Sexual Reproduction in Humans

Sexual Reproduction in Humans(a) the structure and function of the reproductive systems inhumans, including the examination of histology of ovary andtestes Label and describe functions of ovary fallopian hubes(oviducts), uterus, endometrium cervix and vaginga in Females; andscrotum, lestes,epididymia,vas deferens, semiral vesicle, prostategland and urethra and penis.Prostate glandSeminal VesicleVas DeferensEpididymisSeminiferous tubulestestisUrethraejaculatory ductFunctions of male reproductive tractScrotum: to keep the temperature of the testes slightly lowerthan that of the rest of the bodyTestes To produce sperm (spenmatogenesis) and produce androgensprimarily testosterone It contains the seminiferous tubules whichproduce spermatozoa.Epididymis : A long, coiled tube that stores sperm and transports itfrom the testes. The sperm mature in the head of this structure.They then pass to the base where they are stored. was Deferens: Stored semen pass from the base of the epididymisvia the vas Deferens to the urethra during ejaculationSeminal Vesicle Produces a MUCUS secretion that aids spermmobility Empties into the ejaculatory duct (combined with thevas DeferensProstate gland Produces an alkaline secretion that neutralisethe acidity of any urine in the urethra as well as aidingsperm mobilityUrethra : Carries urine from the bladder to outside of the bodyIn males, it has the additional function of ejaculating semenwhen the man reaches orgasm When the penis is erectduring sex, the flow of urine is blocked from the Urethra,allowing only semen to be ejaculated at orgasmPenis External male reproductive organ Also functions toserve as a urinal duct.Histological examination of testesPlacing a TS testes slide under the microscope and using a x4objective, you will see seminiferous tubules, each surrounded by connectivetissue and with a lumen in the centre.Move the slide so that a rubule with a clear immen is seen andusing x10 and x 40 objectives you can :> Identify the germinal epitherial cells at the over margin of the(seminiferous tubule> Identify a sequence OF cells along a radius of the tubuce and moringinwards from the germinal epithelium, identify primary spematocytes>Secondary spermatocytes are rarely seen as they progress rapidlyto spermatids, which can be seen Further in towards the lumon> Adjacent to the lumen, spermatozoa with tails can be seen> Between the strands of developing spermatids are the Sertoli cells.They are columnar, biochemically active and easily recognisedby the large oral nucleus and dense nuclealus They secrete afluid, which nourishes spermatids and protects them from the male's immune system) Groups of cells called interstitial cells (or Leydig cells) are foundbetween the seminiferous tubules They secrete testosterone themale sex hormone, which has roles in sperm formation andmaturation as well as the development of male secondarysexual characteristicsureterOviductOridicaluterusfunnelovarycervixbladdervaginaurethravulvaFunctions of female reproductive tractOvary : Production of ova or eggs and production of someFemale reproductive hormonesFallopian Tubes (oviduck): Conveys ova from the ovaries to theutens. Site of fertilisation of the ovaUterus: Has muscular walls and is lined internally by a mucusmembrane (endometrium). Main function is holding the Foetus untilmaturity Endometrium Mucus membrane lining the utenus well suppliedwith blood and is part of the womb into which theembryo implants during pregnancy Is shed during mensirationVagina: Has several functions during sexual intercourse, childbirthand menstruationUrethra Acts as a passageway for urineHistological examination of ovaryA section of ovary is cut, hold slide up to light and see if thereis an empty area which is likely to be the contrum of a matureGraafian FolliclePlace slide under microscope and use an x4 objective to identifyany maturing Graafian follicles by noting the fluid-filled anfrumThe outer edges of the ovary comprises the germinal epitheliumMoving inwards, using a x10 and a x40 objective, identify primaryfollicles, which appear as clusters of cells, each with a largerprimary oocyte in the middleThe oocyte in a Graafian Follicle will only be seen if the plane ofthe section passes through it. If present, the dark-stainingZona pellucida can be distinguished If the first meiotic divisionhas already taken place it is a secondary oocyte If notit is still a primary oocyteThe Secondary oocyte is surrounded by cumulus cells, whichcontribute to the corona radiata.The antrum may look opaque, despite being fluid-filled assolutes tend to crystalise out during the preparation of theSlideThe matrix of the ovary is called the stroma. It may bepossible to see in in the blood and lymphanic vessels(b): the process of spermatogenesis and oogenesis to producespermatozoa and secondary oocytes; sexual intercourse ;fertilisation and implantation GametogenesisGametogenesis is the production of gametes in sex organs.Spermatogenesis is the formation of sperm in the testis.Oogenesis is the formation of secondary cocytes in the ovaryThe cells of the germinal epithelium OF both the testisand the ovary undergo a sequence of mitotic andmeiotic divisions to form haploid gametes.It is important that the gametes are haploid so thatat fertilisation the diploid number is restared and thechromosome number does not double in every generationSpermatogenesisSpermatogenesis is the process by which spermatozoa areproduced and it takes place in the geminal epitheliumof the seminiferous tubules. The process takes place as follows:1. Cells of the germinal epithelium are diploid so theydivide by mitosis to make diploid spermatoganiaand more germinal epithelium cells2. The spermatogonia divide many times by mitosisand enlarge, making diploid primary spermatocytesand more spermatogonia.3. Primary spermatocytes undergo meiosis 1, makingsecondary spermatocytes, which are haploid4. Secondary spermatocytes undergo meiosis 11, makinghaploid spermatids.5. Spermatids mature into spermatozoa or sperm.Sperm production takes about 70 daysSpermatids are nourished and protected by theSertoli cells.Sertali cells secrete a fluid which nourishes thespermatids and protects them from the immunesystem of the maleInterstitial cells secrete testosterone germinal epithelium cellspermatogoniaprimary spermatocyteso0Osecondary spennatocyk000spermatids0.a0aspenatozoa0(sperm)o0o0Sertori CellSperm StructureThe head contains a haploid nucleus, covered at the anteriorand by a lysasome called the acrosome, which containsenzymes used at Fertilisation.The middle pieces is packed with mitochondria, whichprovide ATP for movement They spiral around themicrotubules, which extend from the centriole into theaxial filament in the tail.The tail, or flagellum, makes lashing movements thatmove the sperm, although the sperm aren't monite untilthey have been modified in the epididymiscellmembranecentrioleacrosomeaxel FilamentnucleusMitxhandriaLOONOODHeadMid-pieceTail5um 8 um50pm OogenesisOogenesis occurs in the ovary and involves both mitosisand meiosis to form a secondary oocyte1. Cells of the geminal epithelium of the ovary, whichare diploid, divide by mitosis to make diploid oogonia,before birth2. The oogonia divide many times by mitosis, makingdiploid primary oocytes3. Primary oocytes start to divide by meiosis I but theprocess stops at prophase 1.4. The germinal epithelium cells divide to form diploidFollicle cells, which surround the primary oocytes,making primary follicles.5. These do not mature until just before ovulation6. At puberty, hormones stimulate the follicles to developfurther. Each month several follicles start to developbut only one matures into a fully developed secondaryFollicle, called a Graafian follicle.7. First the primary oocyte completes the first meisticdivision to form the haploid secondary oocyte anda small polar body.8. The mature Graafian follicle migrates to the surfaceof the ovary where it bursts and the secondaryoocyte is released - ovulation9. The secondary oocyte begins the second meioticdivision this stops at metaphase II unless fertili-sation takes place10. On fertilisation this division is completed to form alarge ovum and a second polar body. Once thisdivison has taken place. the nucleus of the ovumfuses with that OF the sperm to form a zygote,which will when develop in to cen embryo.11. After ovulation, the Graafian Follicle becomes the corpus leuteum This produces the hormonesduring pregnancy but regresses if fertilisationdoes not take place.The Graafian FolliclegerminalepitheliumZonaGraafianpellucidathecaFolliclesecondaryantrumoocytecells ofradiataTStroma of ovary> The zona pellucida is a clear glycoprotein layer that surroundthe cell membrane of the secondary oocyte>The periphery of the cytoplasm contains cortical granules,secretary organelles that prevent entry of more than one sperm>Corona radiata cells surround secondary oocyte and provide nutriersdevelopingOvary Illustrationsecondary FolliclestrumaprimaryFarricle(a)OO@CGraatiandegeneratingcorpus luteumFolliclecorpus Intermsecondary oocytereleased at avvation Differences between spermatogenesis and oogenesisSpermatogenesisGerminal.Epithelium (2n).mitosisSpermatogonium (2n)mitosisPrimary spermatocyte (2n)(meiosis).Secondary spermatocyte (n)meiosis 11.o.spermat ids (n)naturationmaturation..Spermatozoa (n)Oogenesis:GerminalEpithelium (2n)omitosisOoganium (2n)mitosisAtPrimary oocyte (2n)t- birthmeiosis IFirst polarsecondarybody (n)oocy te (n)- After fertilisationmeiosislisecondpolar body(s)ovum (n)Similarities between spermatogenesis and oogenesisBoth involve 2 mitotic divisions and 2 meiotic divisionsBoth products are haploid gametes>Both start from germinal epithelium >Both involve FSH and LHDifferences between spermatogenesis and oogenesis>Ologenesis starts before birth whereas spermatogenesisstarts at puberty>Oogenesis stops at meiosis 1, whereas spermatogenesisdoesn't and continues on to meiosis 11>4 gametes are produced in spermoltogenesis whereasone gamete is produced in oogenesis>Oogenesis produces two polar bodies whereas spermatogenesis doesn't produce any>Oogenesis happens on a monthly cycle whereasspermatogenosis is continuous)Spermatogenesis is continuous throughout life spanwhereas oogenesis stops at menopause)One oocyte is produced from alternate ovaries whereas millions of spermatozoa are released from bothtestesSexual Intercourse)Physical and psycological effects cause the arteriolesentering the penis to dilate and the venules to constrict>Build up of blood in penis causes it to become erect.>During sexual intercourse, penis is inserted into the vaginaand movements of penis result in contraction of smoothmuscle in the walls of the epididymis, vas Deferens andpenis, causing the ejaculation of semen into vagina) The force of ejaculation is sufficient to propel somesperm through the cervix into the uterus, with theremainder being deposited at the top OF the vaginaFertilisation>Fertilisation is the fusion of a female and male gameles producing a zygole It is a process, not a single event:1. Following sexual intercourse, spermatozoa move intothe Fallopian Tubes.2. Sperm can only fertilise a secondary oocyte a Aercapacitation has taken place This is the removalof cholesterol and glycoproteins from the cell membraneover the acrosome in the sperm head. This processincreases the fluidity of the membrane increasesits permability to Ca 2+ ions and makes acrosomemembrane more likely to rupture and leads tothe intiation of the acrosome reaction3. The influx of Ca ions results in the release ofprotolytic enzymes (proteases) which digest thecells of the corona radiata This is the acrosomereaction The sperm cell membrane makes contactwith Zona Pellucinda and acrosome ruptures Acrosiacontaining hydrolytic enzymes, is released andhydrolyses the zona pellucida, enabling the spermand oocyte membranes to fuse4. The cell membranes of the secondary oocyte fuseand sperm cell nucleus enters oocyte cytoplasmFollowing the entry of sperm head, the secondarycocyte is called an ovum.5. Next is the cortical reaction which is the reactionof the oocyte that produces the fertilisation membrane,preventing polyspermy centry of additional sperm). Whenthe sperm attaches to the secondary oocyte, theoocyte's smooth endoplasmic reticulum releases Ca 2+ions into the cytoplasm This makes the corticalgranules fuse with the cell membrane and releasetheir contents of enzymes by exocytosis Thezona pellucida is chemically modified and expandsand hardens, making a fertilisation membrane which prevents polyspermy6. Entry of sperm also stimulates completion ofMeiosis 11 of the ovum nucleus it proceedsthrough ana phase II and telophase 11, dividesand expels the second polar body7 Within 24 hours, the first mitosis combines thegenetic material of the parents to make thediploid cells of the embryo. The sperm chromosomesjoin the own chromosomes on the cell'sequator The cell is now a zygote, as thechrom osomes have combined8 The first mitoric division produces two cells,and until organs have developed at aboutweek 10, this collection of cells is an embryoKey Terms>Capacitation changes in the sperm cell membranethat in crease its fluidity and allow the acrosomereaction to occur>The Acrosome reaction: acrosome enzymes digest thecorona radiata and the zona pellucida, allowingthe sperm and ccaste cell membranes to fuse>The cortical reaction fusion OF cortical granulemembranes with the oocyte cell membrane, releasingtheir contents, which convert the zona pellucida intoa fertilisation membrane>Polyspermy the entry of additional sperm[Diagram of fertilisation following sperm entry onnext page I Flowdiagram showing process following sperm entrycorona radiataSecondaryzona pellucidaoocytecortical granule0First polar bodyOfeechromosomes at metaphase 11spindle fibreFertilisationFertilisation membranesperm headMeiosis 11ofSecond polar bodyFertilisation membraneOFFemale pronucleusokMale pronucleusDNA replication06Second polar body23 oocyte chronwomesand 23 sperm chromosomeson spindleFertilisation membraneMitosisDiploid nucleusFertilisation membrane Implantation)Implantation is the sinking OF the biastocyst into the endometria) As the zygote moves down the oviduct it undergoes repeatedmitoric divisions in a sequence called cleavage, to producea ball of cells called the blastocyst.)cells that form the outer layer of the blastocyst is callerthe trophobiast.>The blastocyst moves from the overuct into the uterus.)After ovulation the endometrium thickens and gets anincreased blood Supply to prepare it to recieve an embryo.)There is an implantation window when the endometiumis receptive which is between 6 and 10 days after orulation.)After 9 days, prohusions from trophobbst cells of the blastocyst,called trophoblastic villi, penetrate the endometrium. The villsincrease the surface area for the absorption of nutrients fromthe endometrium.)A placenta forms between the tissuses of the Mother andthe factus.Blastocyst atTrophoblast cells7 daysBlastcoele (fluid-filled)Inner cell massTrophoblastembedded inendonetriumTrophoblastic willBla shockeleInner cell massTrophoblast cells (c): the endocrine control of reproduction in the Female:including the menstrual cycle, birth and lactation byFSH,LH, Destrogen, progesterone, oxytocin and hCGThe Menstrual Cyclethe menstrual cycle is a system of positive and negativefeedback operating between events involving the brain,the ovaries and the uterus. The cycle lasts 28 days butcan vary between individuals. Significant hormones include:- Follicle stimulating hormone (FSM)- Lutenising hormone (LH)- Oestrogen- Progesterone>FSH :The first day of the cycle is day 0 and is the first dayof menstrual bleedingThe concentrations of all relevant hormones in plasmaare low-Gonadotrophic releasing hormone (GnRH) is secreted bythe hypothalamus and stimulates the antenior pituitarygland to secrete FSHFSH stimulates the development of primary Fullicles intosecondary follicles in the ovaryOnly one fallicle matures fullyOne secondary fallicle fully matures into a Graafian follideFSH also stimulates follicle cells to produce oestrogen)LH:Following menstruation the level of oestrogen secretedby the developing follicle, increases "the blood whichtriggers the repair of the endometrium, which inhibits-FSH production and Stimulates LM productionLH which redeases its maximum concentration just be foreovulation is secreted by the antenior pituitary gland LH induces ovulation On day 14, the high concentrationof LM causes the Graafian fallicle at the surface of theovary to release its secondary oocyte it has a positivefeedback effect on FSHThe remains of the Graafian fallicle develop into thecorpus luteum which secretes Oestrogen and progesteroneThese prevent further secretion of FSM and LH, so theirconcentrations decrease)Destrogen :As FSH concentrations increase at the start of the cycleit stimulates the production of aestrogen As oestrogenbecomes more concentrated in the plasma it- Triggers rebuilding of the endometrium that wasshed during menstruation- Inhibits FSH secretion by negative feedback, whichbrings down its own concentration again- stimulates LH production by positive feelback)ProgesteroneThe conpus Iuteum secretes oestrogen and progesteroneProgesterone causes further development of the endometriumprior to monstration and maintains the newly builtendometrium so if the oocyte (secondary) is fertilised, theembryo can implantIf no implantation occurs, the falling concentrations of FSMand LH cause the conpus iuteum to degenerate so progestroneconcentration declinesThe endometrium is no longer being rebuilt by Destrogenor maintaind by progesterone, so it is shedAs aestrogen is low, it no longer inhibits FSM, 30 themensinal cycle restarts[Graphs on next page] Events in the ovary and uterus in the menstwal cycleFotticlematureEndotheliumGrafiancorpus itemstarts toINmaturingFollicleformsdetachovulationOVARYINUTERUScompanyPERIOD1428FSH and CH:time/dayspositive Feedback= FSMby oestrogen= LHSnegative Feedbackby progesterone&positivefeedback LHbynegative feedbackpositivefeedbackby destrogenby GnRH& toOconsidian28timeldaysOestrogen and Progesterone= oestrogen= progesterone3negativefeedbackby FSHpositive feedbackby FSH1428coveration(hove/days) Follicular PhaseLuteal PhaseLHFSH----Maturing Follicle -Corpus LuteumEstrogenProgesterone13151015202528DayPituitaryOvulationhormone levelsFSHLHOvarianEstradiolhormone levelsProgesterone07142128Day of menstrual cycle Pregnancy>About 6 days after fertilisation, the embryo, at blastocystStage, begins to secrete human chorionic gonadotrophin(hCG)>Following implantation, hCG is made by the charion, theinner layers of the placenta>It is a glycoprotein and it maintains the corpus luteumin its secretion of progesterone, for the First 16 weeksof pregnancy.>Progesherne maintains the endometrium, which is essentialfor embryonic development, as it contributes to the structureof the placenta)Oestrogen and progesterone are secreted from the corpusluteum and then the placent a which increases theconcentration of both in the piasma until the end ofpregnancy>This inhibits FSH so no more fallicles Mahure, LM soovulation is not possible and prolactin so no milk ismade)Progesterone inhibits ovytocin, So the myometrium andmuscles in the milk ducts do not contract>Oestrogen stimulates the growth of the uherus toaccommodate the growing factus and stimulates thegrowth and development of the mammary glandsduring pregnancy.Birth>Just before birth, oestrogen levels increase and adecrease in progesterone levels means that the wheringwall can now contract.>Oxytocin secreted by the posterior pituitary glandstimulates contraction of the uterine wall whichstimulates the secretion of more oxyrocin This is an example of positive feedback and it leads tostronger and more frequent contractions>Prolactin is secreted by the anterior lobe of thepituitury gland and stimulates glandular tissue inthe mammary giands to synthosire milk(d): the role of the placenta including hormone controlThe Placenta>The trophoblast of the blastocyst becomes the charicnand placenta and the inner cell mass forms the embryoand the amnion>The placenta connects the embryo and factus tothe uterus wall. It is made of tissues derived from themother and factus.>The trophoblast develops into the chonicn, an overmenbrane surrouncing the embryu. Cells from thetrophobiast more into the trophoblastic villi and formthe much larger chorionic villi These villi acquireblood capillaries which connect to the umbilicalarteries and vein Projections from the endometriumbetween chononic ville are the maternal tissuesof the placenta.Roles of the placenta>Counter-current Flow between maternal and foetalblood supplies:- A high concentration gradient is maintainedbetween the maternal and Foefal blood suppliesby counter current blood flow- Maternal and foetal blood flow in oppositedirections so that there is always a higherconcentration of nutrients and oxygen in themother's blood than in the foetus'- Therefore diffusion of nutrients/oxygen will be at higher rude >As an endocrine organ, producing hormones to supportthe pregnancy, such as :- hCG : maintains corpus luteum which contributes tothe release of progesterone.- progesterne maintains endometrium and preventcontractions in uterine wall- oestrogen : stimulates growth of when and developmentof mammary glands)Exchange between the mother's and foetus's blood,including nutrients, washe and respiratory gases:- Inter-villous spaces, called lacunae, containing themother's blood, surround the chononic villi. Chorionicvillus cells have micronilli, giving a large area ofcontact with Mother's blood for maximum exchange- The embryo's and the mother's blood do not makecontact The distance between them through thewalls of the ville, is only about 5pm, adistance over which diffusion, facilitated diffusion,achive transport, pinocytosis and osmosis operate effectively)A physical barrier between the foetal and maternalcirculation- Protects the fragile, foetal capillaries from damageby the higher blood pressures of the mother- protects the developing foetus from changes inmaternal blood pressure)Protection from the mother's immune system:- the mother does not make an immuneresponse against the Foetus or placenter, eventhough they contain foreign genes from theFather- One reason is that the cells of the wall ofthe chorionic villi fuse so there are no spacesbetween them making a syncitium. - Then, migratory immune cells, such asgranulocytes cannot get through to the foetalblood- passive immunity is provided Maternal antibodiescross the placenta and attack pathogens butdo not attack the foetal cells, even thoughthey camy the father's antigens, which aredifferent from the mother'sThe umbilical card>The umbilical card develops from the placentaand it transfers blood between the foctus and themothercomes>the blood of the foetus to the placenta throughthe umbilical cord in two umbilical artenes (arteriesas the blood is coming away from factus's heart.))This blood is low in numents and is deoxygenated)The blood exchanges Materials with the mother'sblood at the charionic villi and returns to the Foehusin a single umbilical vein>Returning blood is high in nutrients and isoxygenatedAmniotic Fluid>Amniotic fluid is 98% water, a solution of urea,salts, a little protein and a trace of sugar>Amiotic Fluid- Maintains the foetus's temperature- Provides lubrication- contributes to lung development- Allows movement so muscle, and bones function- Acts as a shock absorber, protecting the foehusfrom injury from outside the wherus, during development