Development of neuromuscular junctions in rat embryos☆

[摘要] Physiologicalpropertiesofnerve-musclejunctionswerestudiedinintercostalmusclesofratembryosof13to21daysgestationandinneonates.NervebundlesgrewintothemuscleregionbyDay13ofgestation.MyotubesbegantoappearonDays13–14.Myotubeswereelectricallycoupledbeforebirth,allowingthespreadofdepolarizationlaterallybetweenfibers.Thestrengthofcouplingdeclinedwithembryonicageanddisappearedafterbirth.Atearlytimes,somefibersofadjacentsegmentswerealsocoupled,endtoend.Restingpotentialsofmyotubeswerehigh(70–90mV)fromthetimeoftheirappearance.Miniatureend-platepotentialswererecordedinsomemyotubesonDay14ofgestation.Atthattimealso,nervestimulationcouldevokeanend-platepotentialwhichwascapableoftriggeringmusclecontraction.Themeanquantalcontentoftransmitterreleasedfromindividualterminalswassmallcomparedtothatinadultmuscle;itremainedsmallthroughthefirstpostnatalweek.Individualmyofibershadasingleend-platesiteneartheircenter,whichcouldreceiveasmanyassixdistinctsynapticinputs.ThenumberofinputsperfiberreachedapeakatDay17ofgestation,andthenbegantodeclinebeforebirth,reachingitsadultvalueofoneinputperfiberwithinthesecondpostnatalweek.Theinternalintercostalmusclescontainedabout30motorunits,eachconfinedtoasmallzoneinthemuscle.Theregionoccupiedbyasinglemotorunitwasnotobviouslyreducedinsizeasthenumberofsynapticinputsperfiberdeclined.AtDay17ofgestation40%ofthemusclescontainedoneormoreaberrantmotorunits,theparentaxonsofwhichprojectedoutthroughtheventralrootsofadjacentsegments.Eliminationoftheseunitscommencedatthesametimeasdidthereductioninnumberofsynapticinputstosinglemyofibers,and70%oftheaberrantunitswereeliminatedbeforebirth.

[学科分类] 生物科学（综合）