Ultrastructural analysis of the apical ectodermal ridge during vertebrate limb morphogenesis: I. The human forelimb with special reference to gap junctions☆
[摘要] Thefinestructureofthehumanforelimbapicalectodermalridgeofstages12–19wasexaminedusingtechniquesoftransmissionelectronmicroscopy,freezefracture,andscanningelectronmicroscopy.Thispaperreportsthepresenceofsubcellularstructuresthatdistinguishtheinductivelyactiveapicalectodermfromadjacentdorsalandventralectoderms.Theapexofthehumanforelimbbeginsdevelopmentwithanepitheliumoftwocelllayers(stage12)whichthickensatthedistaltipduringstages13and14intoamultilayeredapicalectodermalridge.Duringthistransitionwehaveobservedthatthebasallaminadifferentiatesfromabilayeredstructuretothedefinitivesinglelamina.Somecellsintheectodermbecomedetachedfromthebasallaminaasstratificationbegins.Atthesametimethesecellsshowincreasedmitoticactivityandthedevelopingridgecellsacquiregapjunctions.Annulargapjunctionsarealsoobserved.Gapjunctionsarenotobservedinadjacent,presumablynoninductive,epithelia.Finally,theridgecellsnexttothebasallaminaacquirebundlesofmicrofilamentsthatareorientedinthedorsal-ventralplaneinthebasalcytoplasmofthecells.Theapicalridgereachesitsgreatestdimensionsduringstage15.Thenumberandperipheralextentofgapjunctionsalsoappeartobegreatestatthissametime.Atstage17,cellswithintheridgebegintodie,andotherridgecellsengulfthem.Bystage19,gapjunctionsintheapicalepitheliumaresparseandareoflesserdiameterthaninthedefinitiveridge.Inaddition,theorientedbundlesofmicrofilamentspresentatstages14–17areabsent.Thus,atstage19amorphologicallydistinctapicalectodermalridgeisnolongerpresent.Theapexofthelimbiscoveredbytwocelllayerstypicalofhumanembryonicepidermis.
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[效力级别] [学科分类] 生物科学（综合）
[关键词] [时效性]
