Semen and sperm quality

Semen is a white or grey liquid, emitted from the urethra (tube in the penis) on ejaculation. Usually, each millilitre of semen contains millions of spermatozoa (sperm), but the majority of the volume consists of secretions of the glands in the male reproductive organs. The purpose of semen is purely for reproduction, as a vehicle to carry the spermatozoa into the female reproductive tract. Although ejaculation of semen accompanies orgasm and sexual pleasure, erection and orgasm are controlled by separate mechanisms and semen emission is not essential for enjoyable sex in most people.

Where is semen made?

Semen consists of the secretions of several glands but only 5 per cent comes from the testicles.

Where semen is made

Contributing gland

Testicles and epididymes
5 per cent

Seminal vesicles
46 to 80 per cent

Prostate gland
13 to 33 per cent

Bulbourethral and urethral glands
2 to 5 per cent

Testicles and epididymes

The spermatozoa take over 70 days to develop and are produced solely in the testicles. Individual sperm develop within the testicles from a cell called a spermatogonium. The spermatogonium divides to produce spermatocytes, which then develop into spermatids. The spermatid develops its familiar tail and the cell gradually acquires the ability to move by beating its tail.The spermatid eventually develops into a mature spermatozoan. This process takes about 60 days and the sperm then takes a further 10 to 14 days to pass through the ducts of each testicle and its sperm-maturing tube, the epididymis, before it can leave the body in the semen, during ejaculation.The fluid produced by the testicles contains several chemicals, but is particularly rich in testosterone.

Seminal vesicles

The seminal vesicles usually contribute more than half of the seminal volume. Their secretions are particularly rich in a sugar called fructose, which is an essential nutrient for the spermatozoa. They also produce a substance that causes the semen to clot (become sticky or jelly-like) after ejaculation, thought useful in reproduction for keeping the semen at the neck of a woman's womb.

Prostate gland

The secretions of the prostate gland contain several chemicals, of which prostate specific antigen (PSA) is probably the most interesting. PSA is an enzyme, a catalyst that causes biochemical reactions, in this case a type of enzyme called a coagulase. The prime function of PSA is to liquefy the clotted semen, so that the spermatozoa can escape from it and swim off to fertilise the ovum (egg) in the woman's reproductive tract. Prostate gland cells produce PSA and some of it leaks into the blood stream, as well as appearing in semen. The bigger and more active the prostate gland is, the more PSA appears in the blood. Men with big, benign (non-cancerous) prostate glands often have higher blood levels of PSA than average. Men with prostate cancer may have very high levels. However, PSA levels can also be normal in both conditions. It is a useful marker of prostate gland size and activity, but is not a specific test for prostate cancer.

Bulbourethral and urethral glands

The secretions of these glands lubricate the male reproductive tract, but they may also contain antibodies that can affect fertility. These anti-sperm antibodies can prevent the spermatozoa moving properly and prevent them from fertilising the ovum (egg).

What is normal semen?

Semen is usually white or grey, but can occasionally appear yellowish. Pink or red semen suggests that blood is present. Although this is only rarely due to a serious health problem, men with semen that seems bloodstained should seek advice from their family doctor.Semen clots almost immediately after ejaculation, forming a sticky, jelly-like liquid. It will liquefy again in 5 to 40 minutes. It is quite normal for semen to form jelly-like globules and this does not indicate any health or fertility problem. Failure of clotting and subsequent liquefaction can cause fertility problems. The average volume of semen produced at ejaculation is 2 to 5ml. Volumes consistently less than 1.5ml (hypospermia) or more than 5.5ml (hyperspermia) are probably abnormal. Lower volumes may occur after very frequent ejaculation and higher volumes are seen after prolonged abstinence.

The World Health Organisation provides a definition of a 'normal' sperm count:

-the concentration of spermatozoa should be at least 20 million per ml.
-the total volume of semen should be at least 2ml.
-the total number of spermatozoa in the ejaculate should be at least 40 million.
-at least 75 per cent of the spermatozoa should be alive (it is normal for up to 25 per cent to be dead).
-at least 30 per cent of the spermatozoa should be of normal shape and form.
-at least 25 per cent of the spermatozoa should be swimming with rapid forward movement.
-at least 50 per cent of the spermatozoa should be swimming forward, even if only sluggishly.

It is quite surprising how many dead and abnormal sperm can be present in a 'normal' sample.

Measuring sperm count is a very technical business and results can be affected by many factors, including the length of time between ejaculation and semen sample analysis, and how the sample is kept when being transported to the lab.

There can be enormous variation in sperm count in an individual, even over a few days. It is important that at least two, preferably three or more, samples are analysed, each at least two to three weeks apart. A single sample is inadequate to assess semen quality.

Sperm count is only an indication of fertilising capacity and a normal count does not guarantee success. In addition, more is not necessarily better, as too high a sperm count can also result in fertility problems.

Is male fertility declining?

Numerous articles have suggested that sperm counts are falling and that male fertility is in decline. One of the most widely reported was a 1992 paper by Carlson and collaborators1 that looked back at 61 research papers published since 1938, and concluded that the average sperm count had dropped by 42 per cent. However, their analysis was flawed in many ways and their conclusions cannot be considered reliable. Several more retrospective studies of semen quality have been published since, with conflicting results. Some suggest that sperm counts are declining, others that there is no change. Even if sperm counts are falling, the effect on fertility is also unclear. At present, there is no clear evidence one way or the other.

Could environmental factors be affecting male fertility today?

Many more environmental factors that can affect male fertility exist today than 50 years ago.

The factors include:
-endocrine disrupters
-toxic pollutants
-sexually transmitted infections
-zinc deficiency
-alcoholism
-smoking
-anabolic steroid use
-ionising radiation.

Endocrine disrupters are chemicals present in the environment that, by virtue of their ability to adversely affect the endocrine (hormonal) system, cause health consequences. Several industrial pollutants can affect fertility, as can smoking and alcohol use. Some of these toxins can cause reproductive disorders, neurological disease, immune system disorders and cancer.