Cell lineage analysis in ascidian embryos by intracellular injection of a tracer enzyme: III. Up to the tissue restricted stage☆

摘要

CelllineagesduringembryogenesisoftheascidianHalocynthiaroretziwereanalyzedupuntilthestagewhereeachblastomerewasfatedtobeonlyasingletissuetype(i.e.,thetissuerestrictedstage)byintracellularinjectionofhorseradishperoxidaseusingtheiontophoreticinjectionmethod.Initially,thedevelopmentalfatesofallblastomeresofthe64-cellstageembryowereexamined,andthereafter,onlythefatesofdaughterblastomeresofthoseblastomeresthatwerenottissuerestrictedatthe64-cellstageweretraced.Thedevelopmentalfatesofblastomereswerehighlyinvariantexceptfortwocandidatesfora€?equivalencegroupsa€?(J.Kimble,J.Sulston,andJ.White(1979).Ina€?CellLineage,StemCellsandCellDetermination,a€?pp.59a€“68.Elsevier,Amsterdam/NewYork),inwhichcellularinteractionissuggestedtobeinvolvedinthespecificationofthefates.Therightandlefta8.25cellsgaverisetotheotolithandocellus,andtherightandleftb8.17cellsgaverisetothespinalcordandendodermalstrandinacomplementarymanner.Nofixedrelationshipexistedbetweenthepositionoftheblastomereanditsderivative.Mostrestrictionsofcellfatesoccurredearlyincleavage.Thenumbersofblastomereswhichgeneratedasingletypeoftissuewere44atthe64-cellstageand94atthe110-cellstage.Eightpairsofblastomereshadnotyetbecometissuerestrictedbythe110-cellstage.Almostcompletelineagesofepidermis,nervoussystem,muscle,mesenchyme,notochord,andendodermaltissuesweredescribed,andafatemapwasconstructedfortheblastula.Forcertaintissues,theprimordialcellsoccupiedtwodifferentregions.Supplementaryinvestigationsofthelineageofmusclecellswerealsoperformedonembryosofanotherspecies,Cionaintestinalis.