sperm

sperm A mature sperm (spermatozoon) is a complex and highly specialized cell, genetically programmed, and unique in both function and shape. Its production — spermatogenesis — involves cell divisions and reorganization of chromosomal material, which generates genetic diversity. After extensive cell modelling it eventually becomes mobile and capable of penetrating and fertilizing an egg.

Spermatogenesis occurs in the hundreds of seminiferous tubules of the testes, and is dependent on the actions of testosterone produced from cells which lie among these tubules (Leydig cells) and of the gonadotrophic hormones from the pituitary gland. It begins at puberty when the germ cells (spermatogonia), which have been in the testes since fetal life, start dividing by mitosis to produce a small clone of daughter cells with the normal 23 pairs of chromosomes (diploid cells). One of these pairs constitutes the sex chromosomes: in males an X chromosome and a smaller Y chromosome, which carries the male-determining gene. The majority of these cells (now termed primary spermatocytes) push their way through the junctions between the large protective and nourishing cells (Sertoli cells) which lie between them and the lumen of the tubule. In their new environment, created by secretions of the adjacent Sertoli cells, they undergo divisions which halve their number of chromosomes. In the first meiotic division the pairs of chromosomes come together and strands of DNA are swapped between them (crossing over), thus changing the genetic code carried by each chromosome. Eventually the pairs separate and two haploid cells, each containing a single set of 23 chromosomes, are formed. Thus one of these ‘secondary spermaocytes’ contains an X chromosome and the other a Y chromosome. Almost immediately after this first meiotic division a second meiotic division takes place. This involves the separation of the two halves of each single chromosome. These haploid cells — now called spermatids — thus contain 23 single half chromosomes. By this stage the important genetic events have taken place, but these spermatids are still simple round cells and must now undergo extensive remodelling (spermiogenesis) before they are capable of performing their function.

The first stage is the formation of the acrosome, which is an enzyme-rich structure covering the head of the sperm. This is essential for fertilization. Then a tail develops for forward propulsion and mitochondria (energy generators for the cell) form in the midpiece of the sperm. By now, the work of the Sertoli cells in nurturing the primary spermatocytes through the process of spermatogenesis is complete. As these processes have occurred the developing cells have moved closer and closer to the lumen of the seminiferous tubule. Finally they are extruded and released into the tubule (see figure). In humans this whole process of spermatogenesis takes 64 days.

Once in the lumen of the tubule, the sperm are washed away by secretions from the Sertoli cells, and eventually reach the confluence of all the tubules in the single, highly-convoluted tube of the epididymis, which eventually drains into the vas deferens. The passage of sperm from the seminiferous tubules of the testis to the vas deferens takes about 12 days. During this time the sperm are subjected to major environmental changes due to testosterone-dependent secretions within these tubes. As a result, sperm not only acquire the ability to swim (on leaving the testis they are only capable of an infrequent twitch), but they also change in the way they utilize and break down energy substrates, and finally reach full fertilizing capacity. Sperm obtained before their passage through the epididymis are incapable of fertilizing an egg even if implanted directly.

By the time sperm reach the vas deferens they are fully mature and mobile and, due to fluid absorption in the epididymis, they are now densely packed. In fact 1 ml of fluid in the vas deferens contains about 5000 million sperm. Sperm can be stored for as long as five weeks in the tail of the epididyms and the vas deferens until they are released at ejaculation. In the absence of ejaculation sperm dribble into the urethra and are washed away in the urine. In men who have undergone a vasectomy, sperm build up behind the ligation, and are then removed by phagocytes in the epididymis.

Each sperm produced by the testis is only a few thousandths of a millimetre in length and must travel through some 30–40 cm (100 000 times its own length) in the male and then in the female reproductive tract before it reaches the Fallopian tubes and can perform the function for which it was intended. Needless to say, many fewer than 1 in a million ever complete this hazardous journey.

Saffron Whitehead

See urogenital system.See also ejaculation; semen; testes.