Y khoa, y dược - The reproductive system: Part A

27 The Reproductive System: Part AReproductive SystemPrimary sex organs (gonads): testes and ovariesProduce sex cells (gametes)Secrete steroid sex hormonesAndrogens (males) Estrogens and progesterone (females)Accessory reproductive organs: ducts, glands, and external genitaliaReproductive SystemSex hormones play roles inDevelopment and function of the reproductive organsSexual behavior and drivesGrowth and development of many other organs and tissuesMale Reproductive SystemTestes (within the scrotum) produce spermSperm are delivered to the exterior through a system of ducts Epididymis, ductus deferens, ejaculatory duct, and the urethraMale Reproductive SystemAccessory sex glands: seminal vesicles, prostate, and bulbourethral glands Empty secretions into the ducts during ejaculation Figure 27.1PeritoneumSeminalvesicle Ampulla ofductus deferensEjaculatoryduct RectumProstateBulbourethralgland AnusBulb of penisEpididymisUreterUrinary bladderProstaticurethra PubisMembranousurethra Urogenitaldiaphragm Corpuscavernosum Corpusspongiosum Glans penisPrepuceExternalurethral orifice Spongy urethraTestisScrotumDuctus (vas)deferens The ScrotumSac of skin and superficial fasciaHangs outside the abdominopelvic cavityContains paired testes 3C lower than core body temperature (temperature necessary for sperm production)The ScrotumTemperature is kept constant by two sets of musclesSmooth muscle that wrinkles scrotal skin (dartos muscle)Bands of skeletal muscle that elevate the testes (cremaster muscles)Figure 27.2PenisInternal spermaticfasciaSuperficial inguinalring (end of inguinal canal)Urinary bladderSpermatic cordDuctus (vas) deferensAutonomicnerve fibersTesticular arteryEpididymisPampiniformvenous plexusTunica vaginalis(from peritoneum)Tunica albugineaof testisMiddle septumof scrotumCremaster muscleExternal spermaticfasciaSuperficial fasciacontaining dartos muscleSkinScrotumThe TestesEach is surrounded by two tunics Tunica vaginalis, derived from peritoneumTunica albuginea, the fibrous capsuleSepta divide the testis into 250–300 lobules, each containing 1–4 seminiferous tubules (site of sperm production)The TestesSperm are conveyed throughSeminiferous tubulesTubulus rectusRete testisEfferent ductulesEpididymisThe TestesBlood supply comes from the testicular arteries and testicular veins Spermatic cord encloses nerve fibers, blood vessels, and lymphatics that supply the testesFigure 27.3aHead of epididymisBody of epididymisTail of epididymisEfferent ductuleRete testisStraight tubuleDuct of epididymisDuctus (vas)deferensLobuleTestisSeptumTunica albugineaTunica vaginalisCavity oftunica vaginalis Spermatic cordSeminiferoustubuleBlood vesselsand nerves(a)The TestesInterstitial (Leydig) cells outside the seminiferous tubules produce androgensFigure 27.3cSpermatogeniccells in tubuleepitheliumInterstitial cellsAreolarconnective tissueSpermMyoidcells (c)SeminiferoustubuleThe PenisExternal genitalia are the scrotum and the penisPenis is the male copulatory organThe PenisPenis consists ofRoot and shaft that ends in the glans penisPrepuce, or foreskin—the cuff of loose skin covering the glansCircumcision is the surgical removal of the foreskin CruraThe proximal end surrounded by ischiocavernosus muscle; anchors penis to the pubic archThe PenisSpongy urethra and three cylindrical bodies of erectile tissue (spongy network of connective tissue and smooth muscle with vascular spaces)Corpus spongiosum surrounds the urethra and expands to form the glans and bulb Corpora cavernosa are paired dorsal erectile bodies Erection: erectile tissue fills with blood, causing the penis to enlarge and become rigidFigure 27.4Urinary bladderDorsal vesselsand nerves SkinDeep arteries(b)(a)Bulbourethral gland and ductUrogenital diaphragmBulb of penisCorpora cavernosaCorpus spongiosumSection of (b)Spongy urethraGlans penisPrepuce (foreskin)External urethral orificeCorpora cavernosaUrethraTunica albuginea of erectile bodiesCorpus spongiosumCrus of penisBulbourethral duct openingDuctus deferensAmpulla of ductus deferensProstatic urethraOrifices of prostatic ductsProstateMembranous urethraRoot of penisShaft (body) of penisEjaculatory ductSeminal vesicleEpididymisTestisUreterThe Male Duct SystemEpididymisDuctus deferensEjaculatory ductUrethraEpididymisHead: contains the efferent ductules Duct of the epididymisMicrovilli (stereocilia) absorb testicular fluid and pass nutrients to stored spermNonmotile sperm enter, pass slowly through, and become motileDuring ejaculation the epididymis contracts, expelling sperm into the ductus deferensDuctus Deferens and Ejaculatory DuctDuctus deferensPasses through the inguinal canalExpands to form the ampulla and then joins the duct of the seminal vesicle to form the ejaculatory ductPropels sperm from the epididymis to the urethraVasectomy: cutting and ligating the ductus deferens, which is a nearly 100% effective form of birth controlUrethraConveys both urine and semen (at different times)Has three regions Prostatic urethraMembranous urethraSpongy (penile) urethraFigure 27.4Urinary bladderDorsal vesselsand nerves SkinDeep arteries(b)(a)Bulbourethral gland and ductUrogenital diaphragmBulb of penisCorpora cavernosaCorpus spongiosumSection of (b)Spongy urethraGlans penisPrepuce (foreskin)External urethral orificeCorpora cavernosaUrethraTunica albuginea of erectile bodiesCorpus spongiosumCrus of penisBulbourethral duct openingDuctus deferensAmpulla of ductus deferensProstatic urethraOrifices of prostatic ductsProstateMembranous urethraRoot of penisShaft (body) of penisEjaculatory ductSeminal vesicleEpididymisTestisUreterAccessory Glands: Seminal VesiclesProduces viscous alkaline seminal fluid Fructose, ascorbic acid, coagulating enzyme (vesiculase), and prostaglandins 70% of the volume of semenDuct of seminal vesicle joins the ductus deferens to form the ejaculatory ductAccessory Glands: Prostate Encircles part of the urethra inferior to the bladderSecretes milky, slightly acid fluid:Contains citrate, enzymes, and prostate-specific antigen (PSA)Plays a role in the activation of spermEnters the prostatic urethra during ejaculationAccessory Glands: Bulbourethral Glands (Cowper’s Glands)Pea-sized glands inferior to the prostatePrior to ejaculation, produce thick, clear mucusLubricates the glans penisNeutralizes traces of acidic urine in the urethraFigure 27.4Urinary bladderDorsal vesselsand nerves SkinDeep arteries(b)(a)Bulbourethral gland and ductUrogenital diaphragmBulb of penisCorpora cavernosaCorpus spongiosumSection of (b)Spongy urethraGlans penisPrepuce (foreskin)External urethral orificeCorpora cavernosaUrethraTunica albuginea of erectile bodiesCorpus spongiosumCrus of penisBulbourethral duct openingDuctus deferensAmpulla of ductus deferensProstatic urethraOrifices of prostatic ductsProstateMembranous urethraRoot of penisShaft (body) of penisEjaculatory ductSeminal vesicleEpididymisTestisUreterSemenMixture of sperm and accessory gland secretionsContains nutrients (fructose), protects and activates sperm, and facilitates their movement (e.g., relaxin)Prostaglandins in semenDecrease the viscosity of mucus in the cervixStimulate reverse peristalsis in the uterusSemenAlkalinity neutralizes the acid in the male urethra and female vaginaAntibiotic chemicals destroy certain bacteriaClotting factors coagulate semen just after ejaculation, then fibrinolysin liquefies itOnly 2–5 ml of semen are ejaculated, containing 20–150 million sperm/mlMale Sexual ResponseErectionEnlargement and stiffening of the penis from engorgement of erectile tissue with bloodInitiated by sexual stimuli, including:Touch and mechanical stimulation of the penis Erotic sights, sounds, and smellsCan be induced or inhibited by emotions or higher mental activity Male Sexual ResponseErection:Parasympathetic reflex promotes release of nitric oxide (NO)NO causes erectile tissue to fill with bloodExpansion of the corpora cavernosaCompresses drainage veins and maintains engorgementCorpus spongiosum keeps the urethra open Impotence: the inability to attain erectionMale Sexual ResponseEjaculationPropulsion of semen from the male duct systemSympathetic spinal reflex causesDucts and accessory glands to contract and empty their contentsBladder sphincter muscle to constrict, preventing the expulsion of urineBulbospongiosus muscles to undergo a rapid series of contractions SpermatogenesisSequence of events that produces sperm in the seminiferous tubules of the testes Most body cells are diploid (2n) and containTwo sets of chromosomes (one maternal, one paternal) 23 pairs of homologous chromosomesGametes are haploid (n) and contain23 chromosomes Meiosis Gamete formation involves meiosisNuclear division in the gonads in which the number of chromosomes is halved (from 2n to n)Two consecutive cell divisions (meiosis I and II) following one round of DNA replicationProduces four daughter cellsIntroduces genetic variationMeiosisPLAYAnimation: Telephase IPLAYAnimation: Anaphase IPLAYAnimation: Metaphase IPLAYAnimation: Meiosis IIPLAYAnimation: Prophase IPLAYAnimation: InterphaseFigure 27.6 (1 of 2)MEIOSIS IProphase IProphase events occur, as in mitosis. Additionally, synapsis occurs: Homologous chromosomes come together along their length to form tetrads. During synapsis, the “arms” of homologous chromatids wrap around each other, forming several crossovers. The nonsister chromatids trade segments at points of crossover. Crossover is followed through the diagrams below.Anaphase IUnlike anaphase of mitosis, the centromeres do not separate during anaphase I of meiosis, so the sister chromatids (dyads) remain firmly attached. However, the homologous chromosomes do separate from each other and the dyads move toward opposite poles of the cell.Telophase IThe nuclear membranes re-form around the chromosomal masses, the spindle breaks down, and the chromatin reappears as telophase and cytokinesis are completed. The 2 daughter cells (now haploid) enter a second interphase-like period, called interkinesis, before meiosis II occurs. There is no second replication of DNA before meiosis II.Metaphase IThe tetrads align randomly on the spindle equator in preparation for anaphase.CrossoverInterphase cellCentriolepairs NuclearenvelopeChromatin2n = 4SpindleSisterchromatidsNuclear envelopefragments latein prophase ITetradDyadChromosomesuncoilNuclearenvelopesre-formCleavagefurrowCentromereInterphase eventsAs in mitosis, meiosis is preceded by DNA replication and other preparations for cell division.Figure 27.6 (2 of 2)MEIOSIS IIMeiosis II begins with theproducts of meiosis I (2 haploid daughter cells) and undergoes a mitosis-like nuclear division process referred to as the equational division of meiosis.Prophase IIAfter progressing through the phases of meiosis and cytokinesis, the product is 4 haploid cells, each genetically different from the original mother cell. (During human spermatogenesis, the daughter cells remain interconnected by cytoplasmic extensions during the meiotic phases.)Metaphase IIAnaphase IITelophase IIand cytokinesisProducts ofmeiosis:haploiddaughter cellsFigure 27.5 (1 of 2)Mother cell(before chromosome replication)ChromosomereplicationChromosomereplication2n = 4MITOSISReplicatedchromosomeProphaseChromosomesalign at themetaphase plateSister chromatids separate during anaphase2n2nMetaphaseDaughtercells ofmitosisTetrad formed bysynapsis of replicated homologouschromosomesTetrads align at themetaphase plateHomologous chromosomesseparate but sister chromatids remain together during anaphase INo further chromosomalreplication; sister chromatidsseparate duringanaphase IIDaughter cells of meiosis II(usually gametes)nnnnProphase IMetaphase IDaughter cellsof meiosis IMeiosis IIMEIOSISFigure 27.5 (2 of 2)MITOSISMEIOSISOne, consisting of prophase, metaphase,anaphase, and telophase.Number ofdivisions Synapsis of homologouschromosomesDaughter cell number and genetic compositionRoles in the bodyTwo, each consisting of prophase, metaphase, anaphase, and telophase. DNA replication does not occur between the two nuclear divisions.Does not occur.Occurs during mitosis I; tetrads formed, allowing crossovers.Two. Each diploid (2n) cell is identical tothe mother cell.Four. Each haploid (n) cell contains half as many chromosomes as the mother cell and is genetically different from the mother cell.For development of multicellular adultfrom zygote. Produces cells for growthand tissue repair. Ensures constancy ofgenetic makeup of all body cells.Produces cells for reproduction (gametes). Introduces genetic variability in the gametes and reduces chromosomal number by half so that when fertilization occurs, the normal diploid chromosomal number is restored (in humans, 2n = 46).SpermatogenesisSpermatic cells give rise to spermMitosisSpermatogonia form spermatocytesMeiosisSpermatocytes form spermatids SpermiogenesisSpermatids become spermFigure 27.7aFigure 27.7cBasal laminaSpermatogonium(stem cell)Cytoplasm of adjacentsustentacular cellsSustentacularcell nucleusTight junction betweensustentacular cells Lumen ofseminifer-ous tubuleLate spermatidsEarlyspermatids SecondaryspermatocytesCytoplasmicbridgePrimaryspermatocyteSpermatozoaType B daughter cellType A daughter cellremains at basal laminaas a stem cell(c) A portion of the seminiferous tublule wall, showing the spermato- genic cells surrounded by sustentacular cells (colored gold) Mitosis of SpermatogoniaBegins at pubertySpermatogoniaStem cells in contact with the epithelial basal laminaEach mitotic division  a type A daughter cell and a type B daughter cellMitosis of SpermatogoniaType A cells maintain the germ cell line at the basal laminaType B cells move toward the lumen and develop into primary spermatocytesMeiosis: Spermatocytes to SpermatidsMeiosis IPrimary spermatocyte (2n)  two secondary spermatocytes (n)Meiosis IIEach secondary spermatocyte (n)  two spermatids (n)Spermatid: small nonmotile cells close to the lumen of the tubuleFigure 27.7bBasal laminaSpermatogonium(stem cell)MitosisGrowthLate spermatidsEarly spermatidsSecondaryspermatocytesPrimaryspermatocyteSpermatozoaType B daughter cellEnters meiosis I and moves toadluminalcompartmentMeiosis IcompletedMeiosis IIType A daughter cellremains at basal laminaas a stem cell(b) Events of spermatogenesis, showing the relative position of various spermatogenic cells Spermiogenesis: Spermatids to SpermSpermatids lose excess cytoplasm and form a tail, becoming spermatozoa (sperm)SpermMajor regionsHead: genetic region; nucleus and helmetlike acrosome containing hydrolytic enzymes that enable the sperm to penetrate an eggMidpiece: metabolic region; mitochondria Tail: locomotor region; flagellum Figure 27.8a, bCentriolesSpermatidnucleusGolgiapparatusAcrosomalvesicleMitochondriaApproximately 24 daysExcesscytoplasmNucleusAcrosomeMicrotubulesFlagellumTailMidpieceHead(a)(b)1234567Role of Sustentacular Cells Large supporting cells (Sertoli cells)Extend through the wall of the tubule and surround developing cellsProvide nutrients and signals to dividing cells Dispose of excess cytoplasm sloughed off during spermiogenesisSecrete testicular fluid into lumen for transport of spermRole of Sustentacular CellsTight junctions divide the wall into two compartmentsBasal compartment—spermatogonia and primary spermatocytesAdluminal compartment—meiotically active cells and the tubule lumenFigure 27.7cBasal laminaSpermatogonium(stem cell)Cytoplasm of adjacentsustentacular cellsSustentacularcell nucleusTight junction betweensustentacular cells Lumen ofseminifer-ous tubuleLate spermatidsEarlyspermatids SecondaryspermatocytesCytoplasmicbridgePrimaryspermatocyteSpermatozoaType B daughter cellType A daughter cellremains at basal laminaas a stem cell(c) A portion of the seminiferous tublule wall, showing the spermato- genic cells surrounded by sustentacular cells (colored gold) Role of Sustentacular CellsTight junctions form a blood-testis barrier Prevents sperm antigens from escaping into the blood where they would activate the immune systemBecause sperm are not formed until puberty, they are absent during immune system development, and would not be recognized as “self”