Tissue mechanics modulate morphogen signalling to induce the head organiser.

Morphogenetic movements and specification of germ layers during gastrulation are key processes that establish the vertebrate body plan. Despite substantial research into the role of tissue mechanics during gastrulation and detailed characterisation of the molecular signalling networks controlling fate determination, the interplay of mechanical cues and biochemical signals during fate specification is poorly understood. Morphogens that activate Activin/Nodal/Smad2 signalling play a key role in mesoderm induction and axial patterning. We investigate the interplay between a single molecular input and a mechanical input using the well-established ex vivo system of Activin-induced explants of the mid-blastula X. laevis animal cap ectoderm. Activin alone induces mesoderm to form a complex elongating tissue with axial patterning, making this system similar to gastruloids generated in other model organisms. We observed an increase in the expression of dorsal mesoderm markers, such as chordin and goosecoid, and loss of elongation, in Activin-induced explants that were mechanically stimulated through uniaxial compression during the induction period. In addition, head mesoderm specific markers, including cerberus 1, were also increased. We show that mechanical stimulation leads to an increase in nuclear β-catenin activity. Activation of β-catenin signalling is sufficient to induce head Organiser gene expression. Furthermore, inhibition of β-catenin is sufficient to rescue the effect of compression on an early Wnt-signalling response gene siamois. Taken together these observations support the role of mechanical stimulation in modulating Activin-dependent mesoderm induction in favour of head Organiser formation. Given the conserved role of β-catenin in the dorsal specification and the dynamic morphogenetic movements of dorsal gastrula regions, mechanics-dependent Organiser induction may be found in other vertebrate species. Finally, the finding that mechanical cues affect β-catenin-dependent axial specification can be applied in the future development of more biologically relevant and robust synthetic organoid systems.