Each of the two testicles — separated from each other in the scrotum by a membrane, the septum — is made up of two compartments. In terms of volume 95 per cent of the testicle is devoted to sperm production. There are approximately 250 lobules, and if you were to lay all the tubes in the lobules end to end they would have a combined length of about 500 metres. The inner wall of the tubes contains germ cells which after a process of divisions produce young but not yet mature sperm cells. Between ten and a hundred million sperm cells are produced every day.
The unbelievably dense network of fine seminal tubes constituting the sperm-producing section of the testicles was described in the seventeenth century by Reinier de Graaf, though the actual discovery was made by De Graaf’s teacher, Professor Johannes van Horne of Leiden University. During a study placement in France De Graaf had used bull’s testicles for his research. These were easily obtainable, but turned out to be less than ideal for research purposes. He finally opted for the testicles of an unusual little creature, the dormouse. Its body weight is approximately 100 g and the testicles weigh about 1 g each. De Graaf removed the outer membrane from the dormice testicles and
then submerged them in a glass of water. When the glass was gently shaken the testicles simply fell to pieces. ‘One can clearly see that the testicles consist wholly of tiny tubes,’ wrote De Graaf in his book (Van Horne had previously stated that the testicle was nothing but ‘a collection of tiny threads’). This was in fact plagiarism by De Graaf, but after a long correspondence with the Royal Society he was credited with the discovery of the ‘threads’. For the sake of completeness he even had to forward a dissected dormouse testicle that he had preserved in alcohol.
Back to anatomy. Between the tubes, the ‘threads’ in which the sperm cells are formed, there are blood vessels, the Sertoli cells and the Leydig cells. The Sertoli cells, the support cells, form part of the blood-testicle barrier, controlling the emergence of the mature sperm cells: all nutriment for the maturing spermatozoa must first pass through them.
The Italian physiologist Enrico Sertoli (1842-1910) was still a medical student when he observed these ‘nurse’ cells in 1862. In the young embryo they make the Anti-Mullerian hormone (the Mullerian duct produces female sex organs, the Wolffian duct male ones), which ensures that the male embryo actually acquires normal sexual characteristics. After puberty the Sertoli cells produce the hormone inhibine.
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The Leydig cells, named after the German anatomist Franz von Leydig (1842-1910), make up approximately 5 per cent of the total volume of the testicles, and use cholesterol to produce, from puberty onwards, about 7 milligrams per day of the male sex hormone testosterone. They do this in response to the lh (luteinizing hormone) transmitted by the hypophysis or pituitary gland at the base of the brain.
Another regulating substance transmitted by the hypophysis is the Follicle Stimulating Hormone (fsh), which regulates sperm cell formation.
The seminal duct (the ductus deferens, or vas deferens) runs for a distance of between 30 and 40 cm from the epididymis to the verumontanum, an elevation or crest in the wall of the urethra in the centre of the prostate.
Also near the prostate, behind the bladder, are the two seminal glands (vesiculae seminalis), which despite what their name suggests do not store sperm cells but produce fluid, part of the transport medium for the sperm cells. After all, in an ejaculation it is not only sperm cells that are expelled. An ejaculate consists in large part of fluid originating from the seminal glands and the prostate. The sperm cells account for only a small percentage of the total volume. In older medical literature a distinction is made between the section of the sperm cell constituting the ‘noblest part’, the ‘aqueous elements’ from the seminal glands, and the ‘oleagenous’ section from the prostate.
In the late 1950s Japanese researchers conducted experiments with x-ray contrast media that showed clearly how the different sections emerged. First sperm cells were forced from the epididymis in the direction of the ampoule of the seminal duct. The ampoule is a protuberance close to the spot where the seminal duct discharges, in which a quantity of sperm can be stored. Then the muscles around the seminal glands contract, and the sperm cells with fluid originating from the seminal glands are forced together into the prostatic section of the urethra, where they are mixed with prostate liquid. One striking feature is that some sperm cells also return to the ampoule and seminal glands.
The seminal glands particularly contain many nooks and crannies where sperm cells can linger for a considerable period. A practical result of this is that for a long time after sterilization sperms may sometimes be visible in the ejaculate.