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Early EmbryogenesisEmbryogenesis: —formationofbodystructures&organs(organogenesis) —requirescelldivision(proliferation)andcelldifferentiation(specialization) —producesthegreatvarietyofcelltypesandextracellularproductsfoundinthebody.

Cell specialization: —selectivegeneexpression(andresultantproteinproduction)istheultimateexplanationforthecelldifferentiationprocessduringembryogenesis. —geneticexpressionbyaparticularcelldependsonthecell’spreviousgenetichistory(com-mitmentlineage)anditscurrentcellularenvironment(intercellularcommunications).

Cell differentiation istheresultofcellsexpressingsomegenesandsuppressingotherswithinacommongenome.Cellsdifferbecausetheyproduceddifferentproteins/peptides.

Proteins&peptidesare: —structuralcomponents(cytoskeletonorextracellularstructures) —enzymes(controllingcellmetabolism) —secretoryproducts(e.g.,hormones;digestiveenzymes;etc.) —channels&pumps(passageofmoleculesacrossmembranes) —receptors(communication,etc.)

Periods: Embryonic Period — definedasthetimefromfertilizationtotheearliest(primordial)stagesoforgandevelopment(about30daysindog,cat,sheep,pig;almost60daysinhorse,cattle,human).

Fetal Period — thetimebetweentheembryonicperiodandparturition(theendofgesta-tion),duringwhichorgansgrowandbegintofunction.Fertilization: —unionofahaploidoocyteandahaploidspermatozoon,producingadiploidzygote (apleuripotentcellcapableofdevelopingintoanewindividual) —fertilizationbeginswithgametefusion(zygoteformation) —fertilizationendswiththeinitiationofzygotecelldivision(thestartofcleavage) Fertilization related details: —fusionofaspermatozoonwithanoocytetakesplaceintheuterinetube,neartheovary —thespermatozoonmustbindtoaspecificglycoproteinonthezonapellucidasurrounding theoocyte[thisspeciesrecognitionprocesspreventsunionwithforeignsperm];

specialized cell

stellate neuron, etc.

stem cell committed cells

neural epitheliumneuroblast

glioblast

pyramidal neuron

astrocyte

oligodendrocyte

ectoderme.g.,

Cell Differentiation

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—thenthespermatozoonreleasesdegradativeenzymes(acrosomalreaction)[theenzymes denaturethezonapellucida,allowingthespermcelltopenetratethebarrier] —spermatozoonandoocyteplasmamembranesfuse(secondaryoocytecompletesmeiosis) —theoocyteimmediatelycancelsitsmembranepotential(viaCa++influx)andthen denaturesitszonapellucida(viaenzymesarereleasedbyexocytosisfromoocyte cytoplasmicgranules)[thispreventsfusionbyadditionalsperm] —male&femalehaploidpronucleimakecontact,losetheirnuclearmembranes,andbegin mitosis(mitosisbegins12hoursafterspermfusion;DNAsynthesistakesplacebeforemitosis)

Oocyte(envelopedbyazonapellucida(glycoproteinmembrane)andcoronaradiata(granulosacells)atovulation) —selectivefolliclesmatureateachcycle(inresponsetocirculatingFSHhormonefromthepituitary) —oogonia(germcells)giverisetoprimaryoocytesbymitosiswithintheembryo —primaryoocytesinitiateMeiosisI(reductiondivision)withintheembryoandonlyresumeMeiosisI followingovulation(beingsuspendedinMeiosisIbyinhibitorysecretionoffolliclegranulosacells) —secondaryoocytescompletemeiosis(MeiosisII)followingfertilization(ifunfertilizedtheydegenerate), producingafertilizedoocyte(ovum).

Spermatozoa(severalhundredmillionperejaculate) —propelledfromvaginatouterinetubebycontractionoffemalegenitaltract —spermatogonia(germcells)giverisetoprimaryspermatocytesbymitosisrepetitivelyfollowingpuberty —primaryspermatocytesundergoMeiosisI(reductiondivision)producingsecondaryspermatocytes —secondaryspermatocytescompletemeiosis(MeiosisII),producingspermatidsthatundergo transformationintospermatozoa(spermiogenesis) —subsequently,spermatozoaundergocapacitation(removalofsurfaceproteinsthatwouldimpedecontact withanoocyte)

Cleavage: —referstotheinitialseriesofmitoticdivisionsbywhichthelargezygoteisfractionatedinto numerous“normalsize”cells. —eachdaughtercellofthecleavageprocessistermedablastomere. —cleavagebeginswithazygote,progressesthroughcompactiontoamorulastageand terminatesatthestartoftheblastocyst (blastula)stage —thefirsteightblastomeresareundifferentiatedandhaveidenticalpotentialinmammals; thereafter,blastomeresdifferentiateintoinner&outercellswithdifferentmissions

Note: Thefirstcleavagedivisionoccurs1to5daysfollowingovulation(dependingonspecies), thereaftercellsdivideaboutonceevery12hours;Asmanyaseightgenerationsofmitosesmayoccurwithoutinterveningcellgrowth(cytoplasmic increase).Thus,e.g.,one150microndiameterzygotecanbecomesacollectionof256 cells,eachabout7micronsindiameter.

FirstCleavage Division

SecondCleavage Division Morula

Blastula(Blastocyst)

zonapellucida

blastomeres

outerblastomeres

innerblastomeres trophoblasts

blastocoele

innercell mass

5

Morula[L.=smallmulberry] —asolidballofblastomereswithinazonapellucida(typicallyconsistingof16to64blastomeres) —blastomeresbecomecompacted;cellsontheinsidedifferentiatefromthosealongthe surfaceofthemorula: —outer blastomeresbecomeflattenedandformtightjunctions(reducingfluidpermeability); theydevelopthecapacitytosecretefluid(internally);theyaredestinedtobecome trophoblastswhichformthechorion&amnion(fetalmembranes)oftheconceptus; —inner blastomeresformgapjunctionstomaximizeintercellularcommunication;theyare destinedtobecomeinner cell masswhichformstheembryoitself(plustwo fetalmembranes).

Note: •Asfewasthreeinnerblastomeresaresufficienttoproduceanentireembryo(andadult).•Whenamorulaleavestheuterinetubeandenterstheuterus(uterinehorn)itisatabout the16-cellstage,around4to7daysafterfertilization(dependingonspecies).•The32-cellstagemorula(5-7dayspostovulation)isidealforembryotransferincattle.

Blastocyst(orBlastula) —developsduringthesecondweek,afterthezonapellucidaruptures —consistsofalargenumberofblastomeresarrangedtoformahollow,fluid-filled,spherical orcylindricalstructure —containsaninner cell mass(embryoblast),evidentasacollectionofcellslocalizedinside onepolarendoftheblastula —surfacecellsoftheblastocystaredesignatedtrophoblasts(futurechorionoftheconceptus) —thecavityoftheblastocystiscalledablastocoele —eventuallytheblastocystattachestoorimplantswithintheuterinewall(pendingspecies).

Cleavage in fish, reptiles, and birds: Largequantitiesofyolkimpedecelldivisionduringcleavage.Thusablastodisc(ratherthanasphericalorellipticalblastocyst)isformedattheanimalpoleoftheegg. A telolecithal ovum (eggwithlargeamountsofasymmetricallydistributedyolk)hasananimal polewherethenucleusislocatedandanoppositevegetal polewhereyolkisconcentrated.Cleavageispar-tial(meroblastic):cellsdividemorerapidlyattheanimalpolethanatthevegetalpole,resultinginmany,smallblastomeresattheanimalpoleandafew,largemacromeresatthevegetalpole. Incontrast,mammalianovumhasmeageramountsofyolk(oligolecithalovum)whichisuni-formlydistributed(isolecithal).Cleavageisholoblastic(total)andeachblastomeredivisionproducestwoequal-sizedaughtercells.Thusanimalandvegetalpolesarenotevidentinmammalianova.

TwiNS Monozygotic:identical(samegeneticcomposition)twinscanresultfromeither:1]separationofearlyblastomeres(uptothe8-cellstage)—eachoftheseparate

blastomere(s)developsintoanindependentconceptus;or2]separationofinnerblastomereswithinasinglemorula—eachoftheseparate

blastomere(s)developsintoanindependentembryoandbothembryosshareacommonplacenta(thisislesscommonthanthefirstpossibility).

Note:Separationslaterinembryonicdevelopmentresultinconjoinedtwins(diplopagus;Siamesetwins),ordoubleheads,etc.typesofanomalies.

Dizygotic:fraternaltwinsresultwhentwozygotesdevelop“independently”duringthesamepregnancy(independencecanbecompromisedbyfusionoffetalmembranesandbloodsupplies).Itispossibleforfraternalblastomerestomergeandproduceasingleconceptuswithtwodifferentgenotypes(achimera).

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GERM LAYERSEctoderm,mesodermandendodermaredesignatedprimary germ layersbecause originsofallorganscanbetracedbacktothesethreelayers.Ectodermformsepidermisoftheskin,epitheliumoftheoralandnasalcavities,and thenervoussystemandsenseorgans.Mesodermformsmuscleandconnectivetissue,includingbone,andcomponentsof thecirculatory,urinaryandgenitalsystems.Endodermformsmucosalepitheliumandglandsofrespiratoryanddigestivesystems.

Gastrulation: Themorphogenicprocessthatgivesrisetothreegermlayers:ectoderm,mesoderm,andendoderm.(Insomespecies,evidenceofprimitivegutformationcanbeseen[gastrulaGr.=littlestomach].) Gastrulationincludesthefollowingsequence,beginningwithablastocyst:

—Athickenedembryonicdiscbecomesevidentattheblastocystsurface,duetocellprolifera-tionoftheinnercellmasscells.Trophoblastcellsoverlayingtheinnercellmassdegenerateindomesticmammals(inthemouseandhuman,trophoblastcellsoverlayingtheinnercellmassseparateand,insteadofdegenerating,becomeamnionicwall.)

—Fromtheinnercellmass,cellsproliferate,breakloose(delaminate),andmigratetoformanewcelllayerinsidethetrophoblastlayer.Thenewlayerofcells,calledthehypoblast,willformayolksac.Theremaininginnercellmassmaybecalledtheepiblast.

—Ontheepiblastsurface,aprimitive streak formsasdifferentialcellgrowthgeneratesapairofridgesseparatedbyadepression.[NOTE:Theprimitivestreakdefinesthelongitudinalaxisoftheembryoandindicatesthestartofgermlayerformation.]

trophoblastlayer

degeneratingtrophoblast

hypoblastlayer

inner cell mass

embryonic disc

epiblast

Hypoblast Formation (three stages)

delaminatinghypoblast cells

blastocoele

embryonic discmagni�ed

trophoblastlayer

hypoblastlayer

coelom

yolk sac(primitive gut)

coelom

yolk sac(primitive gut)

epiblast

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—Deeptotheprimitivestreak,aspace(coelom/celom)becomesevidentbetweenthehypoblastlayerandepiblast.Subsequently,thecoelomisfilledbymesodermthatundergoescavitationandgivesrisetobodycavities.

—Epiblastcellsproliferatealongprimitivestreakmarginsandmi-gratethroughthestreakintothecoelom.Themigratingcellsformendoderm&mesodermlayers.

—Initialmigratingcellsjointhehypoblastlayer,formingembry-onic endoderm (hypoblastcellsconstitutesyolksacendoderm).

—Themajorityofmigratingcellsenterthecoelomasprimarymesenchymeandbecomemeso-derm.Theprimarymesenchymemigrateslaterallyandcranially(butnotalongthemidlineregiondirectlycranialtotheprimitivestreakwherenotochordwillform).Note:Mesodermdividesinto:paraxial,intermediate,andlateralmesodermalregions.

—Withinthelateralmesoderm,cavitationre-establishesacoelom(hoseshoe-shaped).Themesodermsplitsintotwolayersborderingthecoelom—somatic mesodermisattachedtotheectodermandsplanchnic mesodermisjoinedtoendoderm.

—Theremainingepiblastbecomesectoderm whichformsskinepidermis&nervoussystem.

Dorsal View of Embryonic Disc

NOTE: Arrows indicate the spread of primary mesenchyme through the primitive streak

and between the epiblast and hypoblast

primitivestreak

primitivenode

primarymesenchyme

notochord

epiblast (ectoderm)

hypoblast endoderm primary mesenchyme (mesoderm)

primitive streak

8

NOTE: Mesodermcanexistintwomorphologicforms:mesenchymeandepithelioid: Mesenchymefeaturesaggregatesofstellatecellswithinanabundantextracel-

lularmatrixcomposedoffluidandmacromolecules(polymers). Epithelioid referstoorganizedcellshavingdistinctapicalandbasalsurfaces;

thelattercommonlyrestsonabasallaminaproducedbyepithelioidsecretion.Mesodermcantransformfromamesenchymetoepithelioidandviceversa:The

mesodermthatstreamsthroughtheprimitivestreakisprimary mesenchyme.Somatic,splanchnic,andsomitemesodermcanbetemporarilyepithelioid.Thetemporaryepithelioidtransformstoasecondary mesenchymewhichulti-matelyformsmuscleandconnectivetissue(includingcartilage,bone,liga-ments,tendons,dermis,fascia,andadiposetissue).

Thus,theterm“mesenchyme”referstothemorphologicappearanceofembryonictis-sue.Althoughmostmesenchymeismesoderm,theothergermlayerscanalsoformmesenchyme,e.g.,ectomesenchymefromneuralcrestectoderm.

Formation of the Notochord:•Thenotochordisarod-shapedaggregateofcellslocatedbetweenectodermandendoderm

anteriortotheprimitivestreakoftheembryo.Itoccupiesthemidlinecoelomicspacethatwasnotinvadedbymigratingprimarymesenchyme.

•Thenotochordisimportantbecauseitinduces: formationoftheheadprocess, developmentofthenervoussystem,and formationofsomites•Thenotochordmarksthefuturelocationofthevertebralcolumnandthebaseofthecranium.•Theultimatefateofthenotochordistobecomenucleuspulposusofintervertebraldiscs.Note:Thenotochorddevelopsfromtheprimitive nodelocatedatthecranialendoftheprimitivestreak.Fromthe

node,mesoderm-formingcellsproliferateandmigrateforwardintothefutureheadregionwheretheybecometherod-shapednotochord.

9

Note: Eachorgansystemhasacriticalperiodduringdevelopmentwhenitismostsensitivetoexternalagents(teratogens)thatproducebirthdefects.

Early Formation of the Nervous System (Neurulation): Neurulationreferstonotochord-inducedtransformationofectodermintonervoustissue.Theprocessbeginsduringthethirdweekintheregionofthefuturebrainandthenprogressescaudallyintotheregionofthefuturespinalcord.Theneurulationprocessinvolvesthefollowingsteps:

—ectodermalcellsoverlayingthenotochordbecometallcolumnar(neuroectoderm);theyformathickenedareadesignatedtheneural plate. Theotherectodermalepitheliumisflattened.

—aneural grooveisformedasedgesoftheneuralplatebe-comeraisedoneachsideofamidlinedepression.(Apicalendsofindividualneuroectodermalcellsconstrict.)

—aneural tubeisthenformedastheneuralgrooveundergoesmidlinemergerofitsdorsaledges.Thetubeseparatesfromnon-neuralectodermwhichunitesdorsaltoit.(Tubeformationbeginsinthecranialcervicalregionofthecentralnervoussystemandprogressescraniallyandcaudallyuntilanteriorandposterior neuropores,thelastopenings,finallyclose.)

—bilaterally,wheretheneuralgrooveisjoinedtonon-neuralectoderm,cellsdetachastheneuralgroovecloses;thecellsproliferateandassumeapositiondorsolateraltotheneuraltube—formingneural crest.

NOTE:Neuraltubebecomesthecentralnervoussystem,i.e.,thebrainandspinalcord.

Neuralcrestcellsareremarkablefortherangeofstructurestheyform.Somecellsmi-gratedorsallyandbecomepigmentcellsinskin.Othercellsmigrateventrallyandbe-comeneuronsandglialcellsoftheperipheralnervoussystem,oradrenalmedullacells.Inthehead,neuralcrestformsmesenchyme(ectomesenchyme)whichbecomesmenin-ges,bone,fascia,andteeth.

Neurulation

ectoderm

neuroectoderm

notochord

neural groove

neural tube

paraxial mesodermintermediate mesoderm

lateral mesoderm

coelom

somite

neural crest

somaticsplanchnic

endoderm

10

Somites: •Mesodermblockslocatedjustlateraltothenotochord,whichinducedsomitedevelopment. •Apairofsomitesdevelopsforeveryvertebra,plusahalfdozensomitepairsinthehead. •Numberofsomitesinanembryoisindicativeofage,individualsomitesdevelop

chronologically,incraniocaudalorder.

Somitesdevelopasfollows:—mesoderm,designatedparaxial mesoderm,accumulatesoneachsideofthenotochord—progressingfromrostraltocaudalovertime,transversefissuresdividetheparaxialmesoderm

intoblocks—eachblockbecomesasomite(epithelioidcellswithinasomiteblockre-orient90°,fromtransversetothe

notochordtolongitudinal)

—head(occipital)somitesdevelopfromproliferationoflocalmes-enchymelateraltothecranialendofthenotochord

—rostraltothenotochord,mes-enchymeformsless-developedsomites,calledsomitomeres;thesemigrateintopharyngealarchesandformmusclesofthejaw,face,pharynx,&larynx.

NOTE: Eachsomitedifferentiatesintothreeregions:Sclerotome(ventromedialregion)givesrise

tovertebrae,ribs,andendochondralbonesatthebaseoftheskull.

Dermatome(lateralregion)givesrisetothedermisofskin

Myotome(intermediateregion)givesrisetoskeletalmusclesofthebody

otic placode

optic placode

heart

umbilicalstalk

somites

pharyngealarches

11

Development of a Cylindrical Body: Theearlyembryoisflat,butthevertebratebodyplanfeaturesacylindricaltheme—variouscylindricalstructures(derivativesofthegut,neuraltube,notochord,etc.)enclosedwithinacylindri-calbody.Transitionfromaflatembryotoacylindricaloneinvolvesthefollowingdevelopments:

Head Process Formation: •Thecranialendoftheembryogrowsdorsal-lyandforwardsothatitprojectsabovetheregionoriginallyinfrontoftheembryo. •Thecylindricalheadprocesselongatesbyadditionalgrowthfromitsbase(locatedinfrontoftheprimitivenode).Consequently,themostanteriorpartoftheembryoistheoldest.Theelongationincorporatesthemostanteriorhalf-dozensomitesintothefuturehead.

•Withintheheadprocess,endodermisreflect-edventrallyuponitself,formingablind-endedforegut(futurepharynx).

Tail Fold Formation: •Atthecaudalendoftheembryo,acylindricaltailfoldisformedinamannersimilartothatoftheheadprocess. •Foldedendodermenclosesablindhindgut.

Lateral Body Folds: •Astheheadprocesselongatesupward&forward,asubcephalicpocket(space)isformedventraltotheheadprocess,betweentheheadprocessandextra-embryonictissue.Thebilateralmarginsofthispocketarelateralbodyfolds—

whichconstitutethecontinuitybetweentheelevatedembryoandtherelativelyflatextra-embryonictissue.

•Similarfoldsexistcaudallyinassociationwiththetailprocess.

•Astheembryogrowsandiselevateddorsally,lateralbodyfoldsad-ductandjointogetherventrally,establishingatubularembryosepa-ratedfromflattenedextra-embryonictissue.

•Progressingcaudallyfromtheheadprocessandcraniallyfromthetailfold,ventralfusionoflateralbodyfoldsstopsattheumbilicus—leavingaventralopeninginthebodywallthatallowsvesselsandtheyolksacandallantoistoentertheembryo(andcommunicatewiththegut).

oral plate

head process

subcephalicpocket

pharynx

ectoderm

endodermmesoderm

yolk sac

Three Stages ofHead Process Formation

(longitudinal views)

head process

yolk sacpericardium

DorsalView

primitivestreak

primitivenode

elevatedhead

process

lateralbodyfold

12

•Ventralfusionoflateralbodyfoldsdistinguishestheembryofromextra-em-bryonictissue(fetalmembranes): —embryoniccoelom(futurebodycavitiesofthetrunk)isdistinguishedfromextra-embryoniccoelomwithinfetalmem-branes. —somatopleure(somaticmesoderm+ectoderm)thatformsbodywallisdistin-guishedfromthatformingfetalmembranes(chorionandamnion). —splanchnopleure(splanchnicme-soderm+endoderm)mergesbilaterallytoformgutandmesentery,differentiatedfromextra-embryonicyolksac(andallantois).

Pharyngeal Arches: Intheheadregion,dorso-ventralarchesdemarcatedbygrooves(clefts)appear.Thearchesarecalledpharyngeal arches andtheyareboundedinternallybypharyngeal pouches. Eacharchcontainsavessel(aorticarch).Withineacharch,ectomes-enchyme(derivedfromneuralcrest)givesrisetoboneandfascia.Myotomesofsomitomeresmigratetopharyngealarchestoprovideskeletalmusculature.Eacharchisinnervatedbyonecranialnerve. Onlythefirstthreepharyngeal arches areexternallyevi-dentinmammals.Thefirstarchdevelopsintoupperandlowerjawsandmusclesofmastication.Thesecondgivesrisetohyoidbonesandmusclesoftheface.Theremainingpharyngealarchesformhyoidbones,larynxandassociatedmuscles.Eacharchisinnervatedbyaparticularcranialnerve. Thepharynx(foregut)developsfivebilateraldiverticulathatinternallydemarcatethepharyn-gealarches.Thesepharyngeal pouchesdevelopintoauditorytube,parathyroidglands,thymus,etc.

NOTE:Infish,fiveorsixbranchial[Gr.=gill]archesarewelldeveloped.Cellsdegeneratewherebranchialcleftsandpharyngealpouchesmeetsothatthepharynxcommunicateswiththeoutside(thisoccursonlytemporarilybetweenthefirsttwoarchesinmammals).Thefirstarchformsthejawapparatusandtherestformgillarchesseparatedbygillslits.

Mesoderm = somite = intermediate = lateral

neural tubenotochord

foregut

mesentery

yolksac

embryoniccoelom

extra-embryoniccoelom

somatopleuresplanchnopleure

LateralBody Folds

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Flexures: Thetube-shapedembryoundergoesthreeflexuresthatmakeitC-shaped.Thefirstoccursinthefuturemidbrainregion,thesecondinthefutureneckregion,andthethirdoccursinthetailregion.

Cardiovascular system: •Thecardiovascularsystemdevelopsearly(inthethirdweekafterthestartofthenervous

system),astheembryoenlargesanddiffusionalonebecomesinadequatefortissuepreservation. •Angiogenesis(formationofbloodvessels)beginsinsplanchnicmesodermoftheyolksac,

intheformofbloodislandscomposedofmesenchymeandhemocytoblasts.Thelatterformsbloodcellsandthemesenchymeformsvesicleslinedbyendothelium.Thevesiclescoalescetoformvascu-larchannelsandthenbloodvessels(thelatterareformedbybudding,fusion,&enlargement).

•Vesselsareformedfirstinextra-embryonictissue:vitelline(yolksac)andumbilical(allan-toic)vesselsappearfirst.

•Ventraltothepharynx,bilateralvesselsmergetoformatubularheart;dorsalandventralaortaeareconnectedbyaorticarches.Also,cranialandcaudalcardinalveinsreturnembryonicbloodtotheheartandumbilicalveinsreturnplacentalbloodtotheheart.Noneofthesevesselswillpersistassuchintheadult.

Placentation Placenta=region(s)ofappositionbetweenuterineliningandfetalmembraneswheremetabolitesareexchangedforsustainingpregnancy. •Chorionformsthesurfacefetalmembrane.Appositionareas(placentaltypes)maybe:dif-fuse (pig),zonary (carnivore),discoid (primates&rodents),orinvolveplacentomes. •Aplacentome isadiscreteareaofinterdigitationbetweenamaternalcaruncleandafetalcotyledon.Equineplacentasaremicrocotyledonary(microplacentomesaredistributeddiffusely).Ruminantplacentasconsistofrowsofrelativelylargeplacentomes. •Placentas(placentae)mayalsobeclassifiedaccordingtothetissuelayersseparatingfetalandmaternalblood.Uterineepithelium,uterineconnectivetissueanduterineendotheliummaybeeroded,givingrisetofourplacentaltypes:epitheliochorial(swine,equine,cattle);synepitheliochorial,formerlycalledsyndesmochorial,(sheep,goats);endothelialchorial(carnivore);andhemochorial(primates&rodents).

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Porcine Chorionic Surface(folds; diffuse placental contact)

Equine Chorionic Surface(microcotyledons)

Bovine Chorionic Surface(rows of cotyledons)

Carnivore Chorionic Surface(zonary placental contact)

Human/Rodent Chorionic Surface(discoid placental contact)

Fetal Components of Placentae

marginalhematoma

cervical star(region over cervix)ne

crot

ic ti

p(c

horio

n w

ithou

t alla

ntoi

s)

marginalhematoma

15

Fetal membranes: Fourfetalmembranesdevelopinaconceptus.Twoarisefromthetrophoblastlayeroftheblastocyst(andarecontinuouswiththesomatopleureoftheembryo).Twoarisefromtheinnercellmassoftheblastocyst(andarecontinuouswithsplanchnopleureoftheembryo);thesetwosplanch-nopleuremembranesarevascular.Thefourfetalmembranesare:

1.Chorion—formstheouterboundaryoftheentireconceptus(fromtrophoblast) 2.Amnion—enclosestheembryowithinafluid-filledamnioniccavity;formedbyfoldsofchorionindomesticmammals(inhumans,amnionformsbycavitationdeeptoapersistenttrophoblast). 3.Allantois—developsasanoutgrowthofhindgutsplanchnopleure(originatesfrominnercellmass).Allantoisgrowstofilltheentireextra-embryoniccoelom,withfluid-filledallantoiccavityindomesticmammals.Theoutersurfaceofallantoisbindstotheinnersurfaceofchorion(andtheoutersurfaceofamnion).Theallantoisishighlyvascularandprovidesthefunctionalvesselsoftheplacenta,viaumbilicalvessels. 4.Yolk sac—continuouswithmidgutsplanchnopleure(developsearlywithhypoblastfor-mationfrominnercellmass).Suppliedbyvitellinevessels,itformsanearlytemporaryplacentainthehorseanddog.Yolksacismostimportantinegglayingvertebrates.Note: Thetermconceptusreferstotheembryoorfetusplusitsfetalmembranes.

implantation Theblastocystisinitiallyfreeintheuterinelumen(nourishedbyuterineglands).Im-

plantationoftheblastocyctisagradualprocess,beginningwithapposition,leadingtoadhesion(orinvasioninthecaseofthehuman&GuineaPig).

Approximateimplantationtimesare:oneweek(human);twoweeks(dog,cat,sheep),3-5weeks(cattle),3-8weekshorse;ordelayedupto4mons(deer,bears).

amnion

somatopleure

splanchnopleure

gut

yolk sac

allantoischorion

coelom

embryo