The male reproductive axis has an important "loophole" based on the fact that intratesticular levels of testosterone need to be significantly higher than circulating levels to maintain normal rates of spermatogenesis. However, it is the circulating levels of testosterone and estradiol that provide negative feedback to the pituitary and hypothalamus. This means that exogenous administration of testosterone can raise circulating levels of testosterone and estradiol enough to inhibit LH but not enough to accumulate in the testis at the required concentration for normal spermatogenesis. This "loophole" is currently being investigated as a possible strategy for developing a male oral contraceptive. It is also the basis for sterility in some cases of steroid abuse in men.

The epididymis epithelium is secretory and adds various components to the seminal fluid. Spermatozoa enter the head of the epididymis weakly motile, but become strongly unidirectionally motile by the time they exit the tail. Spermatozoa undergo decapacitation, which involves changes in the cell membrane to prevent the acrosome reaction before contact with an egg. Sperm become capacitated by the female reproductive tract within the oviduct. The function of the epididymis relies on luminal testosterone-ABP complexes from the seminiferous tubules and testosterone from blood.

The normal set point is determined by relatively low levels of circulating testosterone, not intratesticular testosterone. The difference in intratesticular testosterone versus circulating testosterone concentrations is important in the hypothalamic-pituitary-testis axis. Administration of testosterone or an androgenic analogue increases circulating testosterone levels, which in turn increases negative feedback on the release of LH. Decreased LH levels diminish Leydig cell activity and intratesticular production of androgen. Lower intratesticular testosterone levels result in reduced sperm production and can cause infertility.

Sperm are stored in the tail of the epididymis and vas deferens for several months without loss of viability. The vas deferens has a thick muscularis that is richly innervated by sympathetic nerves. Repeated tactile stimulation of the penis during coitus leads to sympathetic stimulation of the muscularis, causing peristaltic contractions.

Emission is the emptying of the contents of the vas deferens into the prostatic urethra and immediately precedes ejaculation, which is the propulsion of semen out of the male urethra.
Thereisanimportant“loophole”inthemalereproductiveaxisthatisbasedonthefactthatintratesticular levels of testosterone needtobegreaterthan100-foldhigherthancirculatinglevelsofthehormonetomaintainnormalratesofspermatogenesis,yetitisthecirculating levels of testosterone (and estradiol) thatprovidethenegativefeedbacktothepituitaryandhypothalamus.ThismeansthatexogenousadministrationoftestosteronecanraisecirculatinglevelsoftestosteroneandestradiolsufficienttoinhibitLHbutnotsufficienttoaccumulateinthetestisattherequiredconcentrationfornormalspermatogenesis.However,thedecreasedLHlevelswilldiminishintratesticularproductionoftestosteronebyLeydigcells,whichresultsinreducedlevels ofspermatogenesis(Fig.44.11 ).This“loophole”iscurrentlybeinginvestigatedasapossiblestrategyfordevelopingamale oral contraceptive. Itisalsothebasisforsterility insomecasesofsteroid abuse inmen.
The epithelium of the epididymis is secretory and adds numerous components to the seminal fluid. Spermatozoa that enter the head of the epididymis are weakly motile but are strongly unidirectionally motile by the time they exit the tail. Spermatozoa also undergo the process of decapacitation, which involves changes in the cell membrane to prevent spermatozoa from undergoing the acrosome reaction before contact with an egg (see later). Sperm become capacitated by the female reproductive tract within the oviduct. The function of the epididymis is dependent on luminal testosterone-ABP complexes that come from the seminiferous tubules and on testosterone from blood.
Normal set point determined by relatively low levels of circulating T, not intratesticular T •Fig. 44.11 Thedifferenceinintratesticulartestosteroneversuscirculatingtestosteroneconcentrationsanditsimportanceinthehypothalamic-pituitary-testisaxis.Upper panel, Feedbackloopinanormaladultman.Lower panel, Administrationoftestosterone(oranandrogenicanalogue)increasescirculatingtestosterone(androgen)levels,whichinturnincreasenegativefeedbackonreleaseofLH.DecreasedLHlevelsdiminishLeydigcellactivityandintratesticularproductionofandrogen.Loweredintratesticulartestosteronelevelsresultinreducedspermproductionandcancauseinfertility.(Theinhibinfeedbackloophasbeenomittedfromthisdiagram.)(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.) 2.
Sperm storage and emission. Sperm are stored in the tail The vas deferens has a very thick muscularis that is richly of the epididymis and vas deferens for several months innervated by sympathetic nerves. Normally in response without loss of viability. The primary function of the vas to repeated tactile stimulation of the penis during coitus, deferens, besides providing a storage site, is to propel the muscularis of the vas deferens receives bursts of symsperm during sexual intercourse into the male urethra. pathetic stimulation that cause peristaltic contractions.
Emptying of the contents of the vas deferens into the prostatic urethra is called emission. Emission immediately precedes ejaculation, which is the propulsion of semen out of the male urethra.
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