4 Preparing for conception
4.3 Gamete production in men
A sexually mature man is producing sperm all the time at a rate of around 300–600 per gram of testis per second. This provides the 500 million or so which are released at each ejaculation. But the formation of an individual sperm takes about nine weeks (64 days). Sperm are produced in the testes, and production is most efficient at a temperature several degrees lower than the normal body temperature of 371°C. For this reason the testes (plural of testis) are suspended outside the body cavity in the scrotum, and if the testes fail to descend from their original position near the kidneys to this location during development, sperm production will not occur.
Figure 10b shows the structure of the adult testis. It consists of a large number of coiled tubes, called seminiferous tubules, which empty into a collecting area called the rete testis. From here larger tubes, the vasa efferentia, carry the mature sperm to the epididymis, where various substances are added to improve sperm ‘quality’ (this will be explained below). From the epididymis leads the muscular-walled vas deferens, contractions of which help ejaculation. The vas deferens empties into the urethra at the base of the penis (Figure 10a).
Figure 10, (a) The male reproductive system. (b) A section of testis; for simplicity, the figure shows only one seminiferous tubule per lobe; in reality, there are thousands. One of the tubules is shown extended
Sperm production takes place in the seminiferous tubules. There are three stages to the process: a mitotic phase, in which several rounds of division mean that cell numbers are increased; a meiotic division which reduces the chromosome number in each cell and provides individual allele combinations; and a maturation and packaging phase, in which the chromosomes are readied for transport in sperm outside the man's body. The cells destined to become sperm are known as spermatogonia, and they undergo several rounds of mitosis until a population of cells known as spermatocytes is produced. Some of these enter meiosis, and after this a population of spermatids results, which are ready to be packaged into sperm. Figure 11 shows a mature sperm: it has a head, a midpiece and a tail. There is also an attachment to the head, an enzyme-containing vesicle called the acrosome, which is vital for fertilization (see below).
The head contains the chromosomes, tightly packed with protein so that they can fit into the small space available. The midpiece is the power supply of the sperm, as it contains the mitochondria which provide the energy needed for the sperm to swim. Swimming is accomplished by means of the tail, which can beat rapidly, although does not do so until ejaculation.
Sperm formation is quite a complicated sequence of events, and of the spermatogonia that begin the process, only a proportion will successfully complete it, the rest dying at different stages along the way. During the whole process, the developing cells move from the outer layer of the seminiferous tubule inwards towards the lumen (the hollow part) of the tube. This means that a cross-section taken across the seminiferous tubule will show a characteristic array of cells at different stages of sperm development, as shown in Figure 12. By the time the cells reach the lumen they are complete sperm, but they are not yet able to fertilize an egg. For this they need to be bathed in secretions from various sources. Just what these various secretions do is not entirely clear, but some which are sugars seem to act as a source of energy for the swimming process. One source of secretion is the epididymis (Figure 10b); another essential source is the prostate gland (shown in Figure 10a). The whole mixture of sperm cells and the secretions they are bathed in is called semen, and this is what is ejaculated.
These, then, are the mechanics of the process, but how is it controlled? We shall now take a brief look at the control mechanisms.