2026 Theses Doctoral

Molecular, mechanical and cellular interactions between germ layers in vertebrate embryonic morphogenesis

Oikonomou, Panagiotis

Vertebrate morphogenesis requires the precise coordination of biochemical signaling, physical force generation, and cellular lineage specification across germ layer boundaries. This thesis investigates the integrated mechanisms driving avian posterior body axis formation, focusing on how the endoderm interacts with adjacent tissues through chemo-mechanical coupling, physical force transmission, and unexpected cellular plasticity.

In the first chapter, we develop a two-dimensional chemo-mechanical model of avian hindgut morphogenesis to investigate how molecular signals are transduced into tissue-scale mechanics. We show that posterior displacement of Fgf8-transcribing cells during axis elongation generates a graded FGF protein distribution via reaction-diffusion-advection dynamics, which the endoderm converts into a spatially graded contractile stress. Treating the endoderm as an active viscous fluid, the model reproduces key morphogenetic features of hindgut elongation and demonstrates that graded isotropic contraction is sufficient to produce large anisotropic cell movements, establishing a chemo-mechanical coupling that coordinates gut and body axis elongation.

In the second chapter, we address a fundamental methodological gap: the lack of accessible, quantitative tools for applying and measuring tissue-scale forces in vivo in epithelia. We introduce a simple micromechanical technique capable of applying forces in the 1–100 𝜇N range to the endodermal epithelium of the early chick embryo, and demonstrate its utility through a series of proof-of-concept experiments. These reveal mechanotype heterogeneity among midgut endoderm cells, non-cell-autonomous responses to cytoskeletal perturbation, and strong mechanical coupling between the endoderm and the adjacent paraxial mesoderm, underscoring that the endoderm is mechanically integrated with neighboring tissues. Applied to primary neurulation, we further show that forces of ∼10 𝜇N suffice to unzip the closing neural tube, demonstrating broad applicability of the method beyond the endoderm.

In the third and final chapter, we report the discovery of a previously unrecognized progenitor subpopulation residing in the node endoderm. At the trunk-to-tail developmental transition, these cells undergo an FGF-dependent epithelial-to-mesenchymal transition and ingress to generate a wide range of cell types classically ascribed to other germ layers, including somitic mesoderm, notochord, and neural tube. Disruption of this ingression leads to a severe reduction in axis elongation rate, revealing an indispensable contribution of the endoderm to axial growth. Single-cell transcriptomics and lineage tracing demonstrate that these cells lose their endodermal identity upon ingression and acquire neuromesodermal progenitor (NMP) characteristics, with individual clones spanning endoderm, mesoderm, and ectoderm fates. This finding fundamentally challenges the classical doctrine of strict germ layer segregation by identifying the endoderm as a source of multipotent axial progenitors.

Together, this work establishes that vertebrate morphogenesis is not a collection of isolated tissue-specific programs, but a highly coupled system where molecular gradients, mechanical stresses, and lineage-crossing cellular flux operate across traditional germ layer boundaries to coordinate embryonic development.