Human Biochemistry Volume 2 - Murray R. 1993

Biochemistry of Intracellular and Intercellular Communications
Sex Hormones
Physiological Effects of Testicular Hormones

Androgens, primarily testosterone and DHT, are involved in 1) sexual differentiation, 2) Spermatogenesis, 3) The Development of secondary sex characteristics and structures, 4) anabolic processes and Gene regulation, and 5) male-typical Sexual Behavior (Fig. 50.4). This vast array of androgen-dependent processes makes it difficult to neatly divide Tissues into target and non-target categories. In a narrower sense, target tissues can be classified based on whether they respond primarily to testosterone or to DHT. Classical DHT target Cells (which accordingly exhibit the highest 5α-reductase activity) include the Prostate Gland, Seminal Vesicles, external genitalia, and genital Skin. Targets for testosterone include embryonic Wolffian structures, spermatogonia, Muscle, bone, Kidneys, and the Brain. The specific androgen responsible for regulating many of the other processes mentioned above has not yet been definitively identified.

Sexual Differentiation

Sexual differentiation is the primary effect of androgens. Because it offers a unique opportunity to understand how individual Hormones influence tissue differentiation, we will examine this topic in detail in a separate section below.

Spermatogenesis

The dependence of spermatogenesis on testosterone is unquestionable. However, the exact role of the hormone in this process remains to be established. It is likely that testosterone acts at relatively late stages of spermatogenesis, since the hormone fails to induce spermatogenesis in immature Testes and, furthermore, does not restore sperm production in hypophysectomized animals in which this function has regressed.

Maturation of Accessory Sex Tissues

Increased testosterone production during Puberty drives the maturation of accessory sex Organs. The Penis, prostate, and seminiferous tubules increase in size. Skin and Hair structures also change: the fine vellus hair typical of childhood becomes coarser, and delicate skin thickens. Facial hair begins to grow, the vocal cords thicken, and Sebaceous Glands enlarge.

Anabolic Processes

The onset of puberty is accompanied by a rapid spurt in musculoskeletal growth. The Cartilage of the epiphyseal growth plates expands, and new Bone Formation begins. This bone elongation is accompanied by a significant increase in Skeletal Muscle mass. Although analyzing these effects at THE MOLECULAR LEVEL is challenging, some information is available. Evidently, androgens induce Protein Synthesis in target tissues via the well-established mechanism characteristic of other Steroid Hormones: the hormone-receptor complex recognizes specific Chromatin regions and selectively activates certain genes (see Ch. 44). This is presumably how the synthesis of ABP, aldolase, and Enzymes involved in polyamine production is triggered. Similar mechanisms likely operate in androgen-induced muscle hypertrophy, although the specific Proteins (genes) responsible have not yet been identified.