a, Freshly collected E5.75 mouse embryos (top) and isolated E5.75 epiblasts (bottom). b, Immunostaining of day 4 epiblast outgrowths. KSR, KnockOut serum replacement. Green, OCT4; red, SSEA-1; blue, DAPI. Insets, higher-magnification images. c, Morphologies of epiblast outgrowths at day 1 and day 4 in EpiSC and rsEpiSC derivation medium. Bottom, EpiSCs and rsEpiSCs at passage 10 (P10). Arrowheads, edges of the epiblast outgrowths. d, Schematic model summarizing the effects of various culture parameters on epiblast explants.