Pofut2 activity in the epiblast is essential for axis elongation. The genotypes of the epiblast (Epi) and extraembryonic (Ext) tissues in the embryo pairs (control on the left and epiblast mutants on the right) are indicated at the top of the figure. To evaluate the effects of deleting Pofut2 in the epiblast using Sox2::Cre, embryos were obtained between E 7.5 (A–B, G) and 8.5 (C–F, H) and were processed for whole-mount in situ hybridization expression or were embedded, sectioned, and stained with hematoxylin and eosin to evaluate tissue morphology (G and H). (A, C, and E) T was expressed in the primitive streak and notochord of Pofut2 wild-type and epiblast mutants, suggesting that loss of POFUT2 activity in the extraembryonic tissue was responsible for blocking gastrulation in the Pofut2 mutants (Fig. 1). Arrowheads in panel E identify discontinuous notochord. (E) Inset shows PCR genotyping of embryos. Lanes correspond to embryo on the left and right. Full litters for panels C and E are shown in Supplementary Fig. 3 with complete genotyping. (B, D, and F) Foxa2 was expressed in the anterior primitive streak and emerging mesendoderm at the late primitive streak stage (B), notochord at the early headfold stage (D), and notochord and floorplate at late headfold stage (F) in Pofut2 wild-type and epiblast mutants. Both T and Foxa2 expression domains were wider and somewhat irregular in shape in the Pofut2 epiblast mutants compared to wild-type littermates. (G and H) Sagittal sections of Pofut2 wild-type and epiblast mutant littermates at E 7.5 (G) E 8.5 (H) stained with hematoxylin and eosin. (G) At E 7.5 (late bud stage), the chorion frequently remains attached to the amnion in Pofut2 epiblast mutants and the epiblast appears disorganized. Persistent attachment of chorion and amnion is indicated by asterisk (*). (H) At E 8.5, there is evidence of heart and somite development in Pofut2 epiblast mutants. Anterior is to the left, posterior to the right, proximal up, and distal down. Amnion, am; allantois, al; blood islands, bl; brachial arch, ba; brain, br; chorion, ch; heart, he; headfold, hf; neural fold, nf; node, n; notochord, nt; neural tube, nr; optic vesicle, ov; somite, so.