Ultrastructural characterization of surface specializations containing high-density acetylcholine receptors on embryonic chick myotubes in vivo and in vitro☆

[摘要] Surfacespecializations(patches)onmyotubesfromlatissimusdorsimusclesofembryonicchickbetween10and15daysofincubationandon5-day-culturedchickthighmyotubeswerecharacterizedbyheavymetalstainsandthebindingofhorseradishperoxidase-labeledα-bungarotoxin(HRP-α-BTX)forthelocalizationofacetylcholinereceptors(AChR).Thespecializationsconsistedofslightlyraisedsurfaceridges(0.1to0.5μminwidth)andwiderregions(2–5μm)sometimescomposedofaseriesofthesmallerridges.Thespecializedzoneswerecharacterizedbyanexternalcoatingofamorphous,fine-texturedmaterial,considerablymoreextensivethanthesparse,developingbasementlaminaoccurringelsewhereonthesurface.Ontheinternalsurfaceandcoextensivewiththeexternallayer,densematerialwasappliedimmediatelyadjacenttotheplasmamembrane.Cytoplasmicmicrofilamentsandmicrotubulescameintocloseproximitytothesubmembranousdensity.HRP-α-BTXbindingoccurredatspecializedregions,indicatingthepresenceofahighdensityofnicotinicAChRatthesesites.Notallofthespecializedregionsnortheentireextentofthebroaderpatchesboundtheconjugate.Allregionsbindingtheconjugate,however,hadassociatedspecializations.Reactivecoatedvesicleswereassociatedwithpatchregions,indicatingtheymaybeinvolvedinthetransportofreceptor-bearingmembraneandcoatmaterialtothesurface.ThespecializedregionsbindingHRP-α-BTXwereusuallywidelyseparatedonuninnervatedsurfaces.Reactivepatcheswereinvariablypresentatsitesofnerve-musclecontactinvivo.Inculturedmyotubes,largereactivepatchesweremoreabundantandmoreextensivethaninvivo.Theseregionscorrespondtothe“hotspots”observedatthelight-microscopiclevel.Reactivepatchesweremostabundantonthelateralandbasalsurfacesofcells,oncellprocesses,andwherecellscontactedthesubstrateorothercells.ThesestudiesshowthatexternalandinternalmembranespecializationsareassociatedwithahighdensityofAChRintheabsenceofinnervation.ConstituentsofthesespecializationsmaybeinvolvedinthelocalizationorpositioningofahighdensityofAChRatthesesitesorcontaincomponentswhichestablishtheseregionsaspreferentialsitesofrecognitionorcontactbythenerve.

[学科分类] 生物科学（综合）