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Fig. 7. Role of early Nodal signaling in overcoming dominant inhibitory signals from the ExE. Nodal initially maintains pluripotent cells in the epiblast (dark red) and must overcome dominant inhibitory signals from the ExE before it can specify DVE. Between E4.5 and 5.0, Nodal signaling (blue) specifies the VE abutting the epiblast to become EmVE and provide an adhesive substratum for the epiblast. Paracrine signals from the ExE (gray) pattern the VE in the extra-embryonic region (ExVE) (Dziadek, 1978), and prevent the conversion of EmVE into DVE (Rodriguez et al., 2005). Therefore, to establish the competence for DVE formation, we propose that EmVE first must promote elongation of the egg cylinder to distance its apex from the ExE. In addition, after overcoming the non-permissive stage, sustained Nodal activity potentiated by the proprotein convertases Furin and Pace4 (Beck et al., 2002) is also essential as an instructive signal, which actively induces DVE-specific genes. In Nodal^lacZ/lacZ null mutants, the entire VE assumes an extra-embryonic character (ExVE) and subsequently fails to form DVE. In addition, the epiblast prematurely downregulates determinants of pluripotency (light red). Whether this is a consequence or a cause of the absence of the EmVE remains to be further investigated.

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