1.3.4 Hormonal control of sperm production
The most important hormone involved in controlling sperm production is a steroid called testosterone. This is produced in the testis itself, by the Leydig cells (see Figure 12a). The testosterone is released from the Leydig cells between the tubules, and taken up by the neighbouring Sertoli cells. The Leydig cells are stimulated to make testosterone by two other hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are both produced by the pituitary gland and reach the testis via the blood. (Although subsequently found to be present in males, these hormones were originally named in relation to their role in females.) Once the Sertoli cells have taken up the testosterone, they convert it to a much more effective substance, dihydrotestosterone (DHT), and this is what stimulates the spermatogonia to enter the sperm production pathway, First by undergoing several rounds of mitosis, then one meiosis. FSH and LH also play a role in maintaining the pathway, and in producing high local levels of DHT within the testes.
Q Why do you think it might be important to have local differences in hormone levels?
A So that only some of the spermatogonia embark on the sperm production pathway.
Sperm do not live for more than a few days once formed, so cannot be stored for long. The production of sperm to ensure a constant supply is illustrated in Figure 12b, where different ‘wedges’ of the tubule are at different stages of development. This shows that different areas were started off at different times.
To summarize, the production of good-quality, fertile sperm requires First that the testis be at the correct temperature. Spermatogonia are triggered to enter the developmental pathway by a cocktail of hormones, in particular DHT, the production of which is controlled by the brain. Sperm production involves three phases: mitosis, meiosis, and packaging. The completed sperm then have to be mixed with a variety of substances before they are ejaculated. Only sperm that contain a correct set of chromosomes, can swim powerfully, and have an intact acrosome, are candidates for fertilization.