Article

Wnt Signaling Mediates Self-Organization and Axis Formation in Embryoid Bodies

Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305, USA.
Cell stem cell (Impact Factor: 22.15). 12/2008; 3(5):508-18. DOI: 10.1016/j.stem.2008.09.013
Source: PubMed

ABSTRACT Embryonic stem cells (ESCs) form descendants of all three germ layers when differentiated as aggregates, termed embryoid bodies. In vivo, differentiation of cells depends on signals and morphogen gradients that provide instructive and positional cues, but do such gradients exist in embryoid bodies? We report here the establishment of anteroposterior polarity and the formation of a primitive streak-like region in the embryoid body, dependent on local activation of the Wnt pathway. In this region, cells undergo an epithelial-to-mesenchymal transition and differentiate into mesendodermal progenitors. Exogenous Wnt3a protein posteriorizes the embryoid body, resulting in predominantly mesendodermal differentiation. Conversely, inhibiting Wnt signaling promotes anterior character and results in neurectodermal differentiation. The activation of Wnt signaling and primitive streak formation requires external signals but is self-reinforcing after initiation. Our findings show that the Wnt pathway mediates the local execution of a gastrulation-like process in the embryoid body, which displays an unexpected degree of self-organization.

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    • "Next, we addressed whether the differentiation-inducing effect of WNT on EpiSCs explains the conflicting reports on the role of WNT in ESCs. While we previously demonstrated that endogenous WNT signals support ESC selfrenewal by inhibiting their differentiation into EpiSCs (ten Berge et al., 2011), we and others also demonstrated that WNT signals induce differentiation of ESCs in EBs (Nostro et al., 2008; ten Berge et al., 2008). However, a transient EpiSC signature has been detected in differentiating EBs (Zhang et al., 2010). "
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    • "Cardiomyocyte-specific staining of TaP4 EBsections with antibodies against sarcomeric a-actinin revealed efficient commitment to the cardiac line (Fig. 1A, B). CMs were found in the inner compartment of the EBs and tended to undergo spatial polarization as described for the mesodermal germ layer during murine EB development [20]. Similar results were found for differentiated aPIG44 EBs (Fig. 1C, D). "
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    • "However they are unable to display the morphogenetic behaviour, germ layer distribution or axial organization that are characteristics of embryos, resulting in spatial disorganization (Leahy et al., 1999; Sajini et al., 2012). In one report, treatment of EBs with Wnt leads to a weak polarization in gene expression in some aggregates but no clear morphogenesis is observed (Berge et al., 2008). In more recent reports, EBs that have been cultured for extended periods of time develop anterior structures, retinas and cortex, which mimic their embryonic counterparts but develop without the context of an axial organization (Eiraku et al., 2011; Lancaster et al., 2013). "
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