Gene expression analysis of bovine embryonic disc, trophoblast and parietal hypoblast at the start of gastrulation

The RNAseq transcriptomes of all four tissues of an early gastrulation (Stage 5) bovine embryo are presented: (i) the top layer of the embryonic disc consisting of embryonic ectoderm (EmE), (ii) the bottom layer of the disc consisting of mesoderm, endoderm and visceral hypoblast (MEH), (iii) the trophoblast (TB) and (iv) the parietal hypoblast. Additionally a whole Stage 4 embryonic disc was analysed, using curated lists of mouse embryonic structures, pathway analyses and receptor-ligand interactions. The EmE transcriptome is similar at Stages 4 and 5, and is characterised by the OCT4/SOX2/NANOG pluripotency network. Expression of genes associated with primordial germ cells suggest their presence in the EmE. Anterior visceral hypoblast genes are transcribed in the Stage 4 disc but no longer by Stage 5. The MEH layer unexpectedly exhibits the greatest similarity to mouse extraembryonic visceral hypoblast. Our data suggests that the first mesoderm to invaginate is fated to become extraembryonic. The MEH layer expresses BMP, WNT, IGF2 and IHH growth factors with WNT receptors exclusively transcribed in the overlying EmE concordant with vertical paracrine signalling. Parietal hypoblast expresses cholesterol pathway genes, potentially related to the trophic role of its descendant, the yolk sac. All hypoblast-containing tissues express PDGFRA with PDGF transcribed only in the overlying tissues (TB/EmE). The TB transcriptome was the most unique and differed significantly from that of mice. FGF signalling in the TB may be autocrine with both FGFR2 and FGF2 expressed.