For objective 1, we have followed up on our previous observations that at the onset of zebrafish morphogenesis, the blastoderm undergoes rapid, pronounced and spatially patterned tissue fluidization, which is important for blastoderm spreading, and that this fluidization is temporally controlled by mitotic cell rounding-dependent cell-cell contact disassembly during the last rounds of cell cleavages (Petridou et al., 2019, Nature Cell Biology). To understand how the blastoderm tissue undergoes fluidization, we have used – in collaboration with Prof. Hannezo at IST Austria – percolation theory to theoretically describe how changes in blastoderm cell-cell contacts can lead to an abrupt and strong tissue fluidization, as observed within the blastoderm at the onset of gastrulation. We found that there is a critical point in cell connectivity within the blastoderm, at which the blastoderm undergoes a rigidity transition leading to its fluidization (Petridou et al, 2021, Cell). This provides a conceptual framework for explaining how cell connectivity within tissues relate to tissue material properties, and what role such relationship might play in embryonic development.
For objective 2, we have analysed how enveloping layer (EVL) morphogenesis/spreading and EVL cell fate specification/differentiation are related to each other. Specifically, we have analysed the interplay between intermediate filament expression, hallmark of EVL specification, and EVL spreading during zebrafish gastrulation. We found that during the spreading process the EVL tissue becomes increasingly stiffer, and that this tissue stiffening process relies on keratin intermediate filament expression and organization into a filamentous network within EVL cells. We also found that interfering with keratin expression within the EVL leads to reduced EVL stiffness and impaired epiboly movements. Finally, we observed that tension within the plane of the EVL tissue, as a result of the yolk syncytial layer (YSL) pulling on the margin of the EVL, triggers keratin expression within the EVL, pointing at the possibility of keratin expression being regulated by mechanical signals. Collectively, these findings suggest that EVL cell specification and differentiation, as evident by their expression of keratin intermediate filaments, is controlled by tissue tension and leads to tissue stiffening, thereby revealing a mechanism by which mechanical tension control cell fate specification and tissue stiffening.
For objective 3, we have focussed on understanding how cell-cell contact formation functions in germ layer self-organisation by studying the influence of actomyosin cell cortex tension on the formation of adhesive contacts between zebrafish progenitor cells. We found that when cortex tension was massively up-regulated, cell-cell contacts did not grow big, as expected from previous models, but, instead, remained very small. This unexpected effect, by which the cell-cell contacts size limits rather than promotes contact expansion, is due to cortical tension reducing turnover of cadherin adhesion molecules at the contact, required for fast contact growth (Slovakova et al., 2022, PNAS).
