to the penis. Unlike women, who are born with around 2 million immature eggs in their ovaries, although they don’t begin to mature and ovulation doesn’t occur until after puberty, a man doesn’t produce sperm at all until puberty, when his reproductive hormones become active.

Hormones are sometimes called the ‘chemical messengers’ of the body. Secreted by a gland in one part of the body, hormones are transported via the bloodstream to another area of the body, where they have an effect. At puberty, for both men and women, an area of the brain called the hypothalamus starts secreting gonadotrophin – releasing hormones, which cause the pituitary gland to produce two other hormones: follicle – stimulating hormone (FSH) and luteinizing hormone (LH). When these two hormones are produced, the gonads – testes in men, ovaries in women – are stimulated, to produce sperm in the man and to stimulate ovulation in a woman. The average age for male puberty is between 12 and 14, although this can vary quite widely and is influenced to some degree by genetic make – up, race and diet. Hormonal stimulation of the testes leads to the production of sperm and, as long as the necessary hormones are available and at the correct levels, and there is no disease or illness affecting production, this is a continuous process from puberty onwards.

The other effect of FSH and LH on the testes is to stimulate the production of testosterone, within the testes, responsible for the development of male characteristics like facial, armpit and pubic hair, and body hair in general. The voice also deepens as the testosterone makes the Adam’s apple at the front of the neck enlarge. Muscle tissue is increased and strengthened, and mood is also influenced – not least in promoting sexual interest. Some men seem to produce more testosterone than others, but this doesn’t mean they are more fertile than other men. Testosterone is responsible for the secondary sex characteristics described above, and isn’t particularly an indicator of the quality of sperm or a man’s fertility.

The process by which sperm are formed in the testes takes around 100 days – 74 for the sperm to develop, and then another 20–30 days to reach maturity, which is what you have to allow before you can expect to see any improvement in sperm quality or quantity. But as sperm production is happening continuously, a man is continuously fertile, unlike a woman who is only fully fertile for about eight hours during each one of her fertility cycles!

Within the testes, the sperm start their journey through the thousands of tiny, coiled seminiferous tubules. Sperm start as spermatocytes, primary cells that divide and develop into spermatids, which are immature, tail – less sperm. During their journey along the seminiferous tubules, where they are nourished by the sertoli cells that line the tubules, each spermatid grows a head that contains all the chromosomal material. Chromosomes carry all of a man’s genetic material ready to pass on to a child, including the chromosome (usually referred to as the sex chromosome) that decides whether a baby will be a boy or a girl. Whereas every one of a woman’s eggs contains the genetic material for a girl (the X chromosome), a man’s sperm will contain either an X chromosome or a Y chromosome (for a boy).

As well as growing a head during their 74-day development, the sperm form a middle piece that contains the energy source, and a tail. It is the mid – piece of the sperm, with its energy source, which is responsible for the tail’s ability to move and transport the sperm independently. Without a properly functioning tail, sperm may swim around in circles rather than forward in a straight line, or may not swim at all.

From the seminiferous tubules, the maturing sperm move into the epididymis, which is a long tube, around 18 feet in length but only a three – hundredth of an inch in diameter, which is coiled and situated at the back of the testicle.

Once produced, sperm then move up into the vas deferens, the tubes that eventually feed into the urethra and on through the penis, via the ejaculatory duct. Here, the sperm mix with secretions from the seminal vesicles and the prostate gland to form the seminal fluid. Once combined, the sperm and seminal fluid are referred to as semen. The secretions of the prostate gland are alkaline, which helps to neutralize the acidity of the woman’s vagina, and protect the sperm. The vas deferens travel from the testes out of the scrotum and through the lower abdomen via the groin, then around to a point underneath the bladder where they join the urethra. They hold an astonishing amount of sperm: it would take around 30 ejaculations to empty the vas deferens of their full load!

Only around 20 per cent of semen is sperm. The seminal fluid that combines with sperm to form semen is composed of more than 22 different chemicals, including sugar, vitamins C, E and B₁₂, prostaglandins (which help stimulate muscle contractions and the dilatation of blood vessels), the minerals zinc, potassium and sulphur, and essential fatty acids (namely DHA). Altogether, ejaculate consists of around 2 to 4 ml of semen, which is quite viscous at first but then liquefies after about 10 minutes. Sperm need to be nourished by this liquid on their journey, and protected from the acidic environment of the woman’s vagina, before any of them can reach an egg. Around 250 million sperm are ejaculated each time, but it takes only one to fertilize an egg.

Why this enormous quantity of sperm? Only a certain proportion – and this varies from man to man – will be normal, active enough and capable of fertilizing an egg in the first place. Then, because the woman’s vagina is acidic, and hostile to sperm, treating it as it would any other ‘foreign body’ and attempting to get rid of it, as it would an infection, this also reduces the available sperm. From here, only around a million sperm will actually get as far as the woman’s cervix, and only around 200 of these, at most, will reach the woman’s Fallopian tube, to fertilize an egg.

If you then consider how many of the remaining sperm are actually of a good enough quality to fertilize an egg – and in sperm analysis only a maximum of 20 per cent pass this test – it’s no wonder that fertilization can be a tricky business. And even if fertilization does occur, if the sperm responsible is faulty in some way then development of the fertilized egg can’t continue, and miscarriage may be the outcome.