Themostprevalentcultivationmethodformaintenanceofmouseandhumanembryonicstem(ES)cellsandinducedpluripotentstem(iPS)cellsisco-cultivationonprimarymouseembryonicfibroblast(mEF)feedercelllayers.FeedercelllayersofNIH3T3fibroblastsorstromalderivedOP9cellsarealsofrequentlyusedinco-culturesystems:NIH3T3fibroblastssupportprimarykeratinocytecultures,OP9cellsareusedforhematopoieticinductionofEScells.