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Formation of The Bilaminar Embryonic Disc THE SECOND WEEK

Formation of The Bilaminar Embryonic Disc THE SECOND WEEK. Prof.Dr.Meltem ÖZGÜNER Department of Histology-Embryology. THE SECOND WEEK. Trophoblast Embryoblast Extra - embryonic mesoderm At the end of the second week - Primitive uteroplacental circulation - Bilaminar embryonic disc

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Formation of The Bilaminar Embryonic Disc THE SECOND WEEK

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  1. Formation of TheBilaminarEmbryonicDiscTHE SECOND WEEK Prof.Dr.Meltem ÖZGÜNER Department of Histology-Embryology

  2. THE SECOND WEEK • Trophoblast • Embryoblast • Extra-embryonicmesoderm At theend of thesecondweek -Primitiveuteroplacentalcirculation -Bilaminarembryonicdisc -Theprochordalplatedevelops

  3. IMPLANTATION • Implantation of theblastocystbegins at theend of thefirstweekand is completedbytheend of thesecondweek

  4. IMPLANTATION Theprocessmay be summarized as follows: • Day 5:The zona pellucidadegenerates . Itsdisappearanceresultsfromtheenlargement of theblastocyst. • Day 6: Theblastocystattachestoendometrialepithelium.

  5. IMPLANTATION • Day 7: Thetrophoblastbeginstodifferentiate in totwolayers, syncytiotrophoblastandcytotrophoblast. 1-thecytotrophoblast, a layer of mononucleatedcellswhich is mitoticallyactiveandformsnewcellsthatmigrateintothesyncytiotrophoblasts 2-thesyncytiotrophoblast, whichrapidlybecomes a large, thick, multinucleatedmass in which no cellboundariesarediscernible.

  6. IMPLANTATION • Thesyncytiotrophoblastbeginstoproducehumanchorionicgonadotrophin (hCG), andenoughhCG is produced at theend of secondweektogive a positivepregnancy test. • hCGinducescorpusluteumforprogesteronsecretion.

  7. IMPLANTATION • Day 8:Thesyncytiotrophoblasterodesendometrialtissues (capilleries, glands, stroma) andtheblastocyststartstoembed in theendometrium. • Day 9: Bloodfilledlacunaappear in thesyncytiotrophoblast.

  8. IMPLANTATION • Day10:Theblastocystsinksbeneaththeendometrialepithelium. • Day 10 and 11: Lacunarnetworks form byfusion of adjacentlacunae.

  9. IMPLANTATION • Day11 and 12: Thesyncytiotrophoblastcontinuestoerodeendometrialbloodvessels, allowingmaternalbloodtoseepintoandout of thelacunarnetworks, therebyestablishing a primitiveuteroplacentalcirculation

  10. IMPLANTATION • Day 12 and 13: Thedefect in theendometrialepitheliumgraduallydisappears as thesurfaceepithelium is repaired. • Day 13 and 14: Primarychorionicvillidevelop

  11. IMPLANTATIONDecidualReaction • Rapidproliferationanddifferentiation of thetrophoblastareimportantfeatures of thesecondweek of development. • Theseprocessesoccur as theblastocystimplants in theendometrium • Thevariousendometrialchangesresultingfromadaptation of thesetissuestoimplantation of theblastocystareknowncollectively as thedecidualreaction.

  12. IMPLANTATIONDecidualReaction • Theblastocystattachtotheendometriallayer at theirembryonicpole; and at thisregion, trophoblastcellsdisplaceendometrialcells in thecentralpart of theimplantation site. • Thestromalcellsaroundtheimplantation site become laden withglycogenandlipidsandnamed as decidualcells. Theseprovide a richsource of embryonicnutrition.

  13. AmnioticCavity, Amnion, BilaminarEmbryonicDiscandYolk Sac • Formation of theAmnioticCavityandAmnion • At thebeginning of thesecondweek, a smallcavityappears at theembryonicpolebetweentheembryoblastandtrophoblast.

  14. Formationof theAmnioticCavityandAmnion • When it enlarges it becomestheamnioticcavity • Floor of theamnioticcavity is formedbytheepiblast

  15. Formation of theAmnioticCavityandAmnion • Amnioblastsoramniogenic (amnion-forming) cells that separate from the epiblast and form a membraneknown as the amnion, which encloses the amniotic cavity.

  16. Formation of theAmnioticCavityandAmnion

  17. Formation of theBilaminarEmbryonicDisc • Concurrently, morphologicalchangesoccur in theembryoblastandresultswiththeformation of a flattened , bilaminarplate of cellscalledembryonicdisc.

  18. Formation of theBilaminarEmbryonicDisc Thethickdiscconsists of twolayers: • 1-theepiblast, consisting of highcolumnarcellsrelatedtotheamnioticcavity, and • 2- thehypoblast, consisting of cuboidalcellsadjacenttotheblastocystcavity

  19. Formation of theExocoelomicMembrane • At days 9 to 10, the cells from the hypoblast begin to migrate to the unembryonic pole forming a layer of cells just beneath the cytotrophoblast, called the Heuser's Membrane (or the exocoelomic membrane). • It surrounds the exocoelomiccavity.i.e. it lines the inner surface of the cytotrophoblast. • Heuser's membrane (or the exocoelomic membrane) is a short lived combination of hypoblast cells and extracellularmatrix.

  20. Formation of theExocoelomicMembrane • At days 9 to 10, the cells from the hypoblast begin to migrate andform a thin layer of cells just beneath the cytotrophoblast, called the exocoelomic membrane (or the Heuser's Membrane).That is continuouswiththehypoblast.

  21. Formation of theExocoelomicMembrane • It surrounds the blastocystcavitywhich is nowcalledtheexocoelomiccavityandit lines the inner surface of the cytotrophoblast. • Heuser's membrane (or the exocoelomic membrane) is a short lived combination of hypoblast cells and extracellularmatrix.

  22. Formation of theExocoelomicMembrane • Theexocoelomicmembraneandcavitysoonbecomemodifiedto form theprimaryyolk sac (primitiveyolk sac) • Theembryonicdiscnowliesbetweentheamnioticcavityandtheprimaryyolk sac.

  23. Formation of theExtraembryonicCavity • At days 11 to 12, cells, probablyfromthehypoblast, giveriseto a layer of loosely arranged cells that inserts aroundtheamnionandprimaryyolk sac, calledtheextraembryonicmesoderm.

  24. Formation of theExtraembryonicCavity • As changesoccur in thetrophoblastandendometrium, theextraembryonicmesodermincreasesandisolatedcoelomicspacesappearwithin it. • Thesespacesrapidlyfuseto form a large, isolatedcavitycalledtheextraembryoniccoelom

  25. Formation of theExtraembryonicCavity • Extraembryoniccoelom(fluid-filledcavity) surroundstheamnionandyolk sac exceptwheretheyareattachedtothechorionbytheconnectingstalk • Extraembryonic somatic mesoderm also forms the connecting stalk that is the primordium of the umbilical cord.

  26. Formation of theSecondaryYolk Sac • At day 13, as theextraembryoniccoelomforms, theprimaryyolk sac decreases in size and a smallersecondaryyolk sacforms.It is oftenreferredtosimply as yolk sac.

  27. Formation of theSecondaryYolk Sac • Secondaryyolk sac is formedbythecellsthatmigratefromthehypoblast inside theprimaryyolk sac. • Theyolk sac containsfluid but no yolk. • Itappearstohave a role in thenutrition of theembryonicdisc.

  28. Blastocyst - day 13 1. Secondaryyolk sac2. Exocoelomiccyst3. Aminoticcavity4. Extra-embryoniccoelom5. Epiblast6. Connectingstalk7. Hypoblast8. Primaryvilli9. Trophoblasticlacunae10. Extraembryonicsomaticmesoderm11. Extraembryonicsplanchnicmesoderm Blastocyst - day 12 1. Syncytiotrophoblast2. Trophoblasticlacunae3. Surfaceepithelium4. Fibrin coagulum5. Epiblast6. Aminoticcavity7. Hypoblast8. Cytotrophoblast9. Exocoelomiccavity

  29. DEVELOPMENT OF THE CHORIONIC SAC • Theend of thesecondweek (at approx. day 13) is characterizedbytheappearance of primitivechorionicvilli.

  30. PRIMITIVE CHORIONIC VILLI • Proliferation of cytotrophoblastcellsproduceslocalmassesthatextendintothesyncytiotrophoblast.

  31. PRIMITIVE CHORIONIC VILLI • Thecellularprojections form primarychorionicvilliwhichrepresentthefirststage in thedevelopment of thechorionicvilli of theplacenta.

  32. DEVELOPMENT OF THE CHORIONIC SAC Theextraembryoniccoelomsplitstheextraembryonicmesodermintotwolayers: • 1-extraembryonicsomaticmesoderm, liningthetrophoblastandcoveringtheamnion • 2-extraembryonicsplanchnicmesoderm, surroundingtheyolk sac.

  33. DEVELOPMENT OF THE CHORIONIC SAC Theextraembryoniccoelomsplitstheextraembryonicmesodermintotwolayers: • 1-extraembryonicsomaticmesoderm, liningthetrophoblastandcoveringtheamnion • 2-extraembryonicsplanchnicmesoderm, surroundingtheyolk sac.

  34. CHORION Theextraembryonıcsomatıcmesoderm and twolayers of trophoblastconstitute CHORION

  35. CHORIONIC CAVITY • Thechorionformsthewall of thechorionic sac, withinwhichtheembryoanditsamnioticandyolksacsaresuspendedbytheconnectingstalk • Theextra-embryoniccoelom is nowcalledtheCHORIONIC CAVITY

  36. PROCHORDAL PLATE • The 14 dayembryostill has the form of a flat, bilaminarembryonicdisc, but thehypoblasticcells in a localizedareaarenowcolumnarand form a thickened, circularareacalledtheprochordalplate.

  37. PROCHORDAL PLATE • Thisplateindicatesthefuture site of themouthand is an importantorganizer of theheadregion.

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