After studying this chapter, you should be able to:
- Name the key hormones secreted by Leydig cells and Sertoli cells of the testes.
- Outline the steps involved in spermatogenesis.
- Outline the mechanisms that produce erection and ejaculation.
- Know the general structure of testosterone, and describe its biosynthesis, transport, metabolism, and actions.
- Describe the processes involved in regulation of testosterone secretion.
The role for a functional, secreting testis in the formation of male genitalia, the action of male hormones on the brain in early development, and development of the male reproductive system through adolescence and into adulthood were discussed in the previous chapter. As observed in the female, male gonads have a dual function: the production of germ cells (gametogenesis) and the secretion of sex hormones. The androgens are the steroid sex hormones that are masculinizing in their action. The testes secrete large amounts of androgens, principally testosterone, but they also secrete small amounts of estrogens. Unlike that observed in females, male gonadotropin secretion is noncyclical, and once mature, male gonadal function slowly declines with advancing age, but the ability to produce viable gametes persists. In this chapter we will focus discussion on the structure and physiology of the mature male reproductive system.
The testes are made up of loops of convoluted seminiferous tubules, in the walls of which the spermatozoa are formed from the primitive germ cells (spermatogenesis). Both ends of each loop drain into a network of ducts in the head of the epididymis. From there, spermatozoa pass through the tail of the epididymis into the vas deferens. They enter through the ejaculatory ducts into the urethra in the body of the prostate at the time of ejaculation (Figure 23–1). Between the tubules in the testes are nests of cells containing lipid granules, the interstitial cells of Leydig (Figures 23–2 and 23–3), which secrete testosterone into the bloodstream. The spermatic arteries to the testes are tortuous, and blood in them runs parallel but in the opposite direction to blood in the pampiniform plexus of spermatic veins. This anatomic arrangement may permit countercurrent exchange of heat and testosterone. The principles of countercurrent exchange are considered in detail in relation to the kidney in Chapter 37.
Anatomical features of the male reproductive system. Left: Male reproductive system. Right: Duct system of the testis.
Seminiferous epithelium. Note that maturing germ cells remain connected by cytoplasmic bridges through the early spermatid stage and that these cells are closely invested by Sertoli cell cytoplasm as they move from the basal lamina to the lumen. (Reproduced with permission ...