Briefly describe the major hormonal and physical changes that take place at puberty in males.

Outline:

·        Onset of puberty

·        Roles of LH, FSH and testosterone

·        Actions of testosterone on:

- sex organs

- growth

Essay:

            Puberty is defined as the period when the endocrine and gametogenic functions of the gonads have first developed to the point where reproduction is possible. The transition from a nonreproductive to a reproductive state during puberty requires maturation of the entire hypothalamic-pituitary-gonadal axis. Before this maturation occurs, plasma LH and FSH levels are low. The gradual maturing of the hypothalamic neurons during puberty leads to an increased synthesis and release of GnRH.

            Beginning at an average age of 10 to 11 years, and ending at an average age of 15 to 17 years, males develop full reproductive function, Leydig cell proliferation, and adult levels of androgenic hormones. They achieve adult size and function of the accessory organs of reproduction, complete secondary sexual characteristics, and adult musculature. The androgens are the hormones responsible for these changes of which the principal hormone is testosterone. The secretion of testosterone is under the control of LH, and the mechanism by which LH stimulates the Leydig cells involves increased formation of cAMP. Testosterone is reduced in peripheral tissues to give rise to two other potent androgens, dihydrotestosterone and 5a-androstenediol.

            The extracellular effects of testosterone and related androgens can be divided into two major categories: effects that pertain specifically to reproductive function and secondary characteristics and effects that pertain more generally to stimulation of nonreproductive tissue growth and maturation. Testosterone diffuses freely into cells. The androgen hormone-receptor complex formed in the cytoplasm then dimerizes and moves into the nucleus, where it interacts with target DNA molecules and transcription factors.

            In androgen target tissues, such as the prostate gland and seminal vesicles, polyamine synthesis is stimulated by testosterone, and these compounds in turn increase RNA synthesis. Androgens also stimulate remarkable growth of these accessory organs of reproduction. This growth is characterized by hypertrophy and hyperplasia of the epithelial cells, stromal components, and blood vessels. Testosterone also causes enlargement of the penis and the seminal vesicles. Dihydrotestosterone stimulates growth of the scrotum and prostate and its secretions.

            Dihydrotestosterone or 5a-androstenediol stimulates the hair follicles and produces a typical male pattern of hair growth characterized by beard growth, a diamond-shaped pubic escutcheon, large amounts of pubic hair, and recession of the temporal hairline. They are also responsible for increased production of sebum by the sebaceous glands, and the consequent development of acne during puberty. On the other hand, testosterone causes enlargement of the larynx and thickening of the vocal cords, which results in a deeper voice. Testosterone also enlarges muscle mass and broadens shoulders.

            Testosterone itself first stimulates the pubertal growth spurt. It then terminates linear growth by closing the epiphyseal growth centers. Estradiol is an essential partner of testosterone’s action on bone maturation in males. Testosterone has important actions on lipid metabolism. It increases levels of LDL and decreases level of HDL. It also favors accumulation of upper body, abdominal and visceral fat.

            Certain other diverse androgenic actions can be ascribed to testosterone. They include stimulation of erythropoietin synthesis and maturation of erythroid precursors; stimulation of renal sodium reabsorption; suppression of hepatic synthesis of sex-steroid binding globulin, cortisol-binding globulin and thyroxine-binding globulin; suppression of mammary gland growth; initiation of sexual drive and stimulation of aggressive behavior.