FIGURE 7 Metabolism of testosterone. Most of the testosterone secreted each day is degraded by the liver and other tissues by reduction of the A ring, oxidation of the 17 hydroxyl group, and conjugation with polar substituents. Conversion to 5a-dihydrotesto-sterone takes place in target cells catalyzed mainly by the type II dehydrogenase and in nontarget cells mainly but not exclusively by the type I dehydrogenase. Aromatization of testosterone to estradiol may occur directly or after conversion to androstenedione. Note that 5-a dihydrotestosterone cannot be aromatized or reconverted to testosterone.

regulates expression of a cadre of genes that are characteristic of each particular target cell. These events are summarized in Fig. 8. Some nongenomic actions of testosterone have also been described and include rapid increases in intracellular calcium. We do not understand the cellular mechanisms of these effects or their physiological importance.

Effects on the Male Genital Tract

Testosterone promotes growth, differentiation, and function of accessory organs of reproduction. Its effects on growth of the genital tract begin early in embryonic life and are not completed until adolescence, after an interruption of more than a decade. Maintenance of normal reproductive function in the adult also depends on continued testosterone secretion. The secretory epithelia of the seminal vesicles and prostate atrophy after castration but can be restored with injections of androgen.

Effects on Secondary Sexual Characteristics

In addition to its effect on organs directly related to transport and delivery of sperm, testosterone affects a variety of other tissues and thus contributes to the morphological and psychological components of masculinity. These characteristics are clearly an integral part of reproduction, for they are related to the attractiveness of the male to the female. During early adolescence, androgens that arise from the adrenals and later from the testes stimulate growth of pubic hair. Growth of chest, axillary, and facial hair is also stimulated, but scalp hair is affected in the opposite manner. Recession of hair at the temples is a typical response to androgen, and adequate amounts allow expression of genes for baldness. Growth

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