Tissue rigidity actively regulates cell polarity and embryonic development

Embryonic development is one of the most dynamic biological processes in nature. Cells and tissues organize and reorganize themselves following incredibly precise patterns, while remaining flexible and robust. Scientists are increasingly probing the role the physical properties of embryonic tissues – such as rigidity or stiffness – play in this process. 

A new set of investigations from EMBL Heidelberg's Petridou Group and collaborators shows that not only is tissue rigidity actively regulated within embryos, but it also strongly influences signaling processes that determine cell polarity and fate, i.e. which cells eventually give rise to which types of tissue.

To reach these conclusions, the researchers used a combination of theoretical modelling, advanced live microscopy, and precise molecular bioengineering. The results were described in two recent papers, published in Nature Physics and Nature Cell Biology, respectively. 

Our lab studies embryogenesis, with a strong integration of biology and physics. Embryos are typically believed to follow genetic determinism – the egg cell contains much of the information needed for early development. However, the execution of this information is not just molecular; it also involves how cells interact physically."

Nicoletta Petridou, Group Leader at EMBL and senior author of the papers

How adhesion regulates rigidity

The team used zebrafish embryos for their studies, which, by virtue of being transparent and amenable to various perturbations, can act as a versatile model system for vertebrate embryo development. Early in development, the embryo transforms from a uniform mass of pluripotent cells – cells which have the potential to become multiple different cell types – to a much more complex, layered structure where cells have specific identities. The researchers focused on the tissue-level changes that occur during this important transition. 

To understand how tissue rigidity is regulated in the developing embryo, the researchers relied on a combination of quantitative measurements, theoretical analysis, and genetic and optogenetic tools. They studied factors such as cell density – how closely cells are packed in a tissue, and cell-cell adhesion – how tightly cells attach to their neighbours. They also used insights from previous studies on the physics of granular materials. 

The scientists found that although both cell-cell adhesion and cell density change during normal development, only cell-cell adhesion is a key regulator of tissue rigidity. Specifically, increasing cell-cell adhesion helps the tissue transition from a fluid-like to a solid-like state, rather like water turning into ice at low temperatures. 

When tissues become 'stiff' or 'frozen' like this, cells pack very tightly and form specialised contacts, making the tissue dense and non-porous. By experimentally uncoupling the contributions of cell-cell adhesion and cell density, the researchers showed that tissues change their organisation significantly upon altering cell-cell adhesion. 

"Strikingly, increasing cell-cell adhesion without a simultaneous increase in cell density led to the formation of large fluid-filled lumens," explained Laura Rustarazo-Calvo, Predoctoral Fellow in the Petridou Group and first author of one of the studies. "Cells lining these cavities became polarised, localising specialised proteins to the lumen-facing surface, suggesting that adhesion alone can initiate aspects of epithelial organisation."

How rigid tissues trap developmental signals

This gave rise to the question of what role this transition plays, functionally, in the embryo. "We found that these properties can also provide instructive cues for development," said Petridou. "Specifically, they can change concentrations of molecules within the embryo, which in turn affects when and how cells change their identity."

To do this, the team looked closely at a process called morphogen signalling. Morphogens are small molecules that diffuse across the embryo, and their concentration at a particular position tells the cells there what identities they should acquire. For this study, the researchers looked at a particular morphogen called Nodal, which helps pattern the mesoderm and endoderm – tissue layers that eventually form most of the body's internal tissues and organs, like muscles, heart, connective tissues and gut.

Just like fish would have trouble swimming across a partially frozen ocean, the scientists found that increasing tissue rigidity results in Nodal becoming trapped in a particular region, limiting the range at which it acts.

"This complements earlier studies on morphogen gradients focused on the biochemical regulation of diffusion," said Petridou. "Here we add that the physical trapping of molecules helps localise signals in space and time, ensuring proper tissue specification. This also provides a way to compartmentalise signals, which is important since many processes occur simultaneously within the embryo."

Interestingly, there exists another layer to this. Since Nodal signalling can directly regulate cell-cell adhesion, this trapping process can also enhance tissue rigidity locally. This essentially gives rise to a feedback loop. 

"The two processes of tissue mechanics and morphogen signalling have been deeply studied but rarely linked," said Camilla Autorino, Bridging Postdoctoral Fellow in the Petridou Group and first author of one of the studies. "During this work, we got growing evidence that they are dynamically tuning one another over development: they need each other to progress. These findings highlight how interdisciplinary work can elevate our understanding of biological mechanisms."

The studies involved collaborations with the labs of Zena Hadjivasiliou at The Francis Crick Institute, UK, and Bernat Corominas-Murtra at the University of Graz, Austria. Hadjivasiliou's team are experts in using reaction-diffusion systems – a type of mathematical model that can help sort out processes involving concentration gradients, such as morphogen signalling – to understand biological organisation. Corominas-Murtra's team, on the other hand, studies the statistical physics that underlie the emergence of biological complexity.

"These studies show that tissue material properties do more than enable mechanical deformation – they actively influence biological information," said Petridou. "Tissue material states and their transitions are deeply integrated within developmental biology, with continuous crosstalk between physical and biochemical processes."