Development of the myotomal neuromuscular junction in Xenopus laevis: An electrophysiological and fine-structural study☆

[摘要] ThenormaldevelopmentofthemyotomalneuromuscularjunctioninXenopusembryosandtadpoleswasinvestigatedelectrophysiologicallyaswellaselectronmicroscopically.Spontaneouspotentials,consideredtobeminiatureend-platepotentials(MEPPs),weredetectedbyintracellularrecordingasearlyasstage21andbystage24theywereobservedineveryembryotested.LikeMEPPSatlaterstagestheywereblockedbycurarebutnotbytetrodotoxin.End-platepotentials(EPPs),subjecttoblockbytetrodotoxin,wereevokedbyelectricalstimulationofthespinalcordinembryosasyoungasstage24andoccurredspontaneouslyasearlyasstage22.ThedurationsofMEPPsandEPPswereinitiallyrelativelylong.Focalexternalrecordingsrevealedaneightfolddecreaseindurationduringthecourseofdevelopment.Nerveprocessesemergedfromthespinalcordandcontacteddevelopingmusclecellsasearlyasstage21,butjunctionalspecializationswerenotapparentandvesicleswererareeveninstage24embryos.Duringthenext24hr,betweenstages25and36,vesiclesincreasedinnumberandbecamelocalizedtowardthejunctionalsurfaceofthenerveending.Basementlaminadevelopedinthecleftandpostjunctionalridgesanddensitieswereobserved.Individualmusclecellsalsobecamecontactedbyseveralnerveprocesses.Bystages48–52therewerefewercontactsonindividualmusclecellsandSchwanncellprocessespartiallycoveredthenerveendings.Gapjunctionswereobservedbetweenthemusclecellsthroughoutdevelopmentbutoccurredlessfrequentlyatthelaterstages.Itisconcludedthatbythetimetheyreachthemusclecells,orveryshortlythereafter,atleastsomeofthegrowingnerveprocessescanreleasetransmitter,andsomeofthemusclecellsaresufficientlysensitivetoacetylcholineintheregionofcontacttorespondwithmillivoltdepolarizations.Theseearliestfunctionalcontacts,however,aremorphologicallyundifferentiated.

[学科分类] 生物科学（综合）