MEK signalling pathway is required for hypoblast specification and migration in ovine.

In brief: MEK signalling pathway is required for hypoblast differentiation in mouse embryos, but its role in ungulate embryos remains controversial. This paper demonstrates that MEK is required for hypoblast specification in the inner cell mass of the ovine blastocyst and that it plays a role during the hypoblast migration occurring following blastocyst hatching. Abstract: Early embryo development requires the differentiation of three cell lineages in two differentiation events. The second lineage specification differentiates the inner cell mass into epiblast, which will form the proper fetus, and hypoblast, which together with the trophectoderm will form the extraembryonic membranes and the fetal part of the placenta. MEK signalling pathway is required for hypoblast differentiation in mouse embryos, but its role in ungulate embryos remains controversial. The aim of this work was to analyse the role of MEK signalling on hypoblast specification at the blastocyst stage and on hypoblast migration during post-hatching stages in vitro in the ovine species. Using well-characterized and reliable lineage markers, and different MEK inhibitor concentrations, we demonstrate that MEK signalling pathway is required for hypoblast specification in the inner cell mass of the ovine blastocyst, and that it plays a role during the hypoblast migration occurring following blastocyst hatching. These results show that the role of MEK signalling pathway on hypoblast specification is conserved in phylogenetically distant mammals.

Transforming growth factor beta (TGFß) pathway is essential for hypoblast and epiblast development in ovine post-hatching embryos.

The developmental failures occurring between blastocyst hatching and implantation in farm ungulates are a major cause of pregnancy losses. At the expanded blastocyst stage, three cell lineages emerge in the embryo: trophoblast, hypoblast and epiblast, the latter being the most vulnerable during post-hatching development. Transforming growth factor beta (TGFß) signaling pathway is involved in hypoblast and epiblast development; however, previous in vitro functional studies are limited to the expanded blastocyst stage. In this study, we have analyzed the effect of TGFß inhibition with 10, 20 or 40 µM SB431542 during ovine post-hatching developmental period using a recently developed culture system able to recapitulate major developmental landmarks. We have found a negative effect of TGFß inhibition on hypoblast and epiblast development that could be partially reverted by Rho-associated protein kinase (ROCK) inhibitor Y-27632. Our findings provide new insights into the molecular networks regulating embryo development beyond the expanded blastocyst and could help to elucidate the causes of early pregnancy losses in farm ungulates.

In vitro culture of ovine embryos up to early gastrulating stages.

Developmental failures occurring shortly after blastocyst hatching from the zona pellucida constitute a major cause of pregnancy losses in both humans and farm ungulates. The developmental events occurring following hatching in ungulates include the proliferation and maturation of extra-embryonic membranes - trophoblast and hypoblast - and the formation of a flat embryonic disc, similar to that found in humans, which initiates gastrulation prior to implantation. Unfortunately, our understanding of these key processes for embryo survival is limited because current culture systems cannot sustain ungulate embryo development beyond hatching. Here, we report a culture system that recapitulates most developmental landmarks of gastrulating ovine embryos: trophoblast maturation, hypoblast migration, embryonic disc formation, disappearance of the Rauber's layer, epiblast polarization and mesoderm differentiation. Our system represents a highly valuable platform for exploring the cell differentiation, proliferation and migration processes governing gastrulation in a flat embryonic disc and for understanding pregnancy failures during the second week of gestation. This article has an associated 'The people behind the papers' interview.

Ovine oocytes display a similar germinal vesicle configuration and global DNA methylation at prepubertal and adult ages.

Epigenetic mechanisms are thought to be involved in the reduced developmental capacity of early prepubertal ewe oocytes compared to their adult counterparts. In this study, we have analyzed the global DNA methylation pattern and in vitro meiotic and developmental competence of oocytes at the germinal vesicle (GV) stage obtained from adult and 3-month-old donors. All oocytes were aspirated from antral follicles with a diameter ≥3 mm, and DNA methylation on 5-methylcytosine was detected by immunofluorescence using an anti-methyl cytosine antibody. The main global chromatin configuration pattern shown by both prepubertal and adult ovine oocytes corresponded to condensed chromatin localized close to the nuclear envelope (the SNE pattern). Immunofluorescence showed that a global bright nuclear staining of 5-methylcytosine (5-mC) occurred in all germinal vesicle stage oocytes and matched the propidium iodide staining pattern. The total fluorescence intensity values of lamb GVs were not lower than those observed in adult GVs. The meiotic competence and cleavage rates were similar in adult and prepubertal oocytes, however, the developmental competence of embryos to reach blastocysts was higher for adult oocytes than lamb oocytes (p<0.0001). In conclusion, our results indicate that adult-size oocytes derived from 3 to 4 month old prepubertal ewes show similar GV morphology and DNA methylation staining patterns to those obtained from adult animals, despite exhibiting a lower developmental competence.