Guts and gastrulation: Emergence and convergence of endoderm in the mouse embryo

Gastrulation is a central process in mammalian development in which a spatiotemporally coordinated series of events driven by cross-talk between adjacent embryonic and extra-embryonic tissues results in stereotypical morphogenetic cell behaviors, massive cell proliferation and the acquisition of distinct cell identities. Gastrulation provides the blueprint of the body plan of the embryo, as well as generating extra-embryonic cell types of the embryo to make a connection with its mother. Gastrulation involves the specification of mesoderm and definitive endoderm from pluripotent epiblast, concomitant with a highly ordered elongation of tissue along the anterior-posterior (AP) axis. Interestingly, cells with an endoderm identity arise twice during mouse development. Cells with a primitive endoderm identity are specified in the preimplantation blastocyst, and which at gastrulation intercalate with the emergent definitive endoderm to form a mosaic tissue, referred to as the gut endoderm. The gut endoderm gives rise to the gut tube, which will subsequently become patterned along its AP axis into domains possessing unique visceral organ identities, such as thyroid, lung, liver and pancreas. In this way, proper endoderm development is essential for vital organismal functions, including the absorption of nutrients, gas exchange, detoxification and glucose homeostasis.