The uptake and localization of radioactive DOPA by amphibian melanoblasts in vitro☆☆☆

[摘要] Neuralcrestexplantsfromdifferentanteroposteriorlevelsofaxolotl(Ambystomamexicanum)embryosweregrowninamodifiedhangingdropsystem.Theculturesweretreatedwith3,4-dihydroxyphenyl-alanine-α-β-3H(DOPA-3H)atvariousages,andtritiumincorporationwasdeterminedbyradioautography.Byday7afterexplantation,allthepotentialmelanophoresinbothtrunkandcranialcultureshavesynthesizedsufficientmelanintomaketheirpositiveidentificationpossible.WhentreatedwithDOPA-3Honday7,8,or9,onlycellsthatcontainmelaninincorporatelabeledDOPA,andallcellsthatcontainmelaninarealsoradioactive.ThisdemonstratesthatdefinitivemelanoblastsselectivelyincorporateDOPA,andthatDOPAservesasaspecificmelaninprecursorinthesecells.SinceDOPAcanariseonlyfromtyrosine,itisconcludedthatatyrosine-DOPA-melaninpathwayisoperativeduringmelanogenesisinamphibianpigmentcells.WhentreatedwithDOPA-3Honday3afterexplantation,one-fourthoftheanteriortrunkoutgrowthsarecomposedalmostentirelyofradioactivecells.Intheremaining3-dayanteriortrunkcultures,andinallthe3-dayposteriortrunk,anteriorcranial,andposteriorcranialcultures,noneofthecellsofanyoftheoutgrowthsarelabeled.Priortothethirdday,thereisnoincorporationofDOPA.Whentreatedonthefourthday,orthereafter,trunkoutgrowthsarecomposedalmostexclusivelyofradioactivecells.Inthecranialoutgrowths,thelabeledcellpopulationdoesnotexceedone-thirdofthetotal.Thetrunkandcranialregionsofneuralcrestarealsosignificantlydifferentfromeachotherwithrespecttopigmentcellpotentialinvitro.Thetrunksegmentsgiverisetooutgrowthswhichconsistalmostexclusivelyofmelanophores,andthecranialsegmentsgiverisetooutgrowthsinwhichmelanophorescompriselessthanone-thirdofthetotalcellpopulation.Theclosecorrespondencebetweenthefrequenciesofradioactivecellsandthefrequenciesofpigmentcellsindicatesthatmelanoblasts,priortovisibledifferentiation,selectivelyincorporateDOPA.ThetimeatwhichpresumptivepigmentcellsarefirstabletoutilizeexogenouslyadministeredDOPAcoincideswiththetimeatwhichmigrationceases,indicatingthatthecapacitytosynthesizemelaninisnotacquireduntilmigrationhasended.Theincorporationoflabelbythepresumptivepigmentcellscontainedincultures3,4,5,and6daysoldpermitstheidentificationofmelanoblastsearlierthanhaspreviouslybeenpossibleanddemonstratesthatthesecellshavedevelopedahighlyspecializedbiochemicalsystembeforeanyoutwardsignsofitaremanifest.Itisclear,moreover,thatamphibianmelanoblastscanutilizeexogenoussubstrateinspiteoftheirampleinternalreserves.

[学科分类] 生物科学（综合）