Electron microscopy of four-cell stages of the ascidians Ciona and Styela☆

摘要

Theinclusionsandsubmicroscopicstructureofanteriorandposteriorblastomeresoffour-cellstagesofCionaintestinalisandStyelabarnhartiwerestudiedbymeansofelectronmicroscopy.Inbothformstheblastomereshaveanouterrimofinclusionsconsistingmainlyofseveraltypesofyolkgranules,mitochondria,andlipiddroplets.Thelargeinnercoreofhyaloplasmconsistsofcloselypackedvesiclesandassociatedsmallgranules.InStyelamanymitochondriacloselyadheretoyellowlipiddroplets,andintheposteriorblastomeresthereisalocalizationoftheseclusterstoformtheyellowcrescent(mesoplasm)describedbyConklin.Threetimesasmanymitochondriaandeighttimesasmanylipiddropletsarefoundinsectionsofposteriorascomparedtoanteriorblastomeres.Itisconcluded,onthebasisofsize,color,density,anddegreeofosmiophilia,thattheyellowlipiddropletscorrespondtoConklin'sa€?yellowmitochondria.a€?Theclearanteriorregionsoftheanteriorblastomeres,probablythechorda-neuroplasm,ischaracterizedbyathinningofthelayerofinclusions.Nolocalizationofspecificinclusionsoccursinthisarea.TheelectronmicrographsindicatethatthevisibleareasofthelivingStyelabarnhartiegg,asidefromthelocalizationofyellowlipiddropletsandthemitochondriatoformtheyellowcrescent,areduetoregionaldifferencesinthethicknessoftheperipheralrimofinclusions.Infour-cellstagesofCionaovertwiceasmanymitochondriaarefound,asindicatedbycountingmitochondriainsections,inposteriorascomparedtotheanteriorblastomeres.Thevegetalcytoplasmiccapsoftheposteriorblastomeres,whichappeartemporarilyduringthesecondcleavage,arecomposedlargelyofmitochondria.Intheanteriorblastomeresmitochondriaarelikewiselocalizedinthevegetalregion,andinbothanteriorandposteriorblastomeresthemitochondriaaredistributedingradientfashionnearthecellmembraneinavegetal-animaldirection.ThereisalsoalocalizationoflargevesiclesofunknownfunctioninthehyaloplasmoftheposteriorblastomeresofCiona.Nodifferenceswerenoted,insectionsoffour-cellstagesofCionaandStyela,intheendoplasmicreticulumofanteriorandposteriorblastomeres.