Table 2.

Determining the mode of germ cell specification across the Metazoa *

PGC originMode of PGC specificationExperimental evidence§PGC identification criteria
StageLocation/derivation
BASAL LINEAGES
PoriferaGastrulationMesenchymal cellsE-LM, TEM, MM
Cnidaria
    AnthozoaPost-embryonicIn coelom from gastrodermal cells of mesentery or endocoelic epithelial cellsE-TEM, LM
    ScyphozoaPost-embryonicWithin ovaries from endodermally derived gastrodermisE-TEM
    HydrozoaGastrulationEndodermal coreE+LM, TEM, MM
CtenophoraEarly larval stageEndodermE-LM
BILATERIA (Triploblasts)
AcoelomorphaLate embryogenesisMesenchymalE-LM, TEM
Lophotrochozoa (Protostomes)
Platyhelminthes
    TurbellariaLate embryogenesisMesenchymalE+LM, TEM, MM
    TrematodaFirst cleavageFirst cleavageP-LM
    CestodaLate embryogenesisMesenchymalE-LM, TEM
RotiferaBefore gastrulation4d cellP-LM
Entoproctandndnd-nd
EctoproctaPost-embryonicMesenchyme: gonadal epitheliumE-LM
NemerteaLate embryogenesisMesodermally derived cells of parenchyma or gonadal epitheliumE-LM, TEM
PhoronidaLate embryogenesisPeritoneal epitheliumE-LM
BrachiopodaLate embryogenesisIleo-parietal epitheliumE-LM, TEM
Gnathostomulidandndnd-nd
PogonophoraPost-embryonicGonadal epitheliumE-LM
EchiuraLarval stageMesodermE-LM, TEM
SipunculidaLarval stageGonadal epitheliumE-LM
Mollusca
    AplacophoraPost-larvalMesodermal?E-LM
    PolyplacophoraPost-embryonicGonadal epitheliumE-TEM
    CephalopodaBlastoderm stageBlastoderm superficial layerP-LM
    GastropodaLate embryogenesis/early cleavage?Mesodermal/early cleavage blastomere?E/P-LM, TEM
    BivalviaEarly cleavage4d cellP-LM
Annelida
    PolychaetaEarly cleavage/post-larval4d cell/peritoneal vascular epithelium/E/P-LM, TEM
    OligochaetaEarly cleavage/late embryogenesis4d cell/unknown source before mesoderm formation/unknown source late in developmentE/P+LM, TEM
    HirudineaEarly cleavageD blastomereP-LM, MM
Ecdysozoa (Protostomes)
Arthropoda
    CollembolaEarly cleavageEarly cleavage blastomeresP-LM, TEM
    InsectaEarly cleavage/late embryogenesisEarly cleavage blastomere/mesodermE/P+LM, TEM, SEM, EM, MM, LI
    CrustaceaEarly cleavage/late embryogenesisEarly cleavage blastomere/mesodermE/P-LM, TEM, MM, LI
    ChelicerataEarly cleavage/late embryogenesisInner blastoderm cells/primary cumulus/secondary cumulus/mesodermE/P-LM, TEM, SEM, MM
    MyriapodaLate embryogenesisMesoderm: coelomic sacsE-LM
TardigradaLate embryogenesisMesoderm: coelomic sacsE-LM
OnychophoraGastrulation/late embryogenesisBlastopore/endoderm/mesodermE/P-LM
NematodaFirst cleavageFirst cleavage blastomereP+LM, TEM, SEM, MM, LI
Priapulandndnd-nd
GastrotrichaLate embryogenesisBase of proctodeumE-LM
KinorhynchandApical cells of gonadE-LM
Deuterostomes
ChaetognathaFirst cleavageFirst cleavage blastomereP+LM, TEM, MM, LI
HemichordataLate embryogenesisEctoderm/mesodermE-LM
Echinodermata
    CrinoideaMetamorphosisWall of stomatocoelE-LM
    AsteroideaMetamorphosisWall of stomatocoelE-LM, TEM
    HolothuroideaPost-larvalGonadal epitheliumE+LM, TEM
    EchinoideaMetamorphosis/16-cell stage?Wall of stomatocoel/small micromeres?E/P+LM, TEM, MM
Chordata
    Urochordata64-cell stage/post-metamorphosisB7.6 cells: posterior of embryo/hemocytesE/P+LM, TEM, MM, LI
    CephalochordataCleavage stages/larval stagesMesoderm of myocoel/gonadal epithelium/single cleavage stage blastomere?E/P-LM, TEM
    AgnathaGastrulationUnclearE-LM
    ChondrichthyesLate cleavage stages/late embryogenesisBlastoderm/mesodermE/P-LM
    ActinopterygiiCleavage stages/late embryogenesisCleavage blastomeres/endodermE/P+LM, TEM, MM, LI
    DipnoiLate embryogenesisUnclearE-MM
    CaudataLate embryogenesisLateral plate mesodermE+LM, TEM, MM
    AnuraCleavage stagesCleavage blastomeres/endodermP+LM, TEM, MM, LI
    ArchosauriaCleavage stagesCleavage stagesP+LM, TEM, EM, MM
    LepidosauriaPrimitive streak formationExtraembryonic endodermE-LM, MM
    TestudinesPrimitive streak formationExtraembryonic endodermE-LM, TEM, MM
    MammaliaPrimitive streak formationProximal epiblastE+LM, TEM, EM, MM, LI
  • * Data compiled from 292 references, which are available in the online version of this table (see Table S2 at http://dev.biologists.org/supplemental/).

  • As comparing the duration of stages of development in different species is often confusing, we describe relative developmental stages rather than absolute time. nd, no data.

  • P, preformation; E, epigenesis.

  • § +, yes; -, no.

  • LM, light microscopic histological analysis, of either whole mounts or sections; TEM, transmission electron microscopy; SEM, scanning electron microscopy; EM, enzymatic markers; MM, molecular markers, usually in situ hybridization or antibody staining; LI, cell lineage studies.