gametogenesis and development of the fertilized ovum · 2018-11-01 · gametogenesis and...
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Gametogenesis and Development of the Fertilized Ovum
InaS.Irabon,MD,FPOGS,FPSRM,FPSGEObstetricsandGynecology
ReproductiveEndocrinologyandInfertilityLaparoscopyandHysteroscopy
References
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7
• ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
outline
• Gametogenesis
• Ovarianfolliculogenesis
• Fertilization
• Blastocystimplantation
• Embryonicperiod
Definitionofterms
• Oogenesis:Thedevelopmentoftheovumfromanoogoniumbymeiosis
• Spermatogenesis:Thedevelopmentofmaturespermfromspermatogoniabymeiosis
• Conceptionalage:Theageoftheconceptusfromfertilization
• Gestationalage:Thestageoftheembryocountingfromthefirstdayofthelastmenstrualperiod.
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
• Fertilization:Thepointatwhichonespermatozoonpenetratestheoocyte
• Conceptus:Thefertilizedoocyteanditsderivativesatallstagesofdevelopmentfromfertilizationuntilbirthandincludingallextraembryonicmembranes
• Embryo:Thedevelopmentalstageoftheconceptusafterthedevelopmentoftheprimitivestreakanduntilallmajororgansaredeveloped
Definitionofterms
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
• Zygote:onecellstageofthefertilizedovumafterpronuclearmembranebreakdownbutbeforethefirstcleavageoccurs
• Morula:Aballofcellscomprisingtheearlyembryothatproducesboththeembryoandtheplacentaandmembranes
• Blastocyst:Thestageoftheconceptusthatfollowsthemorulastage.Itconsistsofafluidfilledcavitysurroundedbytrophoblastswithembryoniccellsatonepole
Definitionofterms
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
Spermatogenesis
• aprocessbywhichprecursorcellstermedspermatogoniaundergoacomplexseriesofdivisionstogiverisetospermatozoa
• Occurswithintheseminiferousepithelium
• Proliferationanddifferentiationofspermatogoniabymitosis
• Primaryspermatocytes(2n)undergo2meioticdivisionstoproducehaploidspermatids(n)
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7;ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
Spermatogenesis
• Approximately72daysarerequiredtoproducematurespermatozoa
• Maturespermatozoa(n)arestoredinthecaudalepididymispriortoejaculation
• Preservationofoptimumspermfunctionduringstoragerequiresadequatetestosteronelevelsincirculationandmaintenanceofnormalscrotaltemperature
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7;ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
Spermatogenesis
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7;ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
OvarianGametogenesis
• Oogenesisistheprocessbywhichfemalegametesdevelopintheovaries
• Unlikeinmales,allprimaryoocytesareproducedduringfetaldevelopment
• Theprecursorstemcellsinfemalesarecalledoogonia(oogonium,sing.)
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7;ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
OvarianGametogenesis(Oogenesis)
• Oogenesisbeginsinfetallifewhentheprimordialgermcells,oroogonia,migratetothegenitalridge(6weeks)
• Thenumberofoogoniaincreasesdramatically:• 2ndmonthAOG:600,000• 10-12thweeksAOG:6-7
million(maximum)
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7;ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
OvarianGametogenesis
(Oogenesis)
• byaprocessofapoptosisandatresiaoftheenvelopingfollicle,thenumberofprimaryoocytesdeclinesfromabout2to4millionatbirthtobecome90%depletedbypuberty.
• byage37:25,000-37,000
• byage50:<1000oocytesremain.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
• Theoogoniathenbeginmeioticdivision(primaryoocytes)
• diplotenestageoftheprophase(thegerminalvesicularstage)
• **primaryoocyctesremainindiplotenestageuntilstimulationbygonadotropinsinadulthoodduringthemenstrualcycle.
OvarianGametogenesis(Oogenesis)
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
OvarianFolliculogenesis
• Theprimaryoocyteissurroundedbyasinglelayerofgranulosacellsinaunitreferredtoastheprimordialfollicle.
• primary(orpreantral)follicles(multiplelayersofgranulosacellssurroundthem)
• **Developmentoffolliclestothisstageappearstoberelativelyindependentofpituitarycontrol
• **occursduringthenonovulatorystagesofchildhood,pregnancy,oralcontraceptiveuse,aswellasduringovulatorycycles.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
OvarianFolliculogenesis
• Withformationofanantrum,thefollicle(secondaryorantralfollicle)entersthefinalstagesoffolliculogenesis
• transitionfromintraovarianregulationtoamajorcontrolbythehypothalamic-pituitaryunit.
• ThisrequiresthepresenceofthecharacteristicincreaseinFSHthatoccursintheearlymenstrualcycle.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
OocyteMeiosis1stmeioticarrest
2ndmeioticarrest
• CunninghamFG,LevenoKJ,BloomSL,SpongCY,DasheJS,HoffmanBL,CaseyBM,SheffieldJS(eds).William’sObstetrics24thedition;2014;chapter7;ComprehensiveGynecology7thedition,2017(LoboRA,GershensonDM,LentzGM,ValeaFAeditors);chapter4
• Thedevelopmentprocessfromprimaryfollicle(preantralfollicle)tosecondaryorantralfollicle,andtoamaturepreovulatoryfollicletakesabout1yeartocomplete.
• Onlyabout400folliclescompletethisprocess
• themajorityoffolliclesundergoprogrammedcelldeath.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
FollicleDevelopment
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
Oogenesisandthefertilizedovum
Oogonium(2n)
Primaryoocyte(2n)
Firstpolarbody(n) Secondaryoocyte(n)
Secondpolarbody Ovum(n)
(Beforebirth)
MeiosisI
MeiosisII
Duringfetaldevelopment,alltheoogoniadifferentiateintoprimaryoocytes.ThesecellsremainfrozeninPROPHASEIofmeiosis(diplotenestage)à FIRSTMEIOTIC ARREST
Whenthefemalereachespuberty,primaryoocytewillcompletemeiosisItoproducesecondaryoocyte,whichwillremainfrozenatmetaphaseIIofmeiosisà 2NDMEIOTICARREST
Duringfertilization,thesecondaryoocytewillcompletemeiosisIItoproducethematureovum(eggcell)
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Follicularphase
• Thefollicularphasecanbesubdividedintothreeperiods:
1. successiverecruitmentofacohortofantralfollicles
2. selectionofadominantfollicle
3. growthoftheselecteddominantfollicle.
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
RecruitmentofaCohortofAntralFollicles
• FSHprovidesthecriticalsignalfortherecruitmentofacohortofpreantralfollicles(cyclicrecruitment)
• Thisisthemajorsurvivalfactorthatrescuesthefolliclesfromtheirprogrammeddeath(atresia)andallowsthemtostartgrowing,increasinginsizeandbeginningtosynthesizesteroids.
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
SelectionofaDominantFollicle“survivalofthefittest”
• usuallyonlyonefollicle(the“dominantfollicle”)isselectedtocompletegrowthtomaturity.(**otherfolliclesinthecohortbecomeatretic)
• HowwastheDOMINANTfollicleselected??• well-vascularizedthecalayerallowing
abetteraccessofthegonadotropinstotheirtargetreceptors.
• Thisresultsinagreaterlocalestradiolsecretion,whichinturnincreasesthedensityofgonadotropinreceptorsandpromotescellmultiplication.
ProductionofEstradiol
“2cell–2g0nadotropintheory”
Thecacellà LHreceptorGranulosacellà FSHreceptor
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
Dominantfollicle
• Asthedominantfolliclegrows,anantrum(cavity)formsintowhichfollicularfluidaccumulates.
• Thisfluidcontainsseveralsteroids,peptideandproteinhormones,andnutrients.
• increasesinbothfolliclediameterandvolumeparalleltheincreaseinestradiollevelsinblood.
• Atmaturation,thedominantfolliclereachesameandiameterrangeof18-25mm.
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
TheOvulatoryGonadotropinSurgeandOvulation
• Maturationofthedominantfollicleismarkedbyhighbloodlevelsofestradiol.
• Whenathresholdisreached,estradiolactivatesthepositivefeedbackloopà signalingtothehypothalamusandanteriorpituitaryglandthatthefollicleisreadyforovulationandthatalargegonadotropinsurgeistobereleased
• AsmallbutsignificantincreaseinprogesteroneisalsosecretedbythefolliclebeforetheLHsurge;
• Duringtheovulatorysurge,LHlevelsincrease10-foldoveraperiodof2to3days,whereasFSHlevelsincreaseabout4-fold.
• Thisgonadotropinsurge(LHSURGE)isanabsoluterequirementforthefinalmaturationoftheoocyteandtheinitiationofthefollicularrupture.
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
LHsurge• TheLHsurgeinitiatesgerminal
vesicle(ornucleus)disruption,andthefullygrownoocyteresumesmeiosis(meioticmaturation).
• Thus,itprogressesfromthediplotenestageofthefirstmeiosistometaphaseIIofthesecondmeioticdivision.
• AstheoocyteentersmetaphaseII,thefirstpolarbodyappears.(Threehaploidpolarbodiesareproducedduringthetwo-stepmeiosisprocess,atovulationandfertilization.)
• Thepolarbodiesgeneratedfromthemeioticeventscontainrelativelylittlecytoplasm,andtheoocyteeventuallydiscardsthem.
• Atovulation,meiosisisarrestedagain(thesecondmeioticarrest).
• Thesecondmeioticdivisionwillonlybecompletedatthetimeoffertilization.
• Theoocyte’sabilitytobefertilizedcoincideswiththecompletionofmeioticmaturation
Ovulation/folliclerupture• Ovulation(folliclerupture)
occursabout32hoursaftertheinitialriseoftheLHsurgeandabout16hoursafteritspeak
• Prostaglandinsthenactlocally—toinducethehyperemiaandedemaseeninthefirsthoursoftheprocessofovulationandwhichresultfromincreasedbloodflowandvascularpermeability.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
LutealPhase• Aftertheoocyteisextrudedfromthe
maturedominantfollicle:
1. theamountoffollicularfluidismarkedlyreduced
2. thefollicularwallbecomesconvoluted
3. folliculardiameterandvolumegreatlydecrease.
CORPUSLUTEUM
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Corpusluteum
• Likethedominantfollicle,growthanddevelopmentofthecorpusluteumoccurrapidly.
• Vasculargrowthplaysacentralroleinthisprocess.
• Angiogenicfactors,suchasvascularendothelialgrowthfactor(VEGF),arepresentinhighquantityintheforminganddevelopingcorpusluteum.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
• Progesteronedominanceduringthelutealphasealsoaffectsthehypothalamicthermoregulatorycenter
• smallincreaseinbasalbodytemperature(BBT)reflectsincreasedprogesteronesecretionduringthelutealphase.
• Thus,thetypicalBBTcurveoftheovulatorymenstrualcycleisbiphasic
EndocrineFactorsandtheCorpusLuteum
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
CorpusLuteumRegression(Luteolysis)
• lifespanofthecorpusluteumislimitedtoaperiodofabout14days.
• thecorpusluteumreachesmaturity8to9daysafterovulation,afterwhichtimelutealcellsstarttodegenerateanditssecretorycapabilitybeginstodecline(corpusalbicans)
• Rapidlyrisingconcentrationsofhumanchorionicgonadotropin[hCG]followingconceptioncanrescuethecorpusluteumandmaintaintheproductionofprogesterone.
TheLuteal-FollicularTransition
• Theendofthelutealphaseischaracterizedbyadramaticdecreaseinprogesterone,estradiol,andinhibinA.
• ThisisaccompaniedbyacharacteristicdivergenceintheFSH:LHratio,nowfavoringaspecificriseinFSH
• TheincreaseintheFSH:LHratioheraldsanewmenstrualcycleandtherecruitmentofanewcohortoffollicles.
• Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe JS, Hoffman BL, Casey BM, Sheffield JS (eds).William’s Obstetrics 24th
edition; 2014; chapter 7• Comprehensive Gynecology 7th edition, 2017 (Lobo RA, Gershenson DM, Lentz GM, Valea FA editors); chapter 4
Fertilizationwithinthefirstweek
Stage1:day1-2
Stage2:day2-3
Stage3:day4-5
Stage4:day5-6
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Fertilizedovum:“zygote”(2n)• Undergoesslowcleavagefor3dayswhilestillin
thefallopiantube
• Formationofasolidmulberry-likeballofcells-themorula.Themorulaenterstheuterinecavityabout3daysafter
• Accumulationoffluidbetweenthecellsofthemorulagivesrisetoanearlyblastocyst
• Between3-5daysafterfertilization,theblastuladifferentiatesintothe58cellblastocyst
• 5embryoproducingcellsknownastheinnercellmassand53cellsdestinedtobetrophoblasts
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Blastocystimplantation• Sixor7daysafterfertilization,the
embryoimplantstheuterinewall.
• (1)apposition—initialcontactoftheblastocysttotheuterinewall;
• (2)adhesion—increasedphysicalcontactbetweentheblastocystanduterineepithelium;
• (3)invasion—penetrationandinvasionofsyncytiotrophoblastandcytotrophoblastsintotheendometrium,innerthirdofthemyometrium,anduterinevasculature.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Blastocystimplantation• Successfulimplantationrequiresa
receptiveendometriumappropriatelyprimedwithestrogenandprogesteronebythecorpusluteum.
• uterinereceptivityislimitedtodays20to24ofthecycle.
• Iftheblastocystapproachestheendometriumaftercycleday24,thepotentialforadhesionisdiminishedbecauseantiadhesiveglycoproteinsynthesispreventsreceptorinteractions
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Blastocystimplantation
• blastocystlooselyadherestotheendometrialepitheliumbyapposition.
• mostcommonlyoccursontheupperposterioruterinewall.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Naegelerule
• add7daystothefirstdayofthelastperiodandsubtract3months,thenadd1year
• Forexample:• LMP:July5,2016• EDD:July5minus3monthsà April5+7days
➔ April12+1yearà April12,2017
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Thetrophoblast
• Humanplacentalformationbeginswiththetrophectoderm,whichappearsatthemorulastage.
• trophoblastplaysacriticalpartatthefetal-maternalinterface.
• Itsinvasivenesspromotesimplantation,nutritionalrolefortheconceptus,andisessentialtomaternalphysiologicaladaptationsandtopregnancymaintenance.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Thetrophoblast• Bytheeighthdaypost-
fertilization,afterinitialimplantation,thetrophoblasthasdifferentiatedintoanoutermultinucleatedsyncytium—primitivesyncytiotrophoblast,andaninnerlayerofprimitivemononuclearcells—cytotrophoblasts
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Thetrophoblast• Thesyncytiotrophoblastisresponsiblefor
thesynthesisofhumanchorionicgonadotropin(HCG)
• HCGisresponsibleformaintainingthecorpusluteum.
• HCGhasbeendetectedintheperipheralbloodofthemotherasearlyas6daysafterovulation,butitisalwaysseenbyday12.
• Theconcentrationdoublesevery1.2to2days,reachingitshighestpointat7to9weeksofpregnancy.
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
EmbryonicPeriod
• Theconceptusistermedanembryoatthebeginningofthethirdweekafterovulationandfertilization.
• Primitivechorionicvilliform
• Embryonicperiodlasts8weeks,duringwhichorganogenesistakesplace.
• Theembryonicdisciswelldefined,andmostpregnancyteststhatmeasurehumanchorionicgonadotropinbecomepositivebythistime
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
• Thebodystalkisnowdifferentiated,andthechorionicsacisapproximately1cmindiameter.
• Duringthethirdweek,fetalbloodvesselsinthechorionicvilliappear.
• Inthefourthweek,acardiovascularsystemhasformed,andthereby,atruecirculationisestablishedbothwithintheembryoandbetweentheembryoandthechorionicvilli.
• Partitioningoftheprimitiveheartbeginsinthemiddleofthefourthweek.Armandlegbudsarepresent,andtheamnionisbeginningtounsheathethebodystalk,whichthereafterbecomestheumbilicalcord.
EmbryonicPeriod
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
Humansexdifferentiation
• Genetic sex is determined at the time of conception. • Presence of a Y chromosome is necessary for the development of
the testes • primary determinant is the SRY (testes determining factor) gene located
on the short arm of the Y chromosome • testes responsible for:
1. organization of the sexual duct system into a male configuration 2. suppression of the paramesonephric (Mullerian) system
In the absence of a Y chromosome or in the absence of a gonad, development will be female in nature
• William’sObstetrics24theditionchapter7;• ComprehensiveGynecology7theditionchapter4
outline
• Gametogenesis
• Ovarianfolliculogenesis
• Fertilization
• Blastocystimplantation
• Embryonicperiod