![]() ![]() To facilitate cross-organism comparisons, we first discuss the origin and fate of the endoderm across different organisms, as well as our understanding of the term ‘mesendoderm’ (see Glossary, Box 1). In this Review, we focus on the earliest stages of endoderm morphogenesis across different organisms, ranging from invertebrate to vertebrate models. Animals that derive from three definitive germ layers: ectoderm (from the Greek εκτοσ, meaning ‘outside’), mesoderm (Greek µεσοσ, ‘middle’) and endoderm (Greek ενδον, ‘inside’). Mesenchymal cells polarize and start expressing adhesion proteins to become epithelial. Cells that can give rise to either mesoderm or endoderm, either by cell division and daughter cells having distinct fates, or in response to inductive signals from environment. ![]() In-pocketing of a sheet of cells for example, the future embryonic gut in several species. Cell neighbour exchange for example, cells joining an epithelium or resident within an epithelium and exchanging neighbours. Cells exiting an epithelium and moving into the body of a tissue mass ( Schöck and Perrimon, 2002). A continuum of states characterized by loss of polarity and adhesive properties of epithelial cells and acquisition of a mesenchymal identity. Cells intercalating into an epithelium ( Schöck and Perrimon, 2002).ĮMT (epithelial-mesenchymal transition). A cell migration phenomenon in which cells migrate in loosely or closely associated groups, and affect one another while doing so ( Rorth, 2012).ĭiplobastic. Blastoderm gives rise to ectoderm, endoderm and mesoderm during gastrulation.Ĭollective cell migration. An epithelial layer that forms within the blastula and encloses blastocoel. The conserved features of early endoderm morphogenesis are somewhat surprising, given that although many of the upstream signals directing cells towards an endoderm identity are conserved between vertebrates, they are not conserved between invertebrates and vertebrates.īlastoderm. Increasingly, endoderm development in different organisms is being used to model these basic cellular processes ( Campbell et al., 2011 Nakaya et al., 2008 Pert et al., 2015 Viotti et al., 2014b), which play key roles in the formation of many tissues and are implicated in several pathogenic events, such as cancer metastasis ( Campbell et al., 2019 Campbell, 2018 Cheung and Ewald, 2016 Friedl and Gilmour, 2009 Nieto et al., 2016). These include a series of tightly coordinated and precisely timed morphogenetic processes, including epithelial-to-mesenchymal transitions (EMTs see Glossary, Box 1), collective cell migration (see Glossary, Box 1) and mesenchymal-to-epithelial transitions (METs see Glossary, Box 1). These approaches have yielded a wealth of new data indicating that, although endoderm organs may vary in their form and function, both within and across species, they share many mechanisms that orchestrate their earliest stages of development. However, in recent years, studies of the endoderm have been aided by the identification of robust molecular markers that identify endoderm cells, coupled with high-resolution time-lapse and, in some cases, deep-tissue imaging techniques. Moreover, in amniotes, the squamous epithelial nature of the nascent endoderm epithelium makes gene expression hard to localize by mRNA in situ hybridization. Furthermore, nascent endoderm cells are not easily morphologically distinguishable from adjacent tissues in many model organisms. For example, the endoderm represents approximately 3.5% of all cells of the mouse embryo-proper at midgestation ( Nowotschin et al., 2019). it being internal) and difficulties in visualizing it during normal and perturbed development, but also because it comprises just a small proportion of the bulk of cells in an embryo at any given stage. ![]() Investigating the endoderm had been hindered, in part due its inaccessibility (i.e. Until recently, the behaviour of endoderm cells during early stages of development had been relatively understudied and poorly understood. ![]()
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