The development of the conduction system in the mouse embryo heart: I. The first embryonic A-V conduction pathway

摘要

Inthe8-,9-,and10-day-oldmouseembryos,theprimitiveatriaareinterconnectedwiththeventriclesviatheatrioventricular(A-V)canal.Duetothetwistingprocessofthetubularheart,thewalloftheA-Vcanalestablishescontinuitynotonlywiththeleftventriclebutalsowiththebulbusandtruncusarteriosus.Atthisstageofheartdevelopment,theA-Vnodeandbundlehavenotyetappeared,and,thus,theatrialimpulsemustbeconveyedtotheventriclebythemuscletissueofthewalloftheA-Vcanal,inwhichtwomusclecelllayershavebeenobserved.Theinnerlayerextendsdeepintotheleftventricularcavityandisinterconnectedwithboththetrabecularsystemandtheventricular(IV)septum,whichbeginstodeveloponthetenthday.IntheinnerdorsalwalloftheA-Vcanal,thecellsarelarge(a??20??mindiameter)andshowastrongPASreaction.Itislikelythattheselargeglycogen-richcellsfromwhichtheA-VnodeprimordiumdevelopsontheeleventhdayplaythemainroleintheA-Vimpulseconduction.Themusclecellsattheventrolateralwallsofthecanalaresmallandformaloosespongymyocardiumintowhichtheconnectivetissuecellsbegintopenetrateonthetenthday,ultimatelytoformtheannulusfibrosus.Atthesametime,theoutercelllayerofthedorsalwallbeginstodeteriorate;thecellsshowvacuolardegeneration,myolysis,andshrinkagenecrosis.Thisprocessappearstorepresentaprogrammedcelldeath,aswasdescribedinthebirdheart(Pexieder,1975).Onthebasisofmorphologicaldata,thesequenceofatrioventricularactivationbeforetheappearanceoftheA-Vnodeandbundleisdiscussed.