Overall, we made an excellent progress in projects addressing three major aims. This resulted in publication of 4 research papers (Fabrèges et al. 2024 Science; Moghe et al. 2025 Nat Cell Biol; Ichikawa et al. 2025 bioRxiv; Guruciaga et al. 2024 arXiv).
The first study (Fabrèges et al. 2024 Science) demonstrated a key role of cell division timing variability in embryonic patterning, hence revealing the coordination between the temporal progression of cell cycle and the spatial cell allocation. To this end, we built a morphomap of embryogenesis in mouse, rabbit and monkey embryos, which revealed that although cell divisions desynchronise passively, 8-cell embryos display robust 3D morphogenesis. Using topological analysis and genetic perturbations in mouse, we showed that embryos progressively change their cellular connectivity to a preferred topology, which can be predicted by a physical model where noise and actomyosin-contractility facilitate topological transitions lowering surface energy. This favours compact embryo packing at the 8- and 16-cell stages and promotes higher number of inner cells. Synchronised division reduced embryo packing and generated more mis-allocated cells and fewer inner-cell-mass cells.
The second study (Moghe et al. 2025 Nat Cell Biol) revealed that coupling of spatio-temporal developmental parameters ensures patterning robustness. This study investigated how precision in patterning is achieved despite the inherent developmental variability, using mouse blastocysts in which salt-and-pepper epiblast (EPI) and primitive endoderm (PrE) cells pattern the inner cell mass. Measuring cell fate and dynamics, we found that PrE cells form apical polarity-dependent protrusions required for migration towards the fluid cavity surface, where they are trapped due to decreased surface tension. Concomitantly, PrE cells deposit an extracellular matrix gradient, breaking the tissue-level symmetry and collectively guiding their own migration. Tissue size perturbations of mouse embryos and their comparison with monkey and human blastocysts further demonstrate that the fixed proportion of PrE/EPI cells is optimal with respect to tissue size and geometry and, despite variability, ensures patterning robustness.
Finally, in the studies under peer-review (Ichikawa et al. 2025 bioRxiv; Guruciaga et al. 2024 arXiv), we showed in experiments and theory that tissue-tissue boundaries guide EPI cell alignment and central lumen positioning, presenting a mechanism and functional significance of EPI tissue patterning in spatial contexts.
