Male reproductive physiology

Question 1

HPG axis in men

Answer 1

• GnRH-> FSH/LH-> testosterone synthesis and release from testicles (T converted to E2 and DHT locally)
• T-> in bone and brain, T->DHT to form secondary sex characteristics
• LH causes leydig cells (located btwn seminiferous tubules) in the testis to synthesize/secrete T
• T then negatively feeds back on the hypothal to reduce GnRH and ant pit to reduce LH secretion
• FSH causes spermatogonia in seminiferous tubules to commence gametogenesis (spermatogenesis), sertoli cells in seminiferous tubules have a supporting role
• In males it is possible to have steroidogenesis w/o gametogenesis (this is not possible in women), however in both men and women gametogenesis is not possible w/o steroidogenesis (requires T)

Question 2

Hormone negative feedback in men

Answer 2

• LH causes production of T by leydig cells, the T then inhibits LH release from pit and GnRH release from hypothal
• Sertoli cells produce inhibin (synthesized in response to high FSH and T from leydig cells) which inhibits FSH release from ant pit
• The frequency of pulses determines the effects of GnRH/LH/FSH (higher frequency pulses-> more sex steroids)

Question 3

Effects of androgens on androgen-dependent tissues 1

Answer 3

• Most of these actions are due to cytoplasmic conversion of T->DHT by 5 alpha reductase at the target tissues
• Hair: stimulates growth of hair on face, chest, armpits, and genitals
• Increases libido
• Brain: increases concentration and memory (partially due to local conversion to E2)
• Causes deepening of voice (permanent)
• Increases lean muscle mass, decreases fat

Question 4

Effects of androgens on androgen-dependent tissues 2

Answer 4

• Increases bone density (due to local conversion to E)
• Triggers nl development of sex organs, enables spermatogenesis (spermatogenesis initiated by FSH and maintained by high levels of T)
• T stimulates production of androgen-binding protein by sertoli cells to increase androgen concentrations in seminiferous tubules
• Testosterone levels are highest in testes/epididymis, where it needs to be concentrated for spermatogenesis and sperm maturation (maturation occurs in epididymis)

Question 5

Testicular descent

Answer 5

• This happens b/c nl spermatogenesis requires lower than body temp
• Under the influence of T the gubernaculum shortens and pulls the testes down into the scrotum from a retroperitoneal position in the lumbar region
• Failure of testes to descend is cryptorchidism, if this is uncorrected and bilateral the man will be infertile
• Testosterone production is unaffected by higher temps, but spermatogenesis cannot occur
• Testes maintain a lower temp via countercurrent heat exchange system of the pampiniform plexus