Urodeles Remove Mesoderm from the Superficial Layer by Subduction through a Bilateral Primitive Streak

[摘要] Urodelesbegingastrulationwithmuchoftheirpresumptivemesoderminthesuperficialcelllayer,allofwhichmustmoveintothedeeplayersduringdevelopment.WestudiedthemorphogenesisofsuperficialmesodermintheurodelesAmbystomamaculatum,Ambystomamexicanum,andTarichagranulosa.Inallthreespecies,somitic,lateral,andventralmesodermmoveintothedeeplayerduringgastrulation,ingressingthrougha“bilateralprimitivestreak”justinsidetheblastopore.Themesodermalepitheliumappearstoslideundertheendodermalepitheliumbyamechanismweterm“subduction.”Subductionremovesthelargeexpanseofsuperficialpresumptivesomiticandlateral–ventralmesodermthatinitiallyseparatesthesub-blastoporalendodermfromthenotochord,leavingtheendodermboundingthestillepithelialnotochordalongthegastrocoelroof.Subductionmaybeacommonfeatureofurodelegastrulation,differinginthisregardfromanurans.Subductingcellsconstricttheirapicesandbecomebottle-shapedastheyapproachthejunctionofthemesodermalandendodermalepithelia.Subductingbottlecellsendocytoseapicalmembraneandwithdrawthetightjunctionalcomponentcingulinfromthecontractingcircumferentialtightjunctions.Eitherinconjunctionwithorimmediatelyaftersubducting,themesodermalcellsundergoanepithelial-to-mesenchymaltransition.Themechanismbywhichepithelialcellsreleasetheirapicaljunctionstobecomemesenchymal,withoutdisruptingtheintegrityoftheepithelium,remainsmysterious,butthissystemshouldproveusefulinunderstandingthisprocessinadevelopmentalcontext.

[学科分类] 生物科学（综合）

[关键词] endocytosis;Keller sandwich explants;giant sandwich explants;ingression;primitive streak;subduction;Ambystoma maculatum;Ambystoma mexicanum;Taricha granulosa;superficial mesoderm;epithelial–mesenchymal transition;epithelial morphogenesis