Background: Body axis formation in animals requires symmetry breaking, with underlying mechanisms often obscured by maternal determinants and tight developmental constraints in embryos. The cnidarian Nematostella vectensis provides a unique system of self-organization to study spontaneous symmetry breaking, as dissociated embryonic cells regenerate into viable polyps with axial polarity and germ layer topology reestablishment via Wnt1/Wnt3+ endodermal organizers.
Key Question: How do initially dispersed organizer cells break symmetry and coordinate with other germ layers to reestablish axial polarity and proper germ layer topology in Nematostella gastruloids?
Key Findings: Using standardized gastruloids, live imaging, FISH, and single-cell RNA-seq, we show that dissociation disrupts local cell-cell signaling, thereby inhibiting Notch signaling as a result. This inhibition leads to a transient cell state change from endoderm (FoxA+) to endomesoderm (FoxA/Cdh1+) thus enabling their early intercalation into mesodermal clusters. This reestablished contact between endoderm (Notch receptor+) and mesoderm (Notch ligand+), reactivates Notch signaling which is subsequently required for resegregation of these germ layers and for endodermal polarization. This was further verified by exogenous Notch inhibition during gastruloid development, resulting in loss of endodermal polarity and subsequent loss of head structures. Additionally, we demonstrate interplay of Notch and Wnt signaling in maintaining this endodermal identity.
Evolutionary Implications: Conservation of Notch-mediated boundary formation between endoderm and mesoderm mirrors bilaterian mechanisms, suggesting deep evolutionary roots for this regulatory axis. The Wnt-Notch circuit in Nematostella gastruloids—where Wnt drives primary patterning and Notch refines germ layer segregation—parallels regulatory interactions in bilaterian organizers, indicating an ancient origin for this developmental logic. Strikingly, Nematostella gastruloids exhibit developmental plasticity (e.g., endoderm-to-endomesoderm conversion) akin to bilaterian systems. The plasticity in the use of such conserved regulatory modules, increases the potential morphospace, which may be responsible for the evolutionary robustness of developmental systems. These findings bridge cnidarian and bilaterian self-organization paradigms, demonstrating how ancestral regulatory networks governing morphogenesis likely enabled the diversification of metazoan body plans.