Foxa2 regulates polarity and epithelialization in the endoderm germ layer of the mouse embryo

Abstract

In the mouse, one of the earliest events in the determination of cell fate is the segregation of cells into germ layers during gastrulation; however, the cellular and molecular details are not well defined due to intrauterine development. We were able to visualize a clear sequence of events occurring in the process of germ-layer formation, using immunohistochemistry and time-lapse confocal imaging. The T-box transcription factor brachyury (T) and the Forkhead transcription factor Foxa2 specify mesoderm and endoderm in the posterior epiblast. Fate-specified epiblast cells lose their polarity and undergo epithelial-mesenchymal transition to invade into the primitive streak region, where these cell populations quickly separate and differentiate into morphologically and molecularly distinct Foxa2-positive endoderm and T-positive mesoderm populations. The endoderm cells flatten and acquire apicalbasal polarity during intercalation into the outside epithelium in order to establish proper intracellular junctions with pre-existing cells. By contrast, the mesodermal cells become spherical during migration and acquire a mesenchymal fate. Interestingly, axial mesodermal cells are descended from Foxa2-positive epiblast cells that upregulate T protein in the anterior primitive streak region. These cells, as well as Foxa2-positive endoderm cells, are highly polarized and epithelialized, suggesting that Foxa2 promotes an epithelial fate and suppresses a mesenchymal fate. This observation is supported by the fact that Foxa2 mutant endodermal cells fail to maintain polarity and do not establish proper cellular junctions, and are thus unable to functionally integrate into the endoderm epithelium. We propose that Foxa2 regulates a molecular program that induces an epithelial cellular phenotype.

NOTCH activation interferes with cell fate specification in the gastrulating mouse embryo

“…Whole mount in situ hybridisation and immunohistochemistry were performed according to standard procedures (Burtscher and Lickert, 2009;Chazaud et al, 2006;Perea-Gomez et al, 2004). For histological analysis, embryos were embedded in paraffin and sectioned at 4 µm.…”

Ets2-dependent trophoblast signalling is required for gastrulation progression after primitive streak initiation

“…2b,f) indicates defective development of posterior and proximal epiblast derivatives. Third, an anatomical node (anterior PS derivative), which at E7.5-7.75 constitutes a bilayered structure at the distal tip of the embryo 13,14 (Fig. 2a), was never observed in the mutants (n ¼ 2; Fig.…”

Gutsy moves in mice: cellular and molecular dynamics of endoderm morphogenesis

“…Upon lineage-specific removal of b-catenin, mesoderm-like cells are present in the endoderm layer of mutant embryos suggesting a transfating of endoderm to mesoderm [47], and deletion of b-catenin specifically in Sox17-expressing cells prevents endoderm formation [48]. Nonetheless, based on marker expression, it has been suggested that DE and mesoderm cell lineages might already be predetermined before gastrulation [49], though it should be noted that markers indicate the state, not fate, of cells. If a mesendodermal state indeed exists within the mouse embryo, it is worth considering how it might tie in with a dispersal model of endoderm morphogenesis.…”