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 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. Between the tubules in the testes are nests of cells containing lipid granules, the interstitial cells of Leydig (Leydig cells), which secrete testosterone into the bloodstream. The walls of the seminiferous tubules are lined by primitive germ cells and Sertoli cells, large, complex glycogen-containing cells that stretch from the basal lamina of the tubule to the lumen. The fluid in the lumen of the seminiferous tubules contains very little protein and glucose but is rich in androgens, estrogens, K+, inositol, and glutamic and aspartic acids.
Spermatogonia, the primitive germ cells next to the basal lamina of the seminiferous tubules, mature into primary spermatocytes beginning during adolescence (Figure 23–1). The primary spermatocytes undergo meiotic division, reducing the number of chromosomes. In this two-stage process, they divide into secondary spermatocytes and then into spermatids, which contain the haploid number of 23 chromosomes. The spermatids mature into spermatozoa (sperm). As a single spermatogonium divides and matures, its descendants remain tied together by cytoplasmic bridges until the late spermatid stage. The formation of a mature sperm from a primitive germ cell by spermatogenesis takes approximately 74 days.
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 from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 10th ed. New York, NY: McGraw-Hill; 2003.)
Each sperm is made up of a head of chromosomal material and a flagellum tail for motility (Figure 23–2). Covering the head like a cap is the acrosome, a lysosome-like organelle rich in enzymes involved in sperm penetration of the ovum. The motile tail of the sperm is wrapped in its proximal portion by a sheath holding numerous mitochondria.
Human spermatozoon, profile view. Note the ...