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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