Primordia black screen5/26/2023 Endodermal cells also migrate toward the midline to form the primitive foregut tube and associated organs, including liver and pancreas ( Warga and Nusslein-Volhard 1999 Ober et al. 2000) and (3) a still unclear mechanism that disrupts intrinsic properties of myocardial migration, affected in miles apart mutant embryos, in which myocardial differentiation and endoderm specification are otherwise normal ( Kupperman et al. 2000 Trinh and Stainier 2004) (2) correct endoderm specification, since mutant embryos that lack endoderm ( bonnie and clyde, casanova, faust, and one-eye pinhead) also display cardia bifida ( Schier et al. Analyses of these mutations have pinpointed several requirements for heart tube assembly (for review, see Stainier 2001): (1) correct differentiation and morphogenesis of myocardial precursors, since mutations that impair myocardial differentiation ( faust and hands off) or epithelial polarity of myocardial precursors ( natter) are associated with cardia bifida phenotypes ( Reiter et al. In the case of heart tube assembly, genetic screens in zebrafish have identified eight mutations that disrupt this process, resulting in the formation of two laterally positioned hearts (a phenotype known as cardia bifida Chen et al. Similar events determine the formation of primitive foregut structures including duodenum, liver, and pancreas ( Warga and Nusslein-Volhard 1999 Ober et al. ![]() For example, myocardial precursors appear bilaterally in the anterior lateral plate mesoderm and migrate toward the midline, where they assemble to form a primitive heart tube in all vertebrates analyzed so far (for review, see Stainier 2001). Overall, our results uncover a late, previously unexpected role of noncanonical Wnt signaling in the control of midline assembly of organ precursors during vertebrate embryo development.ĭuring embryonic development in vertebrates, precursor cells of many organs initially appear as bilateral populations of cells that subsequently migrate toward the embryonic midline, where they assemble to form the primitive organs. ![]() The same ligands are also required, nonredundantly, to control specific aspects of CE that involve interaction of Dishevelled with mediators different from that of RhoA GTPase. We also show that three Wnt noncanonical ligands wnt4a, silberblick/wnt11, and wnt11-related regulate this process by acting in a largely redundant way. Here we show that the key modular intracellular mediator Dishevelled, through a specific activation of RhoA GTPase, controls the process of convergence of endoderm and organ precursors toward the embryonic midline in the zebrafish embryo. However, the complexity of these pathways and the wide patterns of expression and activity displayed by some of their components immediately suggest additional morphogenetic roles beyond the control of CE. Several components of noncanonical Wnt signaling pathways are involved in the control of convergence and extension (CE) movements during zebrafish and Xenopus gastrulation.
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